EP3872193A1 - Flat steel product and method for producing hot-rolled flat steel product - Google Patents

Abstract

The invention relates to a method for producing a hot-rolled flat steel product with a structure of (in% by volume) 4-50% tempered, island-shaped martensite, 30-96% ferrite, <10% retained austenite, the remainder ≤ 66% bainite or bainitic ferrite. Thereby (a) a steel melt from, in mass%, C: 0.05-0.15%, Si: <0.5%, Mn: 0.7-2.1%, Al: <0.1 %, Cr: 0.2 - 1%, in total 0.01 - 0.1% Nb, Ti or V, B: <0.0015%, Mo: <0.2%, Cu: <0.2% , Ni: <0.2%, P: <0.05%, N: <0.01% and the remainder consists of Fe and ≤ 0.1% impurities, cast into a preliminary product that (b) at 1150 - 1380 ° C heated through, (c) optionally descaled and (d), if necessary, pre-rolled to 30 - 50 mm at 1020 - 1150 ° C. Then it is hot rolled to a 1.4 - 6.4 mm thick strip with a hot rolling end temperature ET, for which Ar ₃ ≤ ET ≤ 200 ° C + Ar ₃ - applies. Temperature of the steel. This is followed by (f) cooling to a coiling temperature HT, for which the martensite start temperature T _MS of the steel> HT ≥ room temperature RT applies, with the cooling from ET down to an intermediate temperature Tz, for which Tz <Ar applies ₁ - 50 ° C, with ≥ 20 K / s. After reaching Tz, the strip is either cooled between T _MS and RT at 5-100 K / s or at 10-130 K / s to a cooling stop temperature Tsp of 550-770 ° C, at which the Hot strip is optionally held for ≤ 5 s, and then cooled to a coiling temperature HT lying between Tsp and RT. The hot strip is coiled at HT, then further cooled or held, (g) tempered for 4-16 h at 150-500 ° C and (h) cooled to RT at 4-700 K / h.

Description

The invention relates to a method for producing a hot-rolled flat steel product with a structure whose main components are tempered or freshly formed martensite and ferrite, the remainder of the structure being filled with retained austenite, bainite and / or cementite.

The invention also relates to a flat steel product with a corresponding structure, the flat steel product being able to be produced in particular by the method according to the invention.

In the present text, unless explicitly stated otherwise, information on the content of alloy components is always given in% by mass. The proportions of the structure of a flat steel product are given here in% by volume, unless otherwise noted.

The image analysis for the quantitative determination of the structure is carried out optically by means of light microscopy ("LOM") with a resolution of 1000 times and with a field emission scanning electron microscope ("FE-SEM") with a resolution of 20,000 times. The representation and measurement of the retained austenite fringes present in the flat steel products according to the invention, as explained below, on the martensite islands of the structure, also took place with the FE-SEM at a magnification of 20,000 times as well. The strength and strength properties mentioned here
Elongation properties such as tensile strength Rm, uniform elongation Ag, elongation at break A50 of flat steel products were determined in tensile tests according to DIN-EN 6892-1 Specimen shape 1 determined transversely to the rolling direction (WR), unless otherwise noted. The elongation at break A80 was calculated in accordance with DIN EN 2566-1 (Sept 1999 section 9.3).

The hole expansion behavior or the achievable hole expansion HER of the flat steel products were determined on 100 * 100 samples in accordance with ISO 16630.

The qualitative C distribution in the structure of the flat steel products was determined using an FE microprobe, such as by H. Farivar et al. in the article "Experimental quantification of carbon gradients in martensite and its multi-scale effects in a DP steel", MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 718 (2018) 250-259 described. An example of the evaluation used here can be found in Fig. 1 .

It is known that flat steel products made from dual-phase steels tend to have edge cracks. In particular in the case of flat steel products with high strength, this can lead to failures in the pressing tool. The edge crack tendency is usually characterized by the hole expansion HER determined in accordance with ISO 16630. High values of the hole expansion HER stand for a lower tendency to edge cracking: In order to expand the range of applications of high-strength flat steel products made of dual-phase steels of the type in question, not only a low tendency to edge cracking, but also a high yield point Re is required is associated with a high yield strength ratio Re / Rm formed from the yield strength Re and the tensile strength Rm.

From the EP 2 690 183 A1 a hot-rolled flat steel product is known which, in% by mass, consists of C: 0.10-0.60%, Si: 0.4-2.0%, Al: up to 2.0%, Mn: 0.4 - 2.5%, Ni: up to 1%, Cu: up to 2.0%, Mo: up to 0.4%, Cr: up to 2%, Ti: up to 0.2%, Nb: up to to 0.2%, V: up to 0.5% and the remainder of iron and is composed of unavoidable impurities. The structure of the flat steel product consists of optionally available proportions of up to 5 vol .-% ferrite and up to 10 vol .-% martensite to at least 60 vol .-% of bainite and the remainder of retained austenite, with at least part of the retained austenite in blocky shape and the blocks of the retained austenite present in blocky form have a mean diameter of at least 98% of less than 5 μm. Such a flat steel product can be produced by producing a pre-product in the form of a slab, thin slab or a cast strip from a melt composed in the specified manner, which is then hot-rolled into a hot strip in one or more rolling passes, the hot strip obtained at Leaving the last rolling pass has a hot rolling end temperature of at least 880 ° C. The hot-rolled flat steel product obtained in this way is accelerated and cooled at a cooling rate of at least 5 ° C./s to a coiling temperature which is between the martensite start temperature MS and 600 ° C., and at this temperature it is wound into a coil. The flat steel product is then cooled in the coil, with the temperature of the coil being kept in a temperature range during the cooling for the formation of bainite, the upper limit of which is equal to the bainite start temperature BS, from which bainite is formed in the structure of the hot strip, and the lower limit of which is equal to the martensite start temperature MS from which martensite is formed in the structure of the hot strip until at least 60% by volume of the structure of the hot strip consists of bainite. A hot-rolled flat steel product produced in this way regularly has tensile strengths Rm of more than 1000 MPa, in particular at least 1200 MPa, with elongations A80 which are also regularly above 17%, in particular above 19%. Accordingly, the quality Rm * A80 of the known flat steel products is regularly in the range of 18,000 - 30,000 MPa *%.

