EP2710158B1 - High strength steel flat product and method for its production - Google Patents

Description

The invention relates to a high-strength flat steel product and to a method for producing such a flat steel product.

In particular, the invention relates to a high-strength flat steel product provided with a metallic protective layer and to a method for producing such a product.

When it comes to flat steel products, this refers to steel strips, sheets or sheet metal blanks derived therefrom, such as sinkers.

Unless otherwise stated, in the present text and in the claims, the contents of certain alloying elements in each case in wt .-% and the proportions of certain microstructural constituents in area% indicated.

If cooling or heating rates or rates are mentioned below, then cooling rates are given a negative value because they lead to a decrease in temperature. Accordingly, show Cool down at a rapid cooling a lower value than at a slower cooling. The leading to an increase in temperature heating rates, however, are given a positive.

High strength steels regularly tend to corrode because of their alloying constituents and are therefore typically coated with a metallic protective layer which protects the respective steel substrate from contact with the ambient oxygen. Various methods for applying such a metallic protective layer are known. These include hot-dip coating, also known in technical language as "fire-coating", and electrolytic coating.

Whereas in electrolytic coating, the coating metal is deposited electrochemically on the flat steel product to be coated, which is at most slightly heated in the process, during hot dip coating the products to be coated are subjected to a heat treatment before immersion in the respective melt bath. In this case, the respective flat steel product is heated to high temperatures under a certain atmosphere in order to set the desired structure and to produce an optimum for the adhesion of the metallic coating surface state of the respective flat steel product. Subsequently, the flat steel product passes through the melt bath, which also has an elevated temperature has to keep the coating material molten.

The necessarily high temperatures require an upper limit of the strength of 1000 MPa for flat-rolled steel products provided by hot-dip coating with a metallic protective layer. Steel flat products with an even higher strength can not usually be fire-rated because they suffer considerable strength losses as a result of the associated heating due to tempering effects. High-strength flat steel products are therefore currently regularly electrolytically provided with a metallic protective layer. This step requires a perfectly clean surface, which can be guaranteed in practice only by a to be carried out before the electrolytic coating pickling.

• Warmwalzen eines Warmbands aus einer Bramme,
• Kaltwalzen des Warmbands zu einem Kaltband,
• Wärmebehandeln des Kaltbands, wobei im Zuge dieser Wärmebehandlung
  • das Kaltband mit einer mittleren Erwärmungsgeschwindigkeit von maximal 2 °C/s von einer Temperatur, die um 50 °C niedriger ist als die Ac3-Temperatur des Stahls, aus dem das Kaltband besteht, auf die jeweilige Ac3-Temperatur erwärmt wird,
  • das Kaltband anschließend für mindestens 10 s auf einer Temperatur gehalten wird, die mindestens der jeweiligen Ac3-Temperatur entspricht,
  • daraufhin das Kaltband mit einer mittleren Abkühlgeschwindigkeit von mindestens 20 °C/s auf eine Temperatur abgekühlt wird, die 100 - 200 °C unterhalb der Martensitstarttemperatur des jeweils verarbeiteten Stahls liegt, und
  • schließlich das Kaltband für 1 bis 600 s auf eine 300 - 600 °C betragende Temperatur erwärmt wird.

From the EP 2 267 176 A1 discloses a method for producing a high-strength cold strip provided with a hot-dip coated metallic protective coating, comprising the following steps:

• Hot rolling a hot strip from a slab,
• Cold rolling the hot strip to a cold strip,
• Heat treating the cold strip, being in the course of this heat treatment
  • the cold-rolled strip with an average heating rate of max. 2 ° C / s of a temperature which is 50 ° C lower than the Ac3 temperature of the steel constituting the cold strip heated to the respective Ac3 temperature,
  • The cold strip is then held for at least 10 s at a temperature which corresponds at least to the respective Ac3 temperature.
  • then the cold strip is cooled at a mean cooling rate of at least 20 ° C / s to a temperature which is 100 - 200 ° C below the martensite start temperature of each processed steel, and
  • Finally, the cold strip for 1 to 600 s is heated to a 300-600 ° C temperature.

Finally, the steel strip is dip-coated. The metallic coating applied in this case should preferably be a zinc coating. As a result, a cold strip is to be obtained in this way, the optimized mechanical properties, such as a tensile strength of at least 1200 MPa, an elongation of at least 13% and a hole widening of at least 50%, has.

The cold rolled strip processed in the manner described above is said to be made of a steel containing, in addition to iron and unavoidable impurities (in% by weight) 0.05-0.5% C, 0.01-2.5% Si, 0.5 3.5% Mn, 0.003-0.100% P, up to 0.02% S and 0.010-0.5 A1 contains. At the same time, the steel should have a microstructure comprising (in area%) up to 10% ferrite, up to 10% martensite and 60-95% tempered martensite, and moreover 5-20% retained austenite, as determined by X-ray diffraction , In addition, the steel (in weight%) can be 0.005 - 2.00% Cr, 0.005 - 2.00% Mo, 0.005 - 2.00% V, 0.005 - 2.00% Ni and 0.005 - 2.00% Cu and 0.01-0.20% Ti, 0.01-0.20% Nb, 0.0002-0.005% B, 0.001-0.005% Ca and 0.001-0.005% of rare earths.

In addition to the above-described prior art is from the CA 2 734 976 ( WO 2010/029983 ) a steel with good toughness and ductility, which should have a tensile strength of at least 980 MPa. In addition to iron and unavoidable impurities (in% by weight), the steel contains 0.17-0.73% C, up to 3.0% Si, 0.5- 3.0% Mn, up to 0.1% P, up to 0.07% S, up to 3.0 A1 and up to 0.010% N. The sum of the Al and Si contents should be at least 0.7%. At the same time, the proportion of martensite in the structure of the steel shall be at least 5 - 90%, that of residual austenite in the range of 5 - 50%, and the proportion of ferritic bainite derived from "upper bainite" at least 5, with respect to the totality of all microstructure constituents %. The term "upper bainite" refers to a bainite in which fine carbide grains are distributed evenly, as they are not found in "lower bainite". Higher levels of upper bainite occupied by embodiments of 17% and more are considered to be advantageous to those in this state The technology sought to produce high retained Austenitgehalte in the structure.

Finally, the post-published prior art discloses EP 2 546 368 A1 a method for producing high-strength steel sheet in which the sheet is kept in the range of a cooling stop temperature of Ms to (Ms-150 ° C) for 15-100 seconds to allow a part of the unconverted austenite to proceed to martensitic transformation. Against the background of the prior art explained above, the object of the invention was to provide a low-cost producible high-strength flat steel product which has further optimized mechanical properties, which are expressed in particular in a very good bending behavior.

In addition, a method for producing such a flat steel product should be specified. In particular, this method should be incorporated into a process for hot dip coating of flat steel products.

This object has been achieved in relation to the flat steel product according to the invention in that such a product has the features specified in claim 1. With regard to the method, the solution according to the invention of the abovementioned object consists in that during the production of a flat steel product according to the invention at least the steps mentioned in claim 6 are completed. In order to enable integration of the method according to the invention into a process for hot-dip coating, the operations specified in claim 7 can optionally also be carried out.

Advantageous embodiments of the invention are set forth in the dependent claims and will become hereafter as the general inventive idea explained in detail.

• eine Dehngrenze R_p0,2 von 600 - 1400 MPa,
• ein Streckgrenzenverhältnis R_p/R_m von 0,40 - 0,95,
• eine Dehnung A₅₀ von 10 - 30 %,
• ein Produkt R_m*A₅₀ aus Zugfestigkeit R_m und Dehnung A₅₀ von 15.000 - 35.000 MPa*%,
• eine Lochaufweitung von λ: 50 - 120 % $$\left(\mathrm{\lambda }=\left(\mathrm{df}/\mathrm{d}\mathrm{0}\right)/\mathrm{d}\mathrm{0}\mathrm{in}\left[\%\right]\mathrm{mit\; df}=\mathrm{Lochdurchmesser\; nach\; der\; Aufweitung\; und\; d}\mathrm{0}=\mathrm{Lochdurchmesser\; vor\; der\; Aufweitung}\right)$$
  
  und
• einen Bereich für den zulässigen Biegewinkel α (nach Rückfederung bei einem Biegedornradius = 2 x Blechdicke) von 100° - 180° (ermittelbar gemäß DIN EN 7438)

aus.An inventive flat steel product, which is optionally provided with a metallic protective layer applied by hot-dip galvanizing, has a tensile strength R _m of at least 1200 MPa. In addition, an inventive flat steel product is characterized by regular

• a yield strength R _p0.2 of _600-1400 MPa,
• a yield ratio R _p / R _m of 0.40-0.95,
• an elongation A ₅₀ of 10 - 30%,
• a product R _m * A ₅₀ of tensile strength R _m and elongation A ₅₀ of 15,000-35,000 MPa *%,
• a hole widening of λ: 50 - 120% $$\left(\mathrm{\lambda }=\left(\mathrm{df}/\mathrm{d}\mathrm{0}\right)/\mathrm{d}\mathrm{0}\mathrm{in}\left[\%\right]\mathrm{with\; df}=\mathrm{Hole\; diameter\; after\; widening\; and\; d}\mathrm{0}=\mathrm{Hole\; diameter\; before\; widening}\right)$$
  
  and
• a range for the permissible bending angle α (after springback at a bending mandrel radius = 2 x sheet thickness) of 100 ° - 180 ° (determinable in accordance with DIN EN 7438)

out.