Furthermore, from the WO 2015/158731 A1 a method for producing a cold-rolled flat steel product is known whose structure consists of, in vol .-%, at least 10% hardened martensite, less than 10% bainite, less than 10% retained austenite and the remainder ferrite and which has a yield strength ratio Re / Rm of at least 0.7 with a tensile strength Rm of more than 750 MPa and a hole elongation HER of at least 18%. The steel substrate of this cold-rolled flat steel product consists of iron, unavoidable impurities and, in% by mass, 0.05 - 0.20% C, 0.25 - 1.00% Si, 1.0 - 3.0% Mn, 0, 02 - 1.5% Al, 0.1 - 1.5% Cr, less than 0.02% N, less than 0.03% P, less than 0.05% S and optionally one or more elements from the group "Ti, Mo, Nb, V, B", with the proviso that the Ti content up to 0.15%, the Mo content less than 2%, the Nb content less than 0.1%, the V -Content less than 0.12% and the B-content is 0.0005-0.003%. The cold-rolled flat steel product assembled in this way is subjected to an additional heat treatment in which it is kept for an annealing time of 4.5 - 24 hours at an annealing temperature of up to 150 - 400 ° C. Such long-term annealing, which was carried out at comparatively low temperatures, results in a significant increase in the yield strength Re and an improvement in the hole elongation.

On the basis of the prior art explained above, the task has been to specify a method for producing a flat steel product which is further improved with regard to its mechanical properties and is characterized in particular by a favorable hole expansion behavior.

Likewise, a hot-rolled flat steel product should be specified with a property spectrum that has an optimized combination of high strength and good deformability, in particular good hole expansion behavior.

With regard to the method, the invention has achieved this object in that at least the work steps specified in claim 1 are carried out in the production of a hot-rolled flat steel product.

It goes without saying that when carrying out the method according to the invention, the person skilled in the art not only completes the method steps mentioned in the claims and explained in detail here, but also carries out all other steps and activities that are necessary in the practical implementation of such methods in the prior art Technique should be carried out regularly if the need arises.

A hot-rolled flat steel product that solves the above-mentioned object has at least the features specified in claim 8.

• das aus, in Masse-%, C: 0,05 - 0,15 %, Si: < 0,5 %, Mn: 0,7 - 2,1 %, Al: < 0,1 %, Cr: 0,2 - 1 %, mindestens einem Element der Gruppe "Nb, Ti, V" mit der Maßgabe, dass die Summe der Gehalte an Nb, Ti und / oder V 0,01 - 0,1 % beträgt, B: < 0,0015 %, Mo: < 0,2 %, Cu: < 0,2 %, Ni: < 0,2 %, P: < 0,05 %, N: < 0,01 % und als Rest aus Fe und in Summe höchstens 0,1 Masse-% unvermeidbaren Verunreinigungen besteht und
• dessen Gefüge zu 4 - 50 Vol.-% aus angelassenem, inselförmig im Gefüge vorliegendem Martensit, bei dem mindestens 10 % der Martensitinseln an ihrem Umfang mindestens abschnittsweise von einem Saum umgrenzt sind, und zu 30 - 96 Vol.-% aus Ferrit besteht, wobei der nicht durch Martensit und Ferrit
  eingenommene Anteil aus bis zu 66 Vol.-% Bainit oder bainitischem Ferrit und weniger als 10 Vol.-% Restaustenit besteht und wobei der C-Gehalt des Saums zumindest in einem Abschnitt höher ist als der C-Gehalt des Mittenbereichs der Martensitinsel.

A flat steel product hot-rolled according to the invention accordingly has a hole expansion HER of at least 30% and a steel substrate,

• that from, in mass%, C: 0.05 - 0.15%, Si: <0.5%, Mn: 0.7 - 2.1%, Al: <0.1%, Cr: 0, 2-1%, at least one element from the group "Nb, Ti, V" with the proviso that the sum of the Nb, Ti and / or V contents is 0.01-0.1%, B: <0.0015 %, Mo: <0.2%, Cu: <0.2%, Ni: <0.2%, P: <0.05%, N: <0.01% and the remainder of Fe and in total at most 0.1% by mass of unavoidable impurities and
• the structure of which consists of 4 - 50% by volume of tempered martensite present in the structure in the form of islands, in which at least 10% of the martensite islands are at least partially bordered on their circumference, and 30 - 96% by volume consists of ferrite, being the not by martensite and ferrite
  The proportion occupied consists of up to 66% by volume of bainite or bainitic ferrite and less than 10% by volume of retained austenite, and the C content of the seam is higher than the C content of the central region of the martensite island at least in a portion.

• wobei die Abkühlgeschwindigkeit dT1 in einem Temperaturbereich, der von der Warmwalzendtemperatur ET bis zu einer Zwischentemperatur Tz reicht, die 50 °C unterhalb der Ar₁-Temperatur des Stahls liegt, mindestens 20 K/s beträgt,
• wobei nach Erreichen der Zwischentemperatur Tz
• gemäß einer ersten Variante das Warmband mit einer Abkühlrate dT2' von 5 - 100 K/s abgekühlt wird, bis die Martensitstarttemperatur T_MS des Stahls erreicht ist,
  oder
• gemäß einer zweiten Variante das Warmband mit einer Abkühlrate dT2" von jeweils 10 - 130 K/s zunächst auf eine Kühlstopptemperatur Tsp, die im Bereich 550 - 770 °C liegt und bei der das Warmband optional über eine bis zu 5 s dauernde Haltedauer tH gehalten wird, und anschließend auf die Martensitstarttemperatur T_MS des Stahls abgekühlt wird,
  und
• wobei das Warmband anschließend auf die Haspeltemperatur abgekühlt wird und das so abgekühlte Warmband zu einem Coil gehaspelt wird, wobei im Fall, dass die Haspeltemperatur HT oberhalb der Raumtemperatur liegt, das Stahlflachprodukt abschließend im Coil auf Raumtemperatur abgekühlt wird;

g) Anlassen des warmgewalzten Stahlflachprodukts über eine Anlassdauer t_AN von 4 - 16 h bei einer Anlasstemperatur T_an von 150 - 500 °C unter einer reduzierenden oder inerten Atmosphäre;
h) Abkühlen des angelassenen Stahlflachprodukts mit einer Abkühlgeschwindigkeit von 4 - 700 K/h auf Raumtemperatur.In a production according to the invention of a hot-rolled flat steel product with a structure that consists of 4-50% by volume of tempered, island-shaped martensite and 30-96% by volume of ferrite, the portion not occupied by martensite and ferrite Structure consists of up to 66% by volume of bainite and / or bainitic ferrite and less than 10% by volume of retained austenite, at least the following work steps are carried out:
a) Casting a steel melt to a preliminary product in the form of a slab, thin slab or a cast strip, the melt consisting of, in mass%, C: 0.05 - 0.15%, Si: < 0.5%, Mn: 0.7 - 2.1%, Al: < 0.1%, Cr: 0.2 - 1 %, at least one element from the group "Nb, Ti, V" with the proviso that the sum of the Nb, Ti and / or V contents is 0.01-0.1%, B: <0.0015%, Mon: <0.2%, Cu: <0.2%, Ni: <0.2% P: <0.05% N: <0.01% and the remainder consists of Fe and a total of at most 0.1% by mass of unavoidable impurities;
b) heating through the intermediate product at a temperature of 1150-1380 ° C;
c) optional: descaling of the preliminary product;
d) in the case that the preliminary product is a slab: preliminary rolling of the preliminary product at temperatures of 1020 - 1150 ° C to a thickness of 30 - 50 mm;
e) Hot rolling of the optionally pre-rolled pre-product in one or more rolling passes to form a hot-rolled strip with a thickness of 1.4-6.4 mm, the hot-rolling end _{temperature ET, at which the hot-rolling is ended, at least equal to the Ar 3} temperature of the steel and is at most 200 ° C higher than the Ar ₃ temperature of the steel;
f) cooling the hot strip obtained to a coiling temperature HT which is in a range which _{begins below the martensite start temperature T MS of} the steel and ends at room temperature,

• The cooling rate dT1 in a temperature range that extends from the hot rolling end temperature ET to an intermediate temperature Tz that is 50 ° C below the Ar ₁ temperature of the steel is at least 20 K / s,
• whereby after reaching the intermediate temperature Tz
• According to a first variant, the hot strip is cooled at a cooling rate dT2 'of 5-100 K / s until the martensite start temperature T _{MS of} the steel is reached,
  or
• According to a second variant, the hot strip is initially cooled at a cooling rate dT2 ″ of 10-130 K / s to a cooling stop temperature Tsp, which is in the range 550-770 ° C and at which the hot strip is optionally held for a holding period tH of up to 5 s is then cooled to the martensite start _{temperature T MS of} the steel,
  and
• the hot strip is then cooled to the coiling temperature and the hot strip cooled in this way is reeled into a coil, in the event that the coiling temperature HT is above room temperature, the flat steel product is finally cooled in the coil to room temperature;

g) annealing the hot-rolled flat steel product through a tempering duration t _AN 4-16 h at a tempering temperature T _at 150-500 ° C under a reducing or inert atmosphere;
h) Cooling the tempered flat steel product at a cooling rate of 4-700 K / h to room temperature.

A flat steel product hot-rolled according to the invention has, as mentioned, a hole expansion HER of at least 30%, the hot-rolled flat steel product having a steel substrate which, in mass%, C: 0.05-0.15%, Si: <0.5 %, Mn: 0.7-2.1%, Al: <0.1%, Cr: 0.2-1%, at least one element from the group "Nb, Ti, V" with the proviso that the sum of the Content of Nb, Ti and / or V is 0.01 - 0.1%, B: <0.0015%, Mo: <0.2%, Cu: <0.2%, Ni: <0.2% , P: <0.05%, N: <0.01% and the remainder of Fe and in total a maximum of 0.1% by mass of unavoidable impurities, and its structure to 4 - 50% by volume of tempered, island-shaped Martensite present in the structure and 30-96% by volume of ferrite, the portion not occupied by martensite and ferrite consisting of up to 66% by volume of bainite or bainitic ferrite and less than 10% by volume of retained austenite.

As stated in claim 8, in the structure of the flat steel product according to the invention, at least 10% of the martensite islands are at least partially bordered on their circumference by a seam.

It is particularly evident that in at least 70% of all martensite islands there is a central area covering the center of the respective martensite island, which is delimited by an edge area of the relevant martensite island bordering the edge of the respective martensite island, the C content of the edge area at least in one area Section is higher than the C content of the mid-range.

Typically, the section or sections of the edge area of the martensite islands in which or in which a higher C content is present than in the middle area of the respective martensite island, in total at least 30-70% of the circumference of the martensite island concerned (see also Fig. 1 ).

The "martensite grains" so called in the present text are also called "martensite grains" in technical terminology.

In the process according to the invention, it is possible to produce a hot-rolled flat steel product according to the invention in which, in all martensite islands with an average diameter of more than 3 μm, at least one section of their edge areas has a higher carbon content than in the middle area of the respective martensite island.

According to the invention, the cooling of the hot-rolled flat steel products produced according to the invention is modified in such a way that retained austenite can be produced at the boundaries of the martensite. This allows an extreme improvement in the mechanical properties and the homogeneity of their distribution to be achieved.

With regard to the mechanical properties of a flat steel product according to the invention, it proves to be particularly advantageous that the martensite islands of the structure of a flat steel product according to the invention are at least over part of their circumference from a hem of a decayed residual austenite consisting of residual austenite, which has a greatly increased C content expresses, are delimited. Whose width is typically 10 nm to 1 µm, but can also be up to a third of the diameter of the respective martensite island.

The improved tensile strength and elongation are achieved in a flat steel product according to the invention through the presence of several phases and the associated high level of solidification, and the good hole expansion through the reduction of the shear stresses compared to pure dual-phase structures.

A flat steel product according to the invention thus achieves tensile strengths Rm, hole widenings HER and uniform elongations Ag, the product of which Rm x HER x Ag is regularly at least 300,000 MPa% ² , in particular at least 320,000 MPa% ² .

The tensile strength Rm of a hot-rolled flat steel product according to the invention regularly reaches values of at least 530 MPa, the hole expansion HER regularly values of at least 30% and the uniform elongation Ag regularly values of at least 7%.

• Kohlenstoff ("C") ist im erfindungsgemäßen Stahlflachprodukt in Gehalten von 0,05 - 0,15 Masse-% vorhanden, um das geforderte Festigkeitsniveau zu erreichen. Hierzu sind mindestens 0,05 Masse-% C erforderlich. Besonders sicher werden die erfindungsgemäß genutzten Effekte der Anwesenheit von C dann erreicht, wenn der C-Gehalt mindestens 0,065 Masse-% beträgt. Dabei wird durch die Begrenzung des C-Gehalts auf höchstens 0,15 Masse-%,
• insbesondere weniger als 0,15 Masse-%, sichergestellt, dass sich im Gefüge eines erfindungsgemäßen Stahlflachprodukts eine ausreichende Menge an Ferrit bildet und dass sich der gebildete Martensit in Teilbereichen überhaupt verformen kann und dass somit Scherspannungen abgebaut werden können.
• Diese Wirkung kann insbesondere dann erzielt werden, wenn der C-Gehalt auf höchstens 0,14 Masse-%, insbesondere höchstens 0,12 Masse-%, beschränkt ist.