For this purpose, a steel flat product according to the invention consists of a steel which, in addition to iron and unavoidable impurities (in% by weight) C: 0.10-0.50%, Si: 0.1-2.5%, Mn: 1.0. 3.5%, Al: up to 2.5%, P: up to 0.020%, S: up to 0.003%, N: up to 0.02%, and optionally one or more of the elements "Cr, Mo, V , Ti, Nb, B and Ca "in the following contents: Cr: 0, 1 - 0.5%, Mo: 0, 1 - 0.3%, V: 0.01 - 0.1%, Ti: 0.001 - 0.15%, Nb: 0.02 - 0, 05%, where for the sum Σ (V, Ti, Nb) the contents of V, Ti and Nb apply Σ (V, Ti, Nb) at most equal to 0.2%, B: 0.0005 - 0.005%, Ca: up to 0.01%.

Essential for the superior mechanical properties of the flat steel product according to the invention is that it has a structure with (in area%) less than 5% ferrite, less than 5% bainite, 5-70 unangelassenem martensite, 5-30% retained austenite and 25-80 % annealed martensite. At least 99% of the number of iron carbides contained in tempered martensite has a size of less than 500 nm.

The phase fractions of unstained and tempered martensite, of bainite and of ferrite are determined in the usual way in accordance with ISO 9042 (optical assessment). The retained austenite can additionally be determined by X-ray diffractometry with an accuracy of +/- 1 area%.

Consequently, in a steel flat product according to the invention, the content of so-called "over-tempered martensite" is reduced to a minimum. Over-tempered martensite is characterized in that more than 1% number of carbide grains (iron carbides) are more than 500 nm in size. For example, over-cut martensite can be detected by scanning electron microscopy at 20,000 magnifications on steel samples etched with 3% nitric acid. By avoiding over-leaning Martensite receives a flat steel product according to the invention optimum mechanical properties, which have a favorable effect in particular with regard to its bending properties, which are characterized by the high bending angle α of 100 ° to 180 °.

The C content of the steel of a flat steel product according to the invention is limited to values between 0.10 and 0.50 wt.%. Carbon influences a flat steel product according to the invention in many respects. First, C plays a major role in the formation of austenite and lowering the Ac3 temperature. Thus, a sufficient concentration of C allows complete austenitization at temperatures ≤ 960 ° C, even if at the same time elements, such as Al, are present, which increase the Ac3 temperature. During quenching, the retained austenite is also stabilized by the presence of C. This effect continues during the partitioning step. A stable residual austenite leads to a maximum strain range, in which the effect of the TRIP effect (TRANSformation Induced Plasticity) becomes noticeable. Furthermore, the strength of the martensite is most strongly influenced by the respective C content. Too high a content of C leads to such a strong shift of the martensite start temperature to ever lower temperatures that the production of the flat steel product according to the invention becomes excessively difficult. In addition, too high C contents can adversely affect weldability.

In order to ensure a good surface quality of a flat steel product according to the invention, the Si content in the steel of the flat steel product according to the invention should be less than 2.5% by weight. Silicon is important for suppressing cementite formation. The formation of cementite would break the C as a carbide and would then no longer be available for the stabilization of the retained austenite. In addition, the stretch would be worsened. The effect achieved by the addition of Si can in part also be achieved by alloying aluminum. However, a minimum of 0.1% by weight of Si should always be present in the flat steel product according to the invention in order to utilize its positive effect.

Manganese contents of 1.0-3.5% by weight, in particular up to 3.0% by weight, are important for the hardenability of the flat steel product according to the invention and the prevention of perlite formation during cooling. These properties make it possible to form a starting structure consisting of martensite and retained austenite, and as such is suitable for the partitioning step carried out according to the invention. In addition, manganese proves advantageous with regard to the setting of comparatively low cooling rates of, for example, faster than -100 K / s. On the other hand, an excessively high Mn concentration has a negative influence on the elongation properties and the weldability of a flat steel product according to the invention.

Aluminum is present in the steel of a flat steel product according to the invention in amounts of up to 2.5% Deoxidation and for setting any nitrogen present. As mentioned above, however, Al can also be used for the suppression of cementite and does not have such a negative effect on the surface properties as high contents of Si. However, Al is not as effective as Si and also increases the austenitizing temperature. Therefore, the Al content of a flat steel product according to the invention is limited to a maximum of 2.5% by weight and preferably to values of between 0.01 and 1.5% by weight.

Phosphorus is unfavorable to weldability and should therefore be present in the steel of a flat steel product of the present invention at levels less than 0.02% by weight.

Sulfur leads in sufficient concentration to the formation of MnS or (Mn, Fe) S, which has a negative effect on the elongation. Therefore, the S content in the steel of a flat steel product according to the invention should be below 0.003 wt .-%.

Bonded as nitride, nitrogen in the steel of a flat steel product according to the invention has a detrimental effect on the formability. The N content of a flat steel product according to the invention should therefore be less than 0.02% by weight.

To improve certain properties, "Cr, Mo, V, Ti, Nb, B and Ca" may be present in the steel of a flat steel product according to the invention.

Thus, with regard to optimizing the strength, it may be expedient to add one or more of the micro-alloying elements V, Ti and Nb to the steel of a flat steel product according to the invention. These elements contribute to higher strength through the formation of very finely divided carbides or carbonitrides. A minimum Ti content of 0.001% by weight leads to a freezing of the grain and phase boundaries during the partitioning step. However, too high a concentration of V, Ti and Nb can be detrimental to the stabilization of retained austenite. Therefore, the sum of the contents of V, Ti and Nb in a flat steel product according to the invention is limited to 0.2% by weight.

Chromium is an effective inhibitor of perlite, contributes to the strength and therefore may be added up to 0.5% by weight to the steel of a flat steel product according to the invention. Above 0.5% by weight, there is a risk of pronounced grain boundary oxidation. In order to be able to safely use the positive influence of Cr, the Cr content can be set to 0.1-0.5% by weight.

Like Cr, molybdenum is also a very effective element for suppressing perlite formation. In order to effectively use this favorable influence, the steel of a flat steel product according to the invention can be alloyed with 0.1-0.3% by weight.

Bor sighs on the grain boundaries and slows their movement. At levels of 0.0005% by weight, this leads to a fine-grained microstructure, which has an advantageous effect on the mechanical properties. When alloying B, however, sufficient Ti for the setting of the N must be present. At a level of about 0.005 wt%, saturation of the positive influence of B occurs. Therefore, the B content is set to 0.0005 - 0.005 wt%.

Calcium in contents of up to 0.01% by weight is used in the steel of a steel flat product according to the invention for setting sulfur and for inclusion modification.

mit

• %C: C-Gehalt des Stahls,
• %Mn: Mn-Gehalt des Stahls,
• %Si: Si-Gehalt des Stahls,
• %Cr: Cr-Gehalt des Stahls,
• %Mo: Mo-Gehalt des Stahls,
• %V: V-Gehalt des Stahls,
• %Ni: Ni-Gehalt des Stahls,
• %Cu: Cu-Gehalt des Stahls.

The carbon equivalent CE is an important parameter for the description of weldability. It should be in the range of 0.35 to 1.2 in the case of the steel of a flat steel product according to the invention, in particular 0.5 to 1.0. To calculate the carbon equivalent CE, one developed by the American Welding Society (AWS) and published in D1.1 / D1.1M: 2006, Structural Welding Code - Steel. Section 3.5.2. (Table 3.2). pp. 58 and 66, published formula used: $$\mathrm{CE}=\%\mathrm{C}+\left(\%\mathrm{Mn}+\%\mathrm{Si}\right)/\mathrm{6}+\left(\%\mathrm{Cr}+\%\mathrm{Not\; a\; word}+\%\mathrm{V}\right)/\mathrm{5}+\left(\%\mathrm{Ni}+\%\mathrm{Cu}\right)/\mathrm{15}.$$

With

• % C: C content of the steel,
• % Mn: Mn content of the steel,
• % Si: Si content of the steel,
• % Cr: Cr content of the steel,
• % Mo: Mo content of the steel,
• % V: V content of the steel,
• % Ni: Ni content of the steel,
• % Cu: Cu content of the steel.

The method according to the invention for producing a high-strength steel flat product, optionally provided with a metallic protective coating provided by hot-dip coating, comprises the following working steps:

It is an uncoated, so not yet provided with the respective protective cover flat steel product made available, which is made of the same steel, as the already explained above flat steel product according to the invention. Accordingly, in addition to iron and unavoidable impurities (in% by weight), the steel constituting the flat steel product contains C: 0.10-0.50%, Si: 0.1-2.5%, Mn: 1.0 - 3.5%, Al: up to 2.5%, P: up to 0.020%, S: up to 0.003%, N: up to 0.02%, and optionally one or more of the elements "Cr, Mo, V, Ti, Nb, B and Ca "in the following contents: Cr: 0.1 - 0.5%, Mo: 0.1 - 0.3%, V: 0.01 - 0.1%, Ti: 0.001 - 0.15%, Nb: 0.02 - 0.05%, where for the sum Σ (V, Ti, Nb) the contents of V, Ti and Nb holds Σ (V, Ti, Nb) ≤ 0.2 %, B: 0.0005 - 0.005%, Ca: up to 0.01%. The provided flat steel product may in particular be a cold-rolled flat steel product. However, it is also conceivable to process a hot-rolled flat steel product in accordance with the invention.

The flat steel product thus provided is then at a Austenitisierungstemperatur T _HZ lying above the Ac3 temperature of the steel of the steel flat product and at most 960 ° C at a heating rate θ _H1 , θ _H2 of at least 3 ° C / s heated. Fast heating reduces the process time and improves the overall cost-effectiveness of the process.

The heating to the austenitizing temperature T _HZ can be carried out in two uninterrupted successive stages with different heating rates θ _H1 , θ _H2 .

The heating at lower temperatures, ie below T _W , can be done very quickly to increase the efficiency of the process. At higher temperatures, the dissolution of carbides begins. For this purpose, lower heating rates θ _{H2 are} advantageous in order to ensure a uniform distribution of the carbon and other possible alloying elements, such. Mo or Cr. The carbides are deliberately annealed already below the A _c1 temperature to exploit the faster diffusion in the ferrite over the slower diffusion in austenite. Thus, the dissolved atoms can be distributed more uniformly in the material as a result of a lower heating rate θ _H2 .

In order to produce as homogeneous a material as possible, a limited heating rate θ _{H2 is} also favorable during the austenite transformation, ie between A _c1 and A _c3 . This contributes to a homogeneous starting structure before quenching and thus a uniformly distributed martensite and a fine retained austenite after quenching and ultimately improved mechanical properties of the flat steel product.

It has proved to be useful to reduce the heating rate at temperatures between 200 - 500 ° C. It surprisingly shows that even heating rates of 3 - 10 ° C / s can still be set without endangering the desired result.

Accordingly, in the two-stage heating, in order to achieve the desired characteristics of a flat steel product, the heating rate θ _{H1 of} the first stage may be 5 - 25 ° C / s and the heating rate θ _{H2 of} the second stage 3 - 10 ° C / s, especially 3 - 5 ° C / s. In this case, the flat steel product with the first heating rate θ _H1 can be heated to an intermediate temperature T _W of 200-500 ° C, in particular 250-500 ° C, and the heating can then be continued at the second heating rate θ _H2 up to the austenitizing temperature T _HZ .

After the austenitizing temperature T _{HZ has been} reached, according to the invention, the flat steel product is kept at the austenitizing temperature T _Hz for an austenitizing time t _HZ of 20-180 s. The annealing temperature in the holding zone should be above the A _c3 temperature in order to achieve complete austenitization.

mit

• %C: C-Gehalt des Stahls,
• %Ni: Ni-Gehalt des Stahls,
• %Si: Si-Gehalt des Stahls,
• %Mo: Mo-Gehalt des Stahls,
• %V: V-Gehalt des Stahls.

The A _c3 temperature of the respective steel is analysis-dependent and can either be detected conventionally by measurement or, for example, with the estimate the following empirical equation (alloy contents used in% by weight): $${\mathrm{A}}_{\mathrm{c}\mathrm{3}}\left[\mathrm{°\; C}\right]=\mathrm{910}-\mathrm{203}\sqrt{\%\mathrm{C}}-\mathrm{15}.\mathrm{2}\%\mathrm{Ni}+\mathrm{44}.\mathrm{7}\%\mathrm{Si}+\mathrm{31}.\mathrm{5}\%\mathrm{Not\; a\; word}+\mathrm{104}\%\mathrm{V}$$

With

• % C: C content of the steel,
• % Ni: Ni content of the steel,
• % Si: Si content of the steel,
• % Mo: Mo content of the steel,
• % V: V content of the steel.

After annealing at temperatures above A _c3 , the steel sheet is cooled to a cooling stop temperature T _Q greater than the martensite stop temperature T _Mf and less than the martensite start temperature T _Ms (T _Mf <T _Q <T _Ms ) at a cooling rate θ _Q ,

mit

• %C: C-Gehalt des Stahls,
• %Si: Si-Gehalt des Stahls,
• %Al: Al-Gehalt des Stahls,
• %Mn: Mn-Gehalt des Stahls,
• %Mo: Mo-Gehalt des Stahls,
• %Ti: Ti-Gehalt des Stahls,
• %Nb: Nb-Gehalt des Stahls;

The cooling to the cooling stop temperature T _Q is carried out according to the invention with the proviso that the cooling rate θ _{Q is} at least equal to, preferably faster than a minimum cooling rate θ _{Q (min)} (θ _Q ≤ θ _{Q (min)} ). The minimum cooling rate θ _{Q (min)} can be calculated according to the following empirical formula: $${\mathrm{\theta }}_{\mathrm{Q}\left(\min \right)}\left[\mathrm{°\; C}/\mathrm{s}\right]=-\mathrm{314}.\mathrm{35}\mathrm{°\; C}/\mathrm{s}+\left(\mathrm{268}.\mathrm{74}\%+\mathrm{56}.\mathrm{27}\%\mathrm{Si}+\mathrm{58}.\mathrm{50}\%\mathrm{al}+\mathrm{43}.\mathrm{40}\%\mathrm{Mn}+\mathrm{195}.\mathrm{02}\%\mathrm{Not\; a\; word}+\mathrm{166}.\mathrm{60}\%\mathrm{Ti}+\mathrm{199}.\mathrm{19}\%\mathrm{Nb}\right)\mathrm{°\; C}/\left(\mathrm{weight}\mathrm{,}-\%\cdot \mathrm{s}\right)$$

With

• % C: C content of the steel,
• % Si: Si content of the steel,
• % Al: Al content of the steel,
• % Mn: Mn content of the steel,
• % Mo: Mo content of the steel,
• % Ti: Ti content of the steel,
• % Nb: Nb content of the steel;

Typically, the cooling rate θ _{Q is} in the range of -20 ° C / s to -120 ° C / s. With cooling rates θ _Q of -51 ° C / s to -120 ° C / s, the condition θ _Q ≤ θ _{Q (min)} can be surely satisfied in practice even for steels having a low C or Mn content.

By maintaining the minimum cooling rate θ _{Q (min)} a ferritic and bainitic transformation is reliably avoided and a martensitic microstructure in the flat steel product with up to 30% retained austenite is set.

How much martensite is actually produced during cooling is dependent on how much the steel flat product is cooled in the course of cooling below the martensite start temperature (T _MS ) and on the holding time t _Q over which the flat steel product after the accelerated cooling is maintained at the cooling stop temperature becomes. According to the invention, a span of 10 to 60 seconds, in particular 12 to 40 seconds, is provided for the hold time t _Q. During the first approx. 3 to 5 seconds of holding, thermal homogenization takes place parallel to the martensitic transformation. In the next few seconds, dislocations are pinned by C-diffusion and finest precipitates appear. Thus causes a Extension of the holding time initially an increase in martensite and thus the yield strength. With increasing holding time, this effect weakens, and experience shows that after about 60 seconds, a decrease in the yield strength is observed.

Parallel to the yield strength increase can be achieved by the inventively carried out cooling to the cooling stop temperature and the subsequent holding of the flat steel product at this temperature over the times prescribed by the invention, an improvement of the forming properties. If tensile strength and tensile elongation are to be maximized, the holding time t _{Q should} rather be kept in the lower range, ie between 10 and 30 s. Longer holding times t _Q of 30 - 60 s tend to have a positive effect on the forming properties. This concerns in particular the bending angle.

mit

• %C: C-Gehalt des Stahls,
• %Si: Si-Gehalt des Stahls,
• %Al: Al-Gehalt des Stahls,
• %Mn: Mn-Gehalt des Stahls.