The alloy of the melt produced for the production of a flat steel product according to the invention and the associated steel substrate of a flat steel product according to the invention has been selected as follows:

• Carbon ("C") is present in the flat steel product according to the invention in contents of 0.05-0.15 mass% in order to achieve the required level of strength. For this, at least 0.05 mass% C is required. The effects of the presence of C used according to the invention are achieved particularly reliably when the C content is at least 0.065% by mass. By limiting the C content to a maximum of 0.15% by mass,
• in particular less than 0.15% by mass, ensures that a sufficient amount of ferrite is formed in the structure of a flat steel product according to the invention and that the martensite formed can deform at all in some areas and that shear stresses can thus be reduced.
• This effect can be achieved in particular when the C content is limited to a maximum of 0.14% by mass, in particular a maximum of 0.12% by mass.

Silicon (“Si”) can be present in the steel of a steel flat product according to the invention in order to strengthen the steel. This effect can be achieved reliably with Si contents of at least 0.01% by mass, in particular 0.04% by mass. However, too high Si contents would increase the Ar3 temperature. This would make the hot rolling aimed for according to the invention more difficult in a temperature range in which the flat steel product has a completely austenitic structure. The invention avoids this in that the Si content is limited to less than 0.5% by mass, in particular less than 0.4% by mass.

Manganese ("Mn") is present in the steel of a flat steel product according to the invention in contents of 0.7-2.1% by mass in order to minimize the concentration of C in the structure and the associated formation of undesirable hard martensite. This effect is achieved particularly reliably with Mn contents of at least 0.7% by mass. If the content is more than 2.1% by mass, there is a risk of Mn segregations occurring in the structure of the flat steel product according to the invention, which would impair the mechanical properties. This negative influence of the presence of Mn can be excluded with particular certainty that the Mn content is limited to a maximum of 2.0 mass%.

The aluminum ("Al") content in the steel of a flat steel product according to the invention is limited to less than 0.1% by mass in order to avoid effects of this alloying element on the Ar3 temperature and to ensure optimized castability of the steel melt. However, Al can be used for deoxidation in the course of steel production. This typically requires Al contents of at least 0.02% by mass. Negative effects triggered by the presence of Al can in particular be avoided in that the Al content is limited to less than 0.05 mass%.

Chromium ("Cr") is present in the steel of a flat steel product according to the invention in contents of 0.2-1% by mass in order to increase the hardenability and suppress the formation of pearlite. For this purpose, at least 0.2% by mass of Cr are required, whereby the favorable effects of the presence of Cr can be used particularly reliably with Cr contents of at least 0.25% by mass. At the same time, the Cr content is at most 1% by mass in order to enable the formation of ferrite in the structure of the flat steel product according to the invention, which is aimed at according to the invention. This can be ensured particularly reliably by limiting the Cr content to a maximum of 0.9% by mass.

The steel of a flat steel product according to the invention contains at least one of the microalloying elements niobium (“Nb”), vanadium (“V”) and titanium (“Ti”) in order to increase the fine grain size and strength. According to the invention, the sum of the contents of these elements is 0.01-0.1% by mass. The respectively envisaged content of the micro-alloy elements can be taken up by the micro-elements alone or two or three of the mentioned micro-alloy elements can be present in combination. The positive influences of the micro-alloy elements on the mechanical properties of a flat steel product according to the invention can be used particularly reliably if the sum of their contents is at least 0.01% by mass. At the same time, the contents of the micro-alloy elements are limited to a maximum of 0.1% by mass, in particular a maximum of 0.05% by mass, in order to avoid precipitations and to enable accelerated recrystallization.

Boron ("B") can optionally be present in the steel of a flat steel product according to the invention in contents of up to 0.0015% by mass. It increases the hardenability particularly strongly. However, this must not be too high to prevent it from occurring to enable sufficient amounts of ferrite in the structure of a flat steel product according to the invention. Negative effects of the presence of B can be avoided particularly reliably by limiting the B content to a maximum of 0.0008% by mass.

Molybdenum (“Mo”) can optionally also be added to the steel of a flat steel product according to the invention in contents of less than 0.2% by mass, in particular less than 0.20% by mass, in order to increase the hardenability. For this purpose, at least 0.01 mass% Mo can be provided in practice. An alloy of a steel according to the invention which is particularly balanced in terms of cost / benefit aspects contains up to 0.18% by mass of Mo or up to 0.1% by mass of Mo, in particular up to 0.05% by mass of Mo, in particular up to 0.021% by mass -%, as shown in the exemplary embodiments, or up to 0.018% by mass of Mo.

Copper ("Cu") can optionally also be added to the steel of a flat steel product according to the invention in contents of less than 0.2% by mass in order to further increase the strength (precipitation and mixed crystal strengthening). The positive effect of the presence of Cu at contents of at least 0.1 mass% Cu can be used reliably.

Nickel ("Ni") can optionally also be added to the steel of a flat steel product according to the invention in contents of less than 0.2% by mass, in order to further increase the strength through precipitation and mixed crystal strengthening. The positive effect of the presence of Ni with contents of at least 0.1 mass% Ni can be used reliably.

Phosphorus (“P”) can also optionally be present in the steel according to the invention in contents of less than 0.05% by mass in order to further increase the strength and to control the transformation behavior. Used reliably the positive effect of the presence of P at contents of at least 0.002 mass% P.

Nitrogen ("N") is one of the unavoidable impurities that are present in steel as a result of the manufacturing process. In the steel according to the invention, contents of less than 0.01% by mass are permitted to be harmless for the properties. Higher concentrations would lead to coarse precipitations, which could have a negative effect on the forming behavior.

In step b), the intermediate product, which is composed of a melt in accordance with the above remarks, is cast in an otherwise conventional manner and is heated through at a temperature of 1150-1380 ° C. for a period of typically 60-960 minutes. The maximum temperature and the duration of the heating must be measured in such a way that all the carbides contained in the preliminary product are dissolved. For this purpose, the heating temperature is preferably below 1380 ° C. In the event that a conventional slab is processed as a preliminary product, a heating period of at least 60 minutes has proven to be particularly effective, with heating for a maximum of 8 hours being sufficient in practice for conventional slab dimensions for heating. The lower limit of the range of the through-heating temperature specified according to the invention is at least 1150 ° C., preferably more than 1200 ° C., in order to prevent the formation of precipitates and other undesirable phases in the structure of the preliminary product.