The martensite start temperature T _MS can be estimated by the following equation: $${\mathrm{T}}_{\mathrm{MS}}\left[\mathrm{°\; C}\right]=\mathrm{539}\mathrm{°\; C}+\left(-\mathrm{423}\%\mathrm{C}-\mathrm{30}.\mathrm{4}\%\mathrm{Mn}-\mathrm{7}.\mathrm{5}\%\mathrm{Si}+\mathrm{30}\%\mathrm{al}\right)\mathrm{°\; C}/\mathrm{weight}\mathrm{,}-\%$$

With

• % C: C content of the steel,
• % Si: Si content of the steel,
• % Al: Al content of the steel,
• % Mn: Mn content of the steel.

berechnet werden. Diese Gleichung ist aus der Koistinen-Marburger-Gleichung (s. D. P. Koistinen, R.E. Marburger, Acta Metall.7 (1959), S. 59 ) unter Zugrundelegung folgender Annahmen abgeleitet worden:

1. a) Die Martensitumwandlung wird als abgeschlossen betrachtet, wenn ein Martensitanteil von 95 % erreicht wird.
2. b) Die zusammensetzungunabhängige Konstante α beträgt -0,011.
3. c) Die Martensitstopptemperatur ist gleich der Kühlstopptemperatur.

The martensite _{stop temperature} T _Mf can be determined in practice by means of the equation $${\mathrm{T}}_{\mathrm{mf}}={\mathrm{T}}_{\mathrm{ms}}-\mathrm{272}\mathrm{°\; C}$$

be calculated. This equation is from the Koistinen-Marburger equation (s. DP Koistinen, RE Marburger, Acta Metall.7 (1959), p. 59 ) has been derived on the basis of the following assumptions:

1. a) Martensite transformation is considered completed when a martensite of 95% is achieved.
2. b) The composition-independent constant α is -0.011.
3. c) The martensite stop temperature is equal to the cooling stop temperature.

Typically, the cooling stop temperature T _{Q is} at least 200 ° C.

After cooling and holding the steel flat product at the cooling stop temperature T _Q , the steel flat product is heated to a 400-500 ° from the cooling stop temperature T _Q with a heating rate θ _P1 of 2 - 80 ° C / s, especially 2 - 40 ° C / s C, in particular 450 - 490 ° C, amounting temperature T _P heated.

The heating to the temperature T _P is preferably carried out within a heating time t _A of 1 - 150 s, in order to achieve optimum efficiency.

At the same time, the heating can make a contribution x _Dr to a diffusion length x _D explained below.

The purpose of the heating and a subsequent optional additional holding of the flat steel product at the temperature T _P over a holding period t _Pi of up to 500 s is the enrichment of the retained austenite with carbon from the supersaturated martensite. This is called "partitioning of carbon", also known as "partitioning". The holding period t _Pi is in particular up to 200 s, wherein holding periods t _Pi of less than 10 s are particularly practical.

The partitioning can already during the heating as a so-called "Ramped Partitioning" done by the held after the heating hold at the partitioning temperature T _P (so-called "isothermal" partitioning) or by a combination of isothermal and ramped partitioning. In this way, the high temperatures necessary for the subsequent hot-dip coating can be achieved without causing special tempering effects, ie over-tempering of the martensite. The slower heating rate θ _P1 envisaged for ramped partitioning in comparison to isothermal partitioning permits a particularly precise control of the respectively prescribed partitioning temperature T _P with reduced energy input, since higher temperature gradients require a higher energy expenditure in the system.

The negative effects of überangelassenem martensite, such as coarse carbides that block a plastic strain and have a negative impact on the strength of the martensite and the forming properties bending angle and hole widening, are avoided by the inventive heating to the holding temperature T _P , the optional holding in the Partitioning temperature, the security of avoidance of überangelassenem martensite additionally increased. In particular, the formation of carbides and the decomposition of retained austenite are selectively suppressed by complying with the inventively given total partitioning time t _PT , which is composed of the time t _{PR of} the ramped partitioning and the time of the isothermal partitioning t _PI , and partitioning temperature T _P.

At the same time, the inventively predetermined partitioning temperature T _P ensures sufficient homogenization of the carbon in the austenite, this homogenization being able to be influenced by the heating speed θ _P1 , the partitioning temperature T _P and the optional holding at the partitioning temperature T _P over a suitable holding time t _Pi .

To evaluate the homogenization of carbon in austenite, the so-called "diffusion length x _D " is used. On the basis of the diffusion length x _D different heating rates, partitioning temperatures and possible partitioning times can be compared. The diffusion length x _D is composed of a fraction x _Dr , which follows from the ramped partitioning, and a fraction x _Di , which follows from the isothermal partitioning, together (x _D = x _Di + x _Dr ). It can depend on the In each case the proportions x _Dr or x _{Di are} also "0", with the total diffusion length x _D always being> 0 as a result of the method according to the invention.

mit

• t_Pi = Zeit, über die das isotherme Halten durchgeführt worden ist, angegeben in Sekunden,
• D = D_o * exp(-Q/RT), D_o = 3,72*10^-5 m²/s,
• Q = 148 kJ/mol, R = 8,314 J/(mol·K),
• T = Partitioningtemperatur T_P in Kelvin

The diffusion length x _Di , ie the contribution to the diffusion length x _D obtained in the course of the isothermal hold, can be calculated for the optional isothermal partitioning using the following equation: $${\mathrm{x}}_{\mathrm{di}}=\mathrm{6}*\sqrt{\mathrm{D}*{\mathrm{t}}_{\mathrm{pi}}}$$

With

• t _Pi = time over which the isothermal hold has been performed, expressed in seconds,
• D = D _o * exp (-Q / RT), D _o = 3.72 * 10 ^-5 m ² / s,
• Q = 148 kJ / mol, R = 8.314 J / (mol · K),
• T = partitioning temperature T _P in Kelvin

wobei Δt_Pr,j der Zeitschritt zwischen zwei Berechnungen angegeben in Sekunden und D_j der jeweils aktuelle Diffusionskoeffizient D, berechnet wie voranstehend angegeben, zum Zeitpunkt des jeweiligen Zeitschritts sind. Bei der Bestimmung des Zeitschritts Δt_Pr,j wird beispielsweise davon ausgegangen, dass zwischen zwei Berechnungen jeweils 1 Sekunde vergangen ist (Δt_Pr,j = 1 s).Since redistribution of carbon does not occur isothermally in ramped partitioning, a numerical approximation is used to calculate the diffusion length x _{Dr obtained} over the heating period: $${\mathrm{x}}_{\mathrm{Dr}}={\mathrm{\Sigma }}_{\mathrm{j}}\left(\mathrm{6}*\sqrt{{\mathrm{D}}_{\mathrm{j}}*\mathrm{\Delta }{\mathrm{t}}_{\mathrm{pr}.\mathrm{j}}}\right)$$

where Δt _{Pr, j is} the time step between two calculations given in seconds and D _{j is} the actual diffusion coefficient D calculated as stated above at the time of the respective time step. For example, in determining the time step Δt _{Pr, j} it is assumed that between two Calculations each 1 second has passed (Δt _{Pr, j} = 1 s).

Basically, for the duration t _Pr of partitioning during the heating up to the partitioning temperature T _P : $${\mathrm{t}}_{\mathrm{pr}}\left[\mathrm{s}\right]=\mathrm{0}-{\mathrm{t}}_{\mathrm{A}}\mathrm{,}$$

That is, in cases where the heating to the partitioning temperature T _{P occurs} so rapidly that no significant redistribution of the carbon takes place during the heating, the duration t _Pr = 0 and, accordingly, the contribution x _Dr = 0 can be assumed , A particularly economical mode of operation results if the duration t _{PR of} the partitioning is limited to a maximum of 85 s.

The method according to the invention provides optimum work results if the sum of the respective diffusion lengths x _Di , x _{Dr to be considered is} at least 1.0 μm, in particular at least 1.5 μm.

By setting the operating parameters in the heat treatment so that the diffusion length increases, the bending angle of the respective flat steel product can be improved, while the hole expansion is only slightly affected. With further increasing diffusion length, the hole widening can be improved, but this can be accompanied by a deterioration of the bending properties. Even larger diffusion lengths eventually cause the deterioration of both bending properties and Hole expansion. Optimal work results arise when the operating parameters are set in the method according to the invention so that diffusion lengths of 1.5 to 5.7 microns, in particular from 2.0 to 4.5 microns are achieved.

By means of the diffusion length x _D or via a change in the influencing variables which are significant for their respective value, the yield ratio can also be influenced in cooperation with the cooling and holding step preceding the partitioning. If, for example, a high martensite content of 40% or more is generated by selecting a low cooling stop temperature T _Q and / or a longer hold time t _Q in the cooling step, by selecting a high partitioning temperature T _P and time t _Pt a larger diffusion length x _D and thus ultimately a high yield ratio can be achieved. If less than about 40% martensite is produced, then the influence of the diffusion length x _D on the yield ratio is rather small.