In order to be able to produce a flat steel product with an optimal surface quality in the subsequent hot rolling process, the pre-product can optionally be descaled before it is fed into the hot rolling process.

In any case, in the event that the pre-product is a slab, the pre-product is pre-rolled at temperatures of 1020-1150 ° C to a thickness of 30-50 mm. The pre-rolling compacts the cast structure of the slab so that the best conditions are created for the subsequent finish hot rolling. If the pre-product is a thin slab or a cast strip, pre-rolling can be dispensed with.

The hot rolling of the optionally pre-rolled preliminary product to a thickness of 1.5 - 6.4 mm can be carried out in a conventional manner in one or more steps. The only decisive factor is that the hot rolling end temperature Twe, at which hot rolling is ended, is at least equal to the Ar ₃ temperature of the steel and at most 200 ° C higher than the Ar _{3 of} the steel, with hot rolling end temperatures of 820-900 ° C in particular are practical.

mit %C = C-Gehalt, %Mn = Mn-Gehalt, %Si = Si-Gehalt, des Stahls
in einer für die Erfindung ausreichenden Weise abgeschätzt werden.The Ar ₃ temperature of steels of the type processed according to the invention can be determined experimentally in a conventional manner or the in CHOQUET, P. et al .: Mathematical Model for Predictions of Austenite and Ferrite Microstructures in Hot Rolling Processes. IRSID Report, St. Germain-en-Laye, 1985. 7 p . given formula (1) $${\mathrm{Ar}}_{\mathrm{3}}=\mathrm{902}-\mathrm{527}*\%\mathrm{C.}-\mathrm{62}*\%\mathrm{Mn}+\mathrm{60}*\%\mathrm{Si}$$

with% C = C content,% Mn = Mn content,% Si = Si content, of the steel
can be estimated in a manner sufficient for the invention.

According to the invention, the hot-rolling end temperature is selected such that hot-rolling is carried out as exclusively as possible in a temperature range at which an austenitic structure is present in the hot-rolled flat steel product. For this purpose, the hot rolling end temperature can be set to at least 820 ° C. At the same time, the hot rolling end temperature is at most 200 ° C., in particular less than 200 ° C., above the Ar ₃ temperature, in order to support the development of a fine-grained austenite structure, as far as possible there are many nucleation sites for the subsequent ferrite formation. Particularly suitable hot rolling end temperatures are accordingly in the range of 820-900 ° C.

The strategy of cooling the hot strip obtained by hot rolling to the respective coiling temperature is also of particular importance for the success of the invention. The cooling rate dT1 between the hot rolling end temperature and the intermediate temperature of Ar ₁ -50 ° C must be at least 20 K / s so that a concentration profile of C in the austenite is created, which is later converted to martensite. The cooling can also be continued at the cooling rate dT1 up to an intermediate temperature which is 100 ° C below the A _r1 temperature (intermediate temperature = A _r1 - 100 ° C).

Cooling speeds dT1 of at least 30 K / s are particularly suitable here. In practice, for reasons of efficiency, the upper cooling rate dT1 is limited to 90 K / s, in particular to a maximum of 70 K / s. The cooling down to the intermediate temperature Tz, controlled according to the invention, controls the cooling rate in such a way that, on the one hand, sufficient ferrite is formed and a sufficiently high diffusion of carbon from the ferrite into the adjacent austenite is enabled, through which the residual austenite, which later forms the fringing area of the martensite islands, is enriched with carbon will. In this temperature range, above all, the C diffusion from the ferrite that forms into the adjacent retained austenite can take place and diffuse therein.

mit %C = C-Gehalt, %Mn = Mn-Gehalt, %Si = Si-Gehalt, %Cr = Cr-Gehalt und %Ni = Ni-Gehalt des Stahls abgeschätzt werden, die von LUTSENKO, A. et al. im Artikel "The Definition and Use of Technological Reserves - An Effective Way to Improve the Production Technology of Rolled Metal", erschienen in 9th International Rolling Conference, Associazione Italiana di Metallurgia, Venice, June 2013, 8 p ., angegeben worden ist.The Ar ₁ temperature can be determined experimentally in a conventional manner or according to the formula (2) $${\mathrm{Ar}}_{\mathrm{1}}=\mathrm{741},\mathrm{7th}-\mathrm{7th},\mathrm{13th}\times \%\mathrm{C.}-\mathrm{14th},\mathrm{09}\times \%\mathrm{Mn}+\mathrm{16},\mathrm{26th}\times \%\mathrm{Si}+\mathrm{11},\mathrm{54}\times \%\mathrm{Cr}-\mathrm{49},\mathrm{69}\times \%\mathrm{Ni}$$

with% C = C content,% Mn = Mn content,% Si = Si content,% Cr = Cr content and% Ni = Ni content of the steel be estimated that von LUTSENKO, A. et al. in the article "The Definition and Use of Technological Reserves - An Effective Way to Improve the Production Technology of Rolled Metal", published in 9th International Rolling Conference, Associazione Italiana di Metallurgia, Venice, June 2013, 8 p ., has been specified.

As a result of the cooling controlled according to the invention, an island-like martensite is obtained in the structure of a flat steel product according to the invention, which has an inhomogeneous distribution of the carbon content over its volume. Despite tempering, the inhomogeneous C distribution remains. The martensite edges show a very high C content, which can be easily detected in the FE microprobe. is. This border with increased C concentration typically has a width of 10 nm - 1 μm, whereby its width can also be up to 1/3 of the diameter of the martensite island.

The carbon concentration increasing towards the edge area extends in a flat steel product according to the invention by at least 30% of the circumference of the martensite islands and is in particular not only at least 10% but at least 70% of all martensite islands. The profile of the carbon concentration generated according to the invention can be observed on all martensite islands that have a diameter (half of the shortest length + longest length in Fig 1 . at the martensite island (M)) of> 3 µm.

The border, in which there is a higher C content than in the central area of the respective martensite island, can, as indicated in claim 9, also occupy in total at least 50% of the circumference of the martensite island in question.

The C gradient generated according to the invention in the martensite islands of the structure increases the hole expansion HER, since the formation of large martensitic bees, homogeneous with regard to the carbon distribution, is prevented, which would increase the shear stress in a ferritic matrix and thus minimize the hole expansion. In addition, by the invention between the Ferrite matrix and the residual austenite present on the respective martensite island achieves smoother transitions between the soft ferrite matrix and the hard martensite liners or facilitates the deformation in partial areas of the martensite island. Due to the C distribution, areas in the martensite can be deformed earlier in the event of an external load, which reduces steep hardness jumps that are harmful to the hole expansion HER. Nevertheless, there is still a high degree of consolidation of the structure due to the differences in hardness. Thus, with high strength values, you get good elongation in combination with good hole expansion values HER.

mit %C = C-Gehalt, %Mn = Mn-Gehalt, %Si = Si-Gehalt, %Cr = Cr-Gehalt und %Mo = Mo-Gehalt des Stahls
abgeschätzt werden.The further cooling strategy subordinately promotes the advantageous product properties: After the intermediate temperature Tz has been reached, the cooling rate in the temperature range up to the Martensite start temperature T _MS is controlled in a second cooling section so that the diffusion length of C in austenite remains as limited as possible. The martensite start temperature T _MS can be determined experimentally in a conventional manner or according to that of SMC Van Bohemen in the article "Bainite and martensite start temperature calculated with exponential carbon dependence", Mater. Sci. Technol. 28 (2012) 487-495 , published formula (3) $${\mathrm{T}}_{\mathrm{MS}}=\mathrm{565}-\mathrm{600}\times \left(\mathrm{1}-\mathrm{EXP}\left(-\mathrm{0},\mathrm{96}\times \%\mathrm{C.}\right)\right)-\mathrm{31}\times \%\mathrm{Mn}-\mathrm{13th}\times \%\mathrm{Si}-\mathrm{10}\times \%\mathrm{Cr}-\mathrm{12th}\times \%\mathrm{Mon}$$

with% C = C content,% Mn = Mn content,% Si = Si content,% Cr = Cr content and% Mo = Mo content of the steel
be estimated.

According to a first variant of the second cooling section, the hot strip is cooled starting from the intermediate temperature Tz at a cooling rate dT2 'of at least 5 K / s, in particular more than 5 K / s or at least 20 K / s, until the martite start temperature T _{MS is} reached is. The cooling speed dT2 'in this variant is limited to a maximum of 100 K / s, to ensure that a diffusion of carbon from the previously formed ferrite into the adjacent austenite can take place.

This can be ensured particularly reliably by limiting the cooling rate dT2 'to a maximum of 70 K / s. It is particularly practical if the cooling rate dT2 'is 20 - 70 K / s.

According to the second variant of the second section of cooling, the cooling is completed up to the martensite _{start temperature T MS} with a cooling rate dT2 ″ of 10-130 K / s. A cooling rate dT2 ″ of at least 10 K / s, in particular at least 30 K / s, is limited also the carbon diffusion from the ferrite into the austenite. The diffusion of a sufficient amount of carbon can be supported by interrupting the cooling at a cooling stop temperature of 550 - 700 ° C for up to 5 s. A break of at least 1 s is particularly practical here. At the same time, the cooling rate dT2 ″ should be at most 130 K / s, in particular less than 100 K / s, in order to even allow a sufficient C diffusion length in the austenite is limited to a maximum of 80 K / s. It is particularly practical when the cooling rate dT2 "is 30 - 80 K / s.

The third section of cooling, in which the hot-rolled flat steel _{product is cooled to the coiling temperature after the martensite start temperature T MS} has been reached, is not critical and can take place at a cooling rate in still air. The coiling temperature HT is lower than the martensite start temperature and can reach room temperature. In practice, the coiling temperature HT is typically in a range that extends from room temperature to 100 ° C, in particular 20-80 ° C.

The hot-rolled flat steel product cooled in this way is wound into a coil. In the event that the coiling temperature HT is above room temperature, the flat steel product is finally cooled to room temperature in the coil.

Even the low reel temperature set according to the invention is of particular importance. It enables the carbon (C) contained in the hot-rolled flat steel product to have short distances to diffuse in the last cooling step of hot strip production. The solubility of carbon in alpha-Fe is very low. Therefore, this migrates out of the centers of the islands formed from freshly formed martensite in the direction of their edges. Here, in turn, adjacent phases such as ferrite and bainite make further diffusion difficult. In the case of many martensite islands, due to the high concentrations of carbon present there, a border of retained austenite that at least partially delimits the martensite islands is therefore present in their edge area. This seam can be checked using an FE-SEM microprobe analysis. There are also internal stresses within the martensite grains, which can be determined by EBSD measurements and are much higher in flat steel products according to the invention than in conventional hot strip with a corresponding composition ( FIG 3 ). The EBSD measurements were similar to those at H. Farivar et al. in the article "Experimental quantification of carbon gradients in martensite and its multi-scale effects in a DP steel", MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING 718 (2018) 250-259 described procedure carried out.

The basis for the improved tensile strength and elongation in a flat steel product according to the invention are the different phases contained in its structure and the associated high degree of solidification, as well as the reduction in shear stresses compared to pure dual-phase structures achieved through the special C distribution in the martensite islands and the associated development of residual austenite seams , which lead to optimized hole expansion values HER.

The tempering processes that already take place when cooling from the reel temperature HT to room temperature (∼20 ° C) lead to shorter incubation times, as there is already an increased proportion of dissolved carbon. This can be seen from a publication by Waterschoot ( T. Waterschoot, AK De, S. Vandeputte, DB Coomann, Static Strain Aging Phenomena in Cold-Rolled Dual-Phase Steels, Metall. and Mat. Trans. A (2003) 781 ). Since the incubation temperature and time are mostly linked via diffusion, lower incubation temperatures are also necessary for the further tempering treatment (work steps g), h)) carried out in accordance with the invention after reeling and cooling in the coil.

Here, the flat steel product hot-rolled according to the invention is kept under an inert or reducing atmosphere at a tempering temperature of 150-500 ° C., in particular 150-400 ° C., for a tempering period of 4-16 h. A conventional bell-type annealing furnace is typically used for this, in which the hot-rolled flat steel product is used as a coil.

After tempering, the tempered flat steel product is cooled to room temperature at a rate of 4-700 K / h.

This tempering treatment further reduces the hardness gradient, which is already softened due to the presence of the seam between the hard martensite and the soft ferrite, which in particular consists of retained austenite (reduction of the martensite strength and possible hardening of the ferrite phases due to carbide precipitations). For example, the tempering treatment according to the invention distributes the carbon in the structure to a greater extent. This in turn has the consequence that the seam with the enriched C content widens and the associated sharp hardness gradients between the martensite and ferrite components of the structure are weakened.