The yield ratio is a measure of the solidification potential of the steel. A relatively low yield ratio of about 0.50 has a positive effect on the tensile elongation, but is unfavorable for the hole widening and the bending angle. A higher yield ratio of about 0.90 can improve hole widening and bending properties, but leads to losses in tensile elongation.

After partitioning, the steel flat product is starting from the partitioning temperature T _P with a -3 ° C / s to -25 ° C / s, in particular -5 ° C / s to -15 ° C / s, cooling rate θ _P2 cooled.

If the flat steel product according to the invention is to be additionally provided with a hot-dip coating in the course of the method according to the invention, it is initially cooled to a melt-bath inlet temperature T _B of 400-500 ° C., starting from the partitioning temperature T _P at the cooling rate θ _P2 .

Subsequently, the steel flat product for hot dip coating passes through a melt bath, at the leaving of which the thickness of the protective coating produced on the flat steel product is adjusted in a conventional manner, for example by wiping nozzles.

The coated flat steel product exiting the melt bath is finally cooled to room temperature at the cooling rate θ _D2 to again produce martensite.

The process according to the invention is particularly suitable for the production of flat steel products which are provided with a zinc coating. However, other metallic coatings which can be applied by hot-dip coating to the respective flat steel product, such as ZnAl, ZnMg or comparable protective coatings, are also possible.

The product produced according to the invention has a microstructure which (in each case area%) indicates 25 to 80% tempered martensite (martensite from the first cooling step), 5 to 70% unannealed, new martensite (martensite from the second cooling step), 5 to 30% retained austenite, less than 5% bainite (0% included) and less than 5% ferrite (0% included).

Ferrite: Ferrite is a microstructural constituent which, compared to martensite, only contributes little to the strength of the material produced according to the invention. Therefore, the presence of ferrite in the microstructure of a steel flat product produced according to the invention is undesirable and should always be less than 5 area%.

Bainite: During the phase transformation from austenite to bainite, part of the carbon dissolved in the material accumulates before the austenite-bainite phase boundary, while another part is incorporated into the bainite during bainite transformation. Thus, in the case of bainite formation, a smaller part of the carbon is available for accumulation in the retained austenite than in the case of no bainite formation. To keep as much carbon as possible for the retained austenite, the bainite content must be kept as low as possible. In order to achieve an optimally favorable property profile, the bainite content according to the invention is limited to a maximum of 5 area%. Ideally, the formation of bainite can be completely avoided, ie the bainite content can be reduced to 0% by area.

Decanted martensite: The tempered martensite, as the martensite present before partitioning, is the source of the carbon that diffuses into and stabilizes the retained austenite during the partitioning treatment. In order to provide enough carbon, the proportion of tempered martensite should be at least 25% by area. However, it should not exceed 80 area%, so that after the first cooling, portions of at least 20 area% retained austenite can be adjusted. The proportion of retained austenite present after the first cooling is the basis for the formation of the retained austenite after completion of the heat treatments and the unencumbered martensite from the second cooling process.

Unbacked martensite: Martensite, as a hard structural ingredient, contributes significantly to the strength of the material. In order to achieve high strength values, the proportion of unreinforced martensite should not be less than 5 area%, that of tempered martensite should not be less than 25 area%. The proportion of unripe martensite should not exceed 70% by area and that of martensite tempered should not exceed 80% by area in order to ensure the formation of sufficient retained austenite.

Residual austenite present in the end product at room temperature: retained austenite contributes to the improvement of the elongation properties. The proportion should be at least 5 area% in order to ensure sufficient elongation of the material. On the other hand, if the residual austenite content exceeds 30% by area, this means that little martensite is available to increase strength.

The inventive method thus enables the production of a refined steel flat product with a tensile strength of 1200 to 1900 MPa, a yield strength of 600 to 1400 MPa, a yield ratio of 0.40 to 0.95, an elongation (A ₅₀ ) of 10 to 30% and a very good formability. This is expressed by the fact that for a flat steel product according to the invention, the product R _m * A _{50 is} 15,000-35,000 MPa%. The flat steel product according to the invention simultaneously has a high bending angle α of 100 to 180 ° (with bending mandrel radius = 2.0 * sheet thickness in accordance with DIN EN 7438) and very good values for the hole expansion λ of 50 to 120% (according to ISO-TS 16630) on. Thus, in a steel flat product according to the invention high strength and good forming properties are paired with each other.

In FIG. 1 a variant of the method according to the invention is shown in which the heating time t _A required for heating the steel flat product from the cooling stop temperature T _Q to the partitioning temperature T _{P is} equal to the duration t _{Pr of} the ramped partitioning and the flat steel product in the course of this process a hot dip coating in a Zinc bath ("zinc pot") is subjected.

In principle, the variant comprising a hot-dip coating of the method according to the invention can be carried out in a conventional fire-coating system, if at this certain modifications are made. In order to achieve strip temperatures of above 930 ° C, ceramic radiant tubes may be needed. The high cooling rates θ _Q of up to -120 K / s can be achieved with modern gas jet cooling. The heating to the partitioning temperature T _P after holding at the cooling stop temperature T _Q can be achieved by using a booster. After the partitioning step, the belt passes through the melt bath and is cooled in a controlled manner to regenerate martensite.

The invention has been tested with reference to numerous embodiments.

In the process, samples of cold-rolled steel strips produced from steels A - N shown in Table 1 were tested.

The samples have the inventively given, in FIG. 1 through process steps shown with the process parameters given in Table 2. The process parameters between parameters according to the invention and parameters not according to the invention have been varied in order to demonstrate the effects of a procedure which is outside the scope of the invention. The calculation of the diffusion length was based on time steps of 1 s each.

The mechanical properties of the cold strip samples obtained in this way are summarized in Table 3. The structural components of the obtained Cold strip samples are given in "Area%" in Table 4. Phase fractions of unhardened and tempered martensite, bainite and ferrite have been determined according to ISO 9042 (optical assessment). The retained austenite has additionally been determined by X-ray diffractometry with an accuracy of +/- 1 area%. As traces "Sp." shares of less than 5 area% have been designated.