The solidified ferrite present in the structure of a flat steel product according to the invention in proportions of typically more than 30% by volume also supports accelerated tempering, since dislocations represent fast diffusion paths. These are also found in the hot strips belonging to the invention.

The tempering treatment according to the invention leads to an increase in the hole widening HER of at least 9%, in particular at least 10%, compared to the hole widening HER, which is achieved for a hot-rolled flat steel product according to the invention after coiling and cooling in the coil (steps g) and h)). This applies even to low tempering temperatures and low ferrite content in the structure of the respective flat steel product. The ferrite components present in the structure of a flat steel product according to the invention enable comparatively short tempering times, which can be further shortened by higher proportions of ferrite. For example, with ferrite proportions of 70% by volume, the hole expansion HER, which is already high after coiling and cooling and typically at least 20%, can be increased by a further at least 15%.

A flat steel product according to the invention thus achieves tensile strengths Rm, hole widenings HER and uniform elongations Ag, the product of which Rm x HER x Ag is regularly at least 300,000 MPa% ² , in particular at least 330,000 MPa% ² .

The tensile strength Rm of a hot-rolled flat steel product according to the invention regularly reaches values of at least 530 MPa, the hole expansion HER regularly values of at least 30% and the uniform elongation Ag regularly values of at least 8%

The yield strength ratio Rp / Rm in a hot-rolled flat steel product according to the invention is regularly at least 0.6, where typically yield strength ratios Rp / Rm of at least 0.65 can be achieved.

Fig. 1

eine Fe-Mikrosonde (C-Verteilung), erstellt am Warmband W1 (angelassen bei 400 °C für 10 h);

Fig. 2

eine Fe-REM Aufnahme, erstellt am Stahl W1 (angelassen bei 400 °C für 10 h);

Fig. 3

eine KAM 15° Analyse am Martensit, hier am Stahl W1 (angelassen bei 400 °C für 10 h).

The invention is explained in more detail below on the basis of exemplary embodiments. Show it:

Fig. 1

an Fe microprobe (C distribution), created on the hot strip W1 (tempered at 400 ° C. for 10 h);

Fig. 2

an Fe-REM image taken on steel W1 (tempered at 400 ° C. for 10 h);

Fig. 3

a KAM 15 ° analysis on martensite, here on steel W1 (tempered at 400 ° C for 10 h).

To test the invention, five melts E1-E5, which were composed in accordance with the requirements of the invention and whose compositions are given in Table 1, were melted.

_{In addition, table 1 shows the martensite start temperatures T MS} , Ar ₃ temperatures and Ar ₁ temperatures estimated in accordance with the formulas (1) - (3) explained above for the melts E1-E5.

The melts E1-E5 have been cast in the conventional manner into slabs, which have each been heated through at 1150-1380 ° C. for a period of 60 minutes.

The slabs heated through in this way have been subjected to rough rolling, in which they have been hot rough rolled in the temperature range from 1020 to 1150 ° C. to form a rough strip with a thickness of 30 to 50 mm.

The slabs pre-rolled in this way have been finished hot-rolled in seven passes in a conventional manner to form hot-rolled strips ("hot strip") WV and W1-W11 with a thickness Dw. When leaving the last pass of hot rolling, the hot strips WV and W1-W11 had a hot-rolling end temperature ET which was above the Ar ₃ temperature of the steel E1-E5 of which the hot strips WV and W1-W11 each consisted.

After hot rolling, the hot strips WV and W1 - W11 obtained were cooled, starting from their respective final hot rolling temperature ET, at a cooling rate dT1 to an intermediate temperature Tz which was 50 ° C. below the Ar ₁ temperature of the steel E1 - E5 from which the hot strips were made WV and W1 - W11 each passed.

After the intermediate temperature Tz has been reached, the hot strips WV, W1, W3 to W8, W10 and W11 have each _{been cooled at a cooling rate dT2 'down to the martensite start temperature T MS of} the steel.

The hot strips W2 and W9, on the other hand, were initially cooled at a cooling rate dT2 ″ to a cooling stop temperature Tsp, at which they were each held for a holding period tH, and then to the martensite start _{temperature T MS of} the steel and room temperature.

Starting from the martensite _{start temperature T MS} , the hot strips WV and W1-W11 were then cooled in still air to the respective coiling temperature HT at which they were coiled into a coil. Finally, cooling to room temperature took place in the coil.

For the hot strips WV and W1-W11 obtained in this way, the thickness Dw, as well as the final hot-rolling temperatures ET, cooling rates dT1, cooling rates dT2 ', cooling rates dT2 "and coiling temperatures HT, are shown in Table 2.

While the stipulations of the invention were complied with for the hot strips VW and W1 to W4 and W6 to W11, the cooling rates were too low during the cooling of the hot strip WV after hot rolling and the coiling temperature HT too high and the cooling rate dT1 during the cooling of the hot strip W5 too low.

The tensile strength Rm, the yield point Re, the uniform elongation Ag, the elongation A50, the elongation A80 and the hole expansion HER have been determined for the hot strip WV and W1-W11. The relevant properties as well as the product Rm x λ x Ag and the ratio Re / Rm are listed in Table 3 for the hot strips WV and W1 - W11.

The martensite, ferrite, bainite, pearlite and retained austenite components of the structure have also been determined for the hot strips WV and W1 - W11. For the hot strip W5 not cooled according to the invention, an undesired perlite content of the structure of 15% by volume resulted.

In addition, the martensite and residual austenite grain sizes as well as the width Bras of the residual austenite border that surrounded the martensite islands in the structure of the hot strips WV and W1 - W11 have been determined for some of the hot strips WV and W1 - W11.

The relevant values are listed in Table 4. The hot strips WV and W5 not produced according to the invention accordingly had a structural composition which does not meet the requirements of the invention.

It is found that the hot strips W1-W11 produced according to the invention and alloyed according to the invention reliably have high mechanical parameters Rm, Re, Ag, A50, A80 and HER, which are too high for the product Rm x HER x Ag of more in each case than 200,000 MPa% ² . That However, hot strip WV not produced according to the invention remained below this limit.

After cooling in the coil, the hot strips were subjected to a tempering treatment in a conventional bell-type furnace, in which they were each held for a tempering period t _AN at a tempering temperature T _An under an inert or reducing atmosphere. The relevant parameters are listed in Table 5.