The following abbreviations have been used in the tables, claims and description: abbreviations description unit θ _H1 Heating rate for the first heating phase before austenitizing ° C / s T _W Temperature for change from first to second heating phase before austenitizing ° C θ _H2 Heating rate for the second heating phase before austenitizing ° C / s T _HZ austenitizing ° C t _HZ Austenitisierungsdauer S θ _Q Cooling rate for quenching after austenitizing ° C / s θ _{Q (min)} Mindestabkühlungsgeschw. to avoid ferritic or bainitic transformation ° C / s T _Q Cooling stop temperature for quenching after austenitizing ° C t _Q Holding time at cooling stop temperature S θ _P1 Heating rate to temperature for isothermal partitioning ° C / s t _A Duration of heating to the partitioning temperature T _P S t _PR Duration for partitioning during heating (ramped partitioning) s t _PI Holding time for isothermal partitioning S t _PT Total partitioning time (t _PR + t _PI ) S T _P Temperature for isothermal partitioning ° C X _D Total diffusion length microns x _Dr Diffusion length from the ramped partitioning microns x _Tue Diffusion length from the isothermal partitioning microns θ _P2 Cooling rate after partitioning ° C / s F ferrite % B bainit % M _T tempered martensite (martensite old) % M _N Martensite from cooling after partitioning (martensite new) % RA austenite % R _p0,2 yield strength MPa R _m tensile strenght MPa R _p0,2 / R _m Yield ratio - A ₅₀ strain % R _m * A ₅₀ Product of tensile strength and elongation (= measure of high strength with good formability) MPa *% λ hole expansion % α Bending angle (after springback at bending mandrel radius = 2x sheet thickness) O Table 1 stole C Si Mn al P S N Cr V Not a word Ti Nb B Σ (MLE) CE A 0.169 1.47 1.55 0,038 0,015 0.0006 0.0037 0.011 0.027 0.04 0, 67 B 0.230 1, 66 1.87 0.037 0.009 0.0010 0.0049 0,008 0,040 0.05 0.82 C 0.224 0.16 1.67 1,410 0.016 0.0020 0.0042 0.00 0.53 D 0.452 1.30 1.73 0,041 0,013 0.0020 0.0039 0.00 0.96 e 0.331 1.91 1.52 0,035 0,008 0.0010 0.0041 0,071 0.07 0.90 F 0.193 1.41 1.53 0,460 0.009 0.0020 0.0040 0.00 0.68 G 0.183 1.78 2.34 0.032 0,008 0.0020 0.0047 0.047 0.031 0.08 0.87 H 0.196 1.64 3.14 0,012 0.011 0.0010 0.0040 0,008 0.01 0.99 I 0.306 1.70 1.96 0,018 0,013 0.0010 0.0030 0.00 0.92 J 0,150 1.51 2.01 0,010 0.009 0.0010 0.0060 0.25 0,042 0.0015 0.04 0.79 K 0,150 1.43 1.96 0.024 0.009 0.0022 0.0050 0.32 0,124 0.12 0.78 L 0.276 1.05 1.82 0,021 0,012 0.0020 0.0006 0.22 0,133 0.0030 0.13 0.80 M 0.259 0.85 1.58 0,036 0,010 0.0015 0.0070 0.067 0.084 0.0040 0.15 0.68 N 0.174 0.97 1.47 0.028 0.009 0.0010 0.0040 0.23 0.00 0.63 Data in wt .-%, balance iron and unavoidable impurities Table 2 (part 1) stole Experiment no. θ _H1 [° C / s] T _W [° C] θ _H2 [° C / s] A _c3 [° C] T _HZ [° C] t _HZ [s] θ _{Q (min)} [° C / s] θ _Q [° C / s] T _Q [° C] T _MS [° C] t _Q [s] A 1 11 270 3 892 920 84 -110 -115 250 411 10 A 2 15 300 4 892 920 84 -110 -70 350 411 20 A 3 5 270 5 892 930 50 -110 -120 270 411 12 A 4 10 300 5 892 830 50 -110 -110 460 411 0 A 5 10 270 3 892 910 110 -110 -110 320 411 10 B 6 18 270 3 887 920 75 -67 -70 310 374 0 B 7 12 375 5 887 930 48 -67 -75 310 374 40 B 8th 5 270 5 887 905 115 -67 -70 310 374 40 B 9 14 300 4 887 925 65 -67 -70 250 374 15 B 10 5 300 5 887 820 48 -67 -20 470 374 0 B 11 5 270 5 887 915 80 -67 -75 250 374 10 C 12 11 270 3 821 930 70 -90 -90 290 435 20 C 13 11 270 3 821 930 70 -90 -105 210 435 10 C 14 5 270 5 821 890 125 -90 -95 250 435 12 D 15 6 300 4 832 895 100 -42 -45 250 287 50 D 16 5 270 5 832 880 140 -42 -50 200 287 10 D 17 9 290 3 832 920 55 -42 -50 230 287 15 e 18 5 270 5 879 930 50 -38 -40 310 340 14 e 19 11 290 3 879 920 65 -38 -55 275 340 10 e 20 11 270 4 879 930 55 -38 -10 300 340 0 e 21 10 270 3 879 930 55 -38 -50 300 340 20 F 22 10 350 3 884 930 45 -90 -90 255 414 30 F 23 5 270 5 884 920 55 -90 -50 270 414 15 F 24 5 270 5 884 930 60 -90 -100 310 414 12 F 25 11 270 4 884 890 150 -90 -100 250 414 10 G 26 10 300 5 903 930 60 -48 -60 290 378 10 G 27 11 270 4 903 930 60 -48 -60 250 378 10 H 28 5 270 5 893 930 66 -31 -45 290 348 24 H 29 5 270 5 893 905 80 -31 -40 240 348 24 H 30 10 270 4 893 905 80 -31 -40 240 348 10 H 31 11 300 5 893 930 52 -31 -50 270 348 15 H 32 5 270 5 893 930 52 -31 -30 250 348 0 H 33 9 255 3 893 930 66 -31 -80 210 348 5 H 34 20 295 3 893 920 70 -31 -60 320 348 12 H 35 5 270 5 893 920 70 -31 -60 270 348 70 I 36 14 310 5 874 905 75 -50 -65 200 337 17 I 37 10 270 3 874 900 73 -50 -70 310 337 15 I 38 10 270 3 874 880 98 -50 -50 285 337 0 I 39 15 290 5 874 930 24 -50 -75 230 337 20 J 40 5 270 5 899 930 20 -94 -95 350 403 10 J 41 20 300 3 899 910 46 -94 -100 200 403 0 J 42 5 270 4 899 910 46 -94 -105 265 403 16 J 43 5 270 5 899 905 78 -94 -100 320 403 12 Table 2 (part 2) stole Experiment no. θ _H1 [° C / s] T _W [° C] θ _H2 [° C / s] A _c3 [° C] T _HZ [° C] t _HZ [s] θ _{Q (min)} [° C / s] θ _Q [° C / s] _Q T [° C] T _MS [° C] t _Q [s] K 44 10 300 3 895 920 57 -86 -95 300 406 10 K 45 8th 270 4 895 920 57 -86 -95 350 406 17 K 46 5 270 5 895 910 83 -86 -87 340 406 0 L 47 5 270 5 850 900 60 -79 -80 220 360 14 L 48 10 290 4 850 875 95 -79 -80 275 360 12 L 49 5 270 5 850 890 75 -79 -90 300 360 18 M 50 5 270 3 852 895 80 -112 -120 240 376 10 M 51 5 270 3 852 870 120 -112 -120 285 376 16 M 52 5 270 3 852 890 75 -112 -115 200 376 80 N 53 10 270 3 876 930 38 -103 -105 350 414 12 N 54 11 270 4 876 900 80 -103 -110 250 414 10 N 55 11 270 4 876 900 80 -103 -115 310 414 10 Table 2 (part 3) stole Experiment no. θ _P1 [° C / s] t _PR [s] t _PI [s] _P T [° C] x _D [μm] θ _P2 [° C / s] According to the invention? A 1 6.5 30.8 5 450 2.27 -8th YES A 2 80 1.8 22 490 7.71 -8th NO A 3 8th 27.5 0 490 2.74 -8th YES A 4 0 0.0 34 460 1.14 -8th NO A 5 10 12.0 10 440 2.12 -8th YES B 6 90 2.0 28 490 9.44 -10 NO B 7 90 2.0 16 490 5.83 -10 NO B 8th 75 2.1 20 470 5, 14 -10 YES B 9 12 18.3 5 470 2, 31 -10 YES B 10 0 0.0 218 470 3.40 -10 NO B 11 5 48.0 0 490 3, 98 -10 YES C 12 85 2.4 16 490 5, 83 -7 NO C 13 4.5 62.2 0 490 4, 34 -7 YES C 14 3 66.7 4 450 3, 43 -7 YES D 15 80 3.0 22 490 7.70 -11 NO D 16 6 41.7 5 450 2.31 -11 YES D 17 3.5 68.6 0 470 3.74 -11 YES e 18 5 36.0 0 490 3, 60 -18 YES e 19 4 50.0 10 475 4, 61 -18 YES e 20 85 2.1 25 480 7.49 -18 NO e 21 75 2.4 7 480 2.06 -18 YES F 22 9 26.1 0 490 2.37 -12 YES F 23 90 2.4 15 490 5.51 -12 NO F 24 5 32.0 0 470 2.71 -12 YES F 25 7.5 32.0 0 490 2.86 -12 YES G 26 11 18.2 0 490 3, 27 -11 YES G 27 6.5 34.6 0 475 2.46 -11 YES H 28 75 2.7 15 490 5.33 -20 YES H 29 75 2.8 20 450 3, 61 -20 YES H 30 2.5 84.0 0 450 3.55 -20 YES H 31 3.5 62.9 0 490 5.59 -20 YES H 32 95 2.5 26 490 8, 98 -20 NO H 33 95 2.9 16 490 5, 81 -20 NO H 34 5 26.0 22 450 5, 51 -20 YES H 35 7 30.0 0 480 2, 44 -20 NO I 36 4.5 55.6 0 450 2, 02 -10 YES I 37 5 32.0 0 470 2.59 -10 YES I 38 95 2.2 25 490 8, 66 -10 NO I 39 6 40.8 0 475 2.54 -10 YES J 40 2 45.0 0 440 3.51 -16 YES J 41 80 3.6 28 490 9, 61 -16 NO J 42 6 37.5 5 490 4.86 -16 YES J 43 4 32.5 0 450 2.21 -16 YES Table 2 (part 4) stole Experiment no. θ _P1 [° C / s] t _PR [s] t _PI [s] T _P [° C] x _D [μm] θ _P2 [° C / s] Inventions according to? K 44 4.5 33.3 0 450 2.02 -9 YES K 45 7 17.9 0 475 2.31 -9 YES K 46 95 1.6 27 490 9.29 -9 NO L 47 3 83.3 0 470 4, 33 -18 YES L 48 6 33.3 10 475 2, 60 -18 YES L 49 20 9.5 5 490 2.74 -18 YES M 50 4.5 53.3 5 480 4.81 -13 YES M 51 7 27.9 8th 480 4, 84 -13 YES M 52 85 3.4 22 490 7.72 -13 NO N 53 6 23.3 0 490 3, 62 -15 YES N 54 4 51.3 5 455 3.28 -15 YES N 55 2.5 58.0 5 455 4, 62 -15 YES Table 3 (part 1) stole Experiment No. R _P0,2 [MPa] R _m [MPa] R _P0,2 / R _m [-] A ₅₀ [%] R _m * A ₅₀ [Mpa%] λ [%] α _max [°] According to the invention? A 1 1014 1257 0.81 13 16341 62 133 YES A 2 979 1070 0.91 12 12840 68 117 NO A 3 983 1231 0.80 16 19696 57 147 YES A 4 400 840 0.48 25 21000 ne ne NO A 5 768 1202 0, 64 17 20434 51 139 YES B 6 828 1005 0.82 8th 8040 63 96 NO B 7 958 1245 0.77 11 13695 59 128 NO B 8th 932 1303 0.72 15 19545 56 114 YES B 9 1071 1399 0.77 11 15389 60 125 YES B 10 420 1060 0.40 12 12720 ne ne NO B 11 1143 1276 0, 90 12 15312 74 105 YES C 12 722 1256 0.57 15 18840 26 109 NO C 13 1040 1342 0.77 14 18788 68 117 YES C 14 917 1289 0.71 12 15468 55 133 YES D 15 995 1432 0, 69 14 20048 41 108 NO D 16 912 1484 0.61 16 23744 57 130 YES D 17 874 1320 0.66 13 17160 73 143 YES e 18 935 1541 0, 61 14 21574 55 109 YES e 19 1118 1474 0.76 12 17688 77 121 YES e 20 632 1150 0.55 9 10350 31 90 NO e 21 1093 1405 0.78 15 21075 68 105 YES F 22 914 1236 0.74 14 17304 68 130 YES F 23 702 1149 0.61 15 17235 38 116 NO F 24 727 1371 0.53 16 21936 51 139 YES F 25 1064 1206 0.88 13 15678 81 127 YES G 26 1101 1497 0.74 13 19461 59 114 YES G 27 1272 1522 0.84 11 16742 72 137 YES ne = not determined Table 3 (part 2) stole Experiment No. R _P0,2 [MPa] R _m [MPa] R _P0,2 / Rm [-] A ₅₀ [%] R _m * A ₅₀ [MPa%] λ [%] α _max [°] According to the invention? H 28 760 1357 0.56 13 17641 52 111 YES H 29 874 1412 0.62 12 16944 57 106 YES H 30 826 1398 0.59 16 22368 78 128 YES H 31 797 1261 0, 63 17 21437 63 135 YES H 32 893 1056 0.85 13 13728 48 98 NO H 33 1114 1199 0.93 13 15587 86 125 NO H 34 650 1315 0.49 18 23670 61 120 YES H 35 852 1194 0.71 15 17910 49 109 NO I 36 1066 1476 0.72 14 20664 53 102 YES I 37 898 1384 0.65 18 24912 59 117 YES I 38 978 1132 0.86 8th 9056 72 103 NO I 39 933 1447 0, 64 15 21705 55 129 YES J 40 788 1273 0, 62 21 26733 51 122 YES J 41 1068 1102 0, 97 4 4408 57 93 NO J 42 1037 1463 0.71 17 24871 75 131 YES J 43 985 1379 0.71 19 26201 54 114 YES K 44 1202 1576 0.76 13 20488 58 112 YES K 45 954 1398 0, 68 16 22368 66 130 YES K 46 1017 1255 0.81 8th 10040 71 108 NO L 47 1263 1642 0.77 12 19704 56 119 YES L 48 991 1482 0.67 15 22230 51 131 YES L 49 870 1451 0, 60 17 24667 68 139 YES M 50 1126 1401 0.80 16 22416 62 109 YES M 51 930 1529 0.61 13 19877 51 123 YES M 52 1242 1297 0, 96 6 7782 76 117 NO N 53 905 1386 0, 65 19 26334 63 129 YES N 54 1132 1475 0.77 12 17700 77 136 YES N 55 1063 1458 0.73 16 23328 69 125 YES ne = not determined Table 4 (part 1) stole Experiment no. F [%] M _T [%] Does excessive martensite contain? RA [% -] M _N [%] B [%] According to the invention? A 1 0 80 NO 10 10 Sp. YES A 2 0 55 YES 5 40 Sp. NO A 3 0 80 NO 13 7 Sp. YES A 4 76 0 NO 9 15 Sp. NO A 5 0 69 NO 16 15 Sp. YES B 6 4 45 YES 11 40 0 NO B 7 0 55 YES 9 25 11 NO B 8th 0 55 NO 16 29 0 YES B 9 0 78 NO 12 10 0 YES B 10 62 0 NO 18 5 5 NO B 11 0 79 NO 8th 8th 5 YES C 12 Sp. 55 YES 15 30 0 NO C 13 0 80 NO 11 9 0 YES C 14 0 75 NO 14 11 0 YES D 15 Sp. 45 YES 21 34 Sp. NO D 16 0 70 NO 18 12 Sp. YES D 17 0 56 NO 19 25 Sp. YES e 18 0 35 NO 24 41 Sp. YES e 19 0 60 NO 14 26 Sp. YES e 20 20 30 YES 9 21 20 NO e 21 0 50 NO 14 36 Sp. YES F 22 0 80 NO 13 7 0 YES F 23 17 65 NO 8th 10 0 NO F 24 0 59 NO 16 25 0 YES F 25 0 80 NO 7 13 0 YES G 26 0 65 NO 12 23 0 YES G 27 0 80 NO 5 15 0 YES Sp. = Tracks Table 4 (part 2) stole Experiment No. F [%] M _T [%] Does excessive martensite contain? RA [% -] M _N [%] B Invention [%] According to? H 28 Sp. 50 NO 15 35 0 YES H 29 0 74 NO 11 15 0 YES H 30 Sp. 72 NO 18 10 0 YES H 31 Sp. 66 NO 14 20 0 YES H 32 0 75 YES 8th 17 0 NO H 33 0 85 YES 8th 7 0 NO H 34 Sp. 23 NO 17 60 0 YES H 35 Sp. 70 NO 10 20 0 NO 1 36 Sp. 77 NO 18 5 0 YES I 37 Sp. 40 NO 19 41 0 YES I 38 Sp. 55 YES 6 39 0 NO I 39 Sp. 75 NO 12 13 0 YES J 40 0 51 NO 9 40 0 YES J 41 0 95 YES 3 2 0 NO J 42 0 80 NO 10 10 0 YES J 43 0 61 NO 14 25 0 YES K 44 0 67 NO 12 21 0 YES K 45 0 40 NO 17 43 0 YES K 46 0 48 YES 7 46 Sp. NO L 47 0 80 NO 11 9 0 YES L 48 0 64 NO 16 20 0 YES L 49 Sp. 51 NO 19 30 0 YES M 50 0 78 NO 13 9 0 YES M 51 0 65 NO 14 21 0 YES M 52 0 90 YES 5 5 0 NO N 53 0 45 NO 17 38 0 YES N 54 0 80 NO 11 9 0 YES N 55 0 70 NO 12 18 0 YES Sp. = Tracks

Claims (17)

1. Flat steel product which has a tensile strength R_m of at least 1200 MPa and which consists of a steel that contains (in wt%)

   C: 0.10 - 0.50%,

   Si: 0.1 - 2.5%,

   Mn: 1.0 - 3.5%

   Al: up to 2.5%,

   P: up to 0.020%,

   S: up to 0.003%,

   N: up to 0.02%,

   and optionally one or more of the elements "Cr, Mo, V, Ti, Nb, B and Ca" in the following quantities:

   Cr: 0.1 - 0.5%,

   Mo: 0.1 - 0.3%,

   V: 0.01 - 0.1%,

   Ti: 0.001 - 0.15%,

   Nb: 0.02 - 0.05%,

   wherein Σ(V, Ti, Nb) ≤ 0.2% for the sum Σ(V, Ti, Nb) of the quantities of V, Ti and Nb,

   B: 0.0005 - 0.005 %, and

   Ca: up to 0.01%

   in addition to Fe and unavoidable impurities, and a microstructure with (in surface percent) less than 5% ferrite, less than 5% bainite, 5 - 70% untempered martensite, 5 - 30% residual austenite and 25 - 80% tempered martensite, at least 99% of the iron carbide contained in the untempered martensite having a size of less than 500 nm.
2. Flat steel product according to claim 1, characterised in that (in wt%) the Al content is 0.01 - 1.5%, the Cr content is 0.20 - 0.35 wt%, the V content is 0.04 - 0.08%, the Ti content is 0.008 - 0.14%, the B content is 0.002 - 0.004% or the Ca content is 0.0001 - 0.006%.
3. Flat steel product according to any one of the preceding claims, characterised in that for the carbon equivalent CE of its steel the following is valid: $$\mathrm{0.35}\mathrm{wt}\mathrm{.}-\%\le \mathrm{CE}\le \mathrm{1.2}\mathrm{wt}\mathrm{.}-\%$$

   

   wherein $$\mathrm{CE}=\%\mathrm{C}+\left(\%\mathrm{Mn}+\%\mathrm{Si}\right)/\mathrm{6}+\left(\%\mathrm{Cr}+\%\mathrm{Mo}+\%\mathrm{V}\right)/\mathrm{5}+\left(\%\mathrm{Ni}+\%\mathrm{Cu}\right)/\mathrm{15},$$

   

   %C: C content of the steel,

   %Mn: Mn content of the steel,

   %Si: Si content of the steel,

   %Cr: Cr content of the steel,

   %Mo: Mo content of the steel,

   %V: V content of the steel,

   %Ni: Ni content of the steel,

   %Cu: Cu content of the steel.
4. Flat steel product according to claim 3, characterised in that for the carbon equivalent CE the following is valid: $$\mathrm{0.5}\mathrm{wt}\mathrm{.}\%\le \mathrm{CE}\le \mathrm{1.0}\mathrm{wt}\mathrm{.}\%$$

   
5. Flat steel product according to any one of the preceding claims, characterised in that it is provided with a metallic protective layer applied by hot-dip coating.
6. Method for producing a high-strength flat steel product, comprising the following work steps:

   - providing an uncoated flat steel product of a steel that contains (in wt%)

   C: 0.10 - 0.50%,

   Si: 0.1 - 2.5%,

   Mn: 1.0 - 3.5%,

   Al: up to 2.5%,

   P: up to 0.020%,

   S: up to 0.003%,

   N: up to 0.02%,
   and optionally one or more of the elements "Cr, Mo, V, Ti, Nb, B and Ca" in the following quantities:

   Cr: 0.1 - 0.5%,

   Mo: 0.1 - 0.3%,

   V: 0.01 - 0.1%,

   Ti: 0.001 - 0.15%.

   Nb: 0.02 - 0.05%.
   wherein Σ(V,Ti,Nb) ≤ 0.2% for the sum Σ(V,Ti,Nb) of the quantities of V, Ti and Nb,

   B: 0.0005 - 0.005%,

   Ca: up to 0.01% in addition to Fe and unavoidable impurities;

   - heating the flat steel product to an austenitisation temperature T_HZ above the A_C3 temperature of the steel of the flat steel product and with a maximum of 960 °C at a heating speed θ_H1, θ_H2 of at least 3°C/s;

   - holding the flat steel product at the austenitisation temperature for an austenitisation period t_HZ of 20 - 180 seconds;

   - cooling of the flat steel product to a cooling stop temperature T_Q, greater than the martensite stop temperature T_Mf and less than the martensite start temperature T_Ms (T_Mf < T_Q < T_MS), at a cooling speed θ_Q for which the following is valid: $${\mathrm{\theta }}_{\mathrm{Q}}\le {\mathrm{\theta }}_{\mathrm{Q}\left(\min \right)}$$

   

   where $${\mathrm{\theta }}_{\mathrm{Q}\left(\min \right)}\left[\mathrm{°C}/\mathrm{s}\right]=-\mathrm{314.35}\mathrm{°C}/\mathrm{s}+\left(\mathrm{268.74}\%+\mathrm{56.27}\%\mathrm{Si}+\mathrm{58.50}\%\mathrm{Al}+\mathrm{43.40}\%\mathrm{Mn}+\mathrm{195.02}\%\mathrm{Mo}+\mathrm{166.60}\%\mathrm{Ti}+\mathrm{199.19}\%\mathrm{Nb}\right)\mathrm{°C}/\left(\mathrm{wt}\mathrm{.}\%\cdot \mathrm{s}\right),$$

   

   %C: C content of the steel,

   %Si: Si content of the steel,

   %Al: Al content of the steel,

   %Mn: Mn content of the steel,

   %Mo: Mo content of the steel,

   %Ti: Ti content of the steel,

   %Nb: Nb content of the steel;

   - holding the flat steel product at the cooling stop temperature T_Q for a holding time t_Q of 10 - 60 seconds;

   - starting from the cooling stop temperature T_Q, heating the flat steel product at a heating speed θ_P1 of 2 - 80 °C/s to a partitioning temperature T_P of 400 - 500 °C;

   - optionally holding the flat steel product isothermally at the partitioning temperature T_P for a holding time t_Pi of up to 500 seconds;

   - starting from the partitioning temperature T_P cooling the flat steel product at a cooling speed θ_P2 of between -3 °C/s and -25 °C/s.
7. Method according to claim 6, characterised in that in the cooling starting from the partitioning temperature T_P at a cooling speed θ_P2

   - the flat steel product is initially cooled to a molten bath entry temperature T_B of 400 to <500°C;

   - then the flat steel product cooled to the molten bath entry temperature T_B is hot-dip coated by being passed through a molten bath and the thickness of the protective layer created on the flat steel product is set;

   - and finally the flat steel product leaving the molten bath with the protective layer is cooled to ambient temperature at a cooling speed θ_P2.
8. Method according to claim 6 or 7, characterised in that heating to the austenitisation temperature T_HZ takes place in two consecutive stages without interruption at different heating speeds θ_H1, θ_H2.
9. Method according to any one of claims 6 to 8, characterised in that the heating speed θ_H1 of the first stage is 5 - 25 °C/s and the heating speed θ_H2 of the second stage is 3 - 10 °C/s.
10. Method according to any one of claims 6 to 9, characterised in that the flat steel product is heated at the first heating speed θ_H1 to an intermediate temperature T_W of 200 - 500°C and in that the heating is then continued at the second heating speed θ_H2 to the austenitisation temperature T_HZ.
11. Method according to any one of claims 6 to 10, characterised in that the cooling speed θ_Q is -20°C/s to -120°C/s.
12. Method according to any one of claims 6 to 11, characterised in that the cooling stop temperature T_Q is at least 200 °C.
13. Method according to any one of claims 6 to 12, characterised in that the holding time t_Q, for which the flat steel product is held at the cooling stop temperature T_Q is 12 - 40 seconds.
14. Method according to any one of claims 6 to 13, characterised in that the heating speed θ_P1 at which the heating takes place from the cooling stop temperature T_Q is 2 - 80°C/s.
15. Method according to any one of claims 6 to 14, characterised in that heating to the partitioning temperature T_P takes place within a heating time t_A of 1 - 150 seconds.
16. Method according to claim 15, characterised in that for the time t_Pr of partitioning during heating to partitioning temperature T_P the following is valid: $${\mathrm{t}}_{\mathrm{Pr}}\left[\mathrm{s}\right]=\mathrm{0}-{\mathrm{t}}_{\mathrm{A}}\mathrm{.}$$

    
17. Method according to any one of claims 6 to 16, characterised in that for a diffusion length x_D the following is valid: $${\mathrm{x}}_{\mathrm{D}}\ge \mathrm{1.0}\mathrm{\mu m}$$

    

    where $${\mathrm{x}}_{\mathrm{D}}={\mathrm{x}}_{\mathrm{Di}}+{\mathrm{x}}_{\mathrm{Dr}}$$

    

    x_Di: the contribution obtained in the course of isothermic holding to the diffusion length x_D, calculated according to the formula $${\mathrm{x}}_{\mathrm{Di}}=\mathrm{6}*\sqrt{\mathrm{D}*{\mathrm{t}}_{\mathrm{Pi}}}$$

    

    where t_Pi = time for which isothermal holding is performed, in seconds,

    D = D₀ * exp (-Q/RT), D₀ =3.72 *10^-5 m²/S

    Q = 148 kJ/mol, R = 8.314 J/(mol·K)

    T = partitioning temperature T_P in Kelvin

    and

    x_Dr: the contribution obtained in the course of heating to the partitioning temperature to the diffusion length x_D, calculated according to the formula $${\mathrm{x}}_{\mathrm{Dr}}={\mathrm{\Sigma }}_{\mathrm{j}}\left(\mathrm{6}*\sqrt{{\mathrm{D}}_{\mathrm{j}}*\mathrm{\Delta }{\mathrm{t}}_{\mathrm{Pr},\mathrm{j}}}\right)$$

    

    where Δt_Pr,j = is the time step between two calculations in seconds,

    D_j = D₀ * exp(-Q/RT_j), D₀ = 3.72 *10^-5 m²/s,

    Q = 148 kJ/mol, R = 8.314 J/(mol·K)

    T_j = current partitioning temperature T_P in each case in Kelvin.

    wherein x_Di or x_Dr can also be 0.

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