The tensile strength Rm, the uniform elongation Ag, the elongation A50 and the hole expansion HER, as well as the product Rm x HER x Ag and the% increase delta HER, by which the hole expansion determined after the tempering, were determined on the hot strips WV - W11 tempered in this way HER has increased compared to the hole widening HER; which the respective hot strip WV and W1-W11 had after cooling in the coiler but before tempering. The relevant data are also given in Table 5. It turns out that the tempering treatment led to a considerable increase in the hole expansion HER and the product Rm x HER x Ag, which makes it clear that a hot-rolled flat steel product can be produced with the method according to the invention, which has superior mechanical properties.

The invention thus provides a method for producing a hot-rolled flat steel product with a structure of (in% by volume) 4-50% tempered, island-shaped martensite, 30-96% ferrite, <10% retained austenite, the remainder ≤ 66% bainite or bainitic ferrite to disposal. Thereby (a) a steel melt from, in mass%, C: 0.05-0.15%, Si: <0.5%, Mn: 0.7-2.1%, Al: <0.1 %, Cr: 0.2 - 1%, in total 0.01 - 0.1% Nb, Ti or V, B: <0.0015%, Mo: <0.2%, Cu: <0.2% , Ni: <0.2%, P: <0.05%, N: <0.01% and the remainder of Fe and ≤ 0.1% impurities, cast into a preliminary product that (b) at 1150-1380 ° C, (c) optionally descaled and (d), if necessary, pre-rolled at 1020 - 1150 ° C to 30 - 50 mm. Then it will Hot-rolled to a 1.4 - 6.4 mm thick strip with a hot rolling end temperature ET, for which Ar ₃ ≤ ET ≤ 200 ° C + Ar ₃ temperature of the steel applies. This is followed by (f) cooling to a coiling temperature HT, for which the martensite start temperature T _{MS of} the steel> HT ≥ room temperature RT, with the cooling from ET to an intermediate temperature Tz, for which Tz <Ar ₁ - 50 ° C applies ≥ 20 K / s takes place. After reaching Tz, the strip is either _{cooled between T MS} and RT at 5-100 K / s or at 10-130 K / s to a cooling stop temperature Tsp of 550-770 ° C, at which the hot strip is optionally held for ≤ 5 s and then cooled to a reel temperature HT lying between Tsp and RT. The hot strip is coiled at HT, then cooled further or held, (g) tempered for 4-16 h at 150-500 ° C and (h) cooled to RT at 4-700 K / h. Table 1: Chemical analyzes, *) not according to the invention stole Figures in% by mass, remainder iron and unavoidable impurities ° C C. Si Mn P. S. Al Cr Mon N Ti + Nb + V B. T _MS A _r3 A _r1 E1 * 0.08 0.6 1.7 0.009 0.0014 0.04 0.3 0.018 0.0048 0.124 0.0002 456 788 730 E2 0.08 0.11 1.0 0.009 0.0008 0.027 0.5 0.021 0.005 0.044 0.0002 485 807 735 E3 0.07 0.06 1.0 0.003 0.0007 0.035 0.4 0.012 0.0087 0.038 0.0001 489 806 733 E4 0.12 0.30 1.7 0.003 0.0007 0.036 0.8 0.014 0.0051 0.044 0.0002 435 751 731 E5 0.14 0.10 1.0 0.003 0.0006 0.033 0.4 0.01 0.0052 0.038 0.0001 455 774 733 Hot strip stole thickness Oven temperature Lay time Opening stitch Sliver thickness ET HT dT1 dT2 ' dT2 " Tsp tH [mm] [° C] [min] [° C] [mm] [° C] [K / s] [K / s] [° C [s] WV * E1 * 3.2 1240 130 1030 41 820 520 30th 2 W1 E2 4th 1260 140 1080 40 860 60 40 50 690 2 W2 E2 2 1280 240 1140 41 850 50 35 30th W3 E2 1.5 1260 130 1130 42 850 30th 30th 40 W4 E2 2 1230 150 1050 46 910 50 135 50 W5 * E3 1.6 1180 160 1040 47 850 30th 2 30th W6 E3 1.6 1290 120 1140 44 860 50 51 60 W7 E3 2 1260 145 1130 43 855 90 62 40 W8 E3 3 1230 165 1110 42 900 30th 30th 40 W9 E4 1.5 1260 125 1130 41 860 40 80 70 675 3 W10 E4 2 1250 135 1125 42 850 80 31 30th W11 E5 3.4 1270 140 1120 38 860 40 42 30th Hot strip stole thickness re Rm Re / Rm Ag A50 HER Rm * HER * Ag [mm] [MPa] [%] MPa *% ² WV * E1 3.2 762 856 0.89 7.9 15.5 26th 175822 W1 E2 4th 501 702 0.71 12.6 20.7 34 300737 W2 E2 2 474 711 0.67 12.4 19.4 38 335023 W3 E2 1.5 562 752 0.75 11.8 18.5 31 275082 W4 E2 2 750 934 0.80 12.4 15.2 15th 173724 W5 * E3 1.6 390 590 0.66 12th 20.5 41 290280 W6 E3 1.6 438 652 0.67 11.8 20.0 48 369293 W7 E3 2 401 612 0.66 12.6 20.6 56 431827 W8 E3 3 480 687 0.70 11.5 18.4 45 355523 W9 E4 1.5 750 1014 0.74 8.5 13.4 25th 215475 W10 E4 2 689 975 0.71 9.1 13.9 27 239558 W11 E5 3.4 508 804 0.63 10.3 20.8 30th 248436 *) not according to the invention Hot strip stole Martensite ferrite Bainite Perlite RA Wide RA hem Vol% µm WV * E1 5 10 75 5 2 - W1 E2 12th 83 5 - 2 0.3 W2 E2 15th 80 5 - 2 - W3 E2 20th 75 0 - 2 0.3 W4 E2 30th 65 5 - 3 nb W5 * E3 5 80 15th 0 - W6 E3 15th 80 5-10 <1 1.5 0.4 W7 E3 10 85 5 1.5 - W8 E3 15th 85 - 1.5 - W9 E4 35 60 5 - 1 0.9 W10 E4 35 65 - - <1 - W11 E5 25th 75 5 - 1.0 - *) not according to the invention Hot strip stole thickness T _AN t _ON re Rm Re / Rm Ag A50 HER Delta HER Rm * HER * Ag [mm] [° C] [H] [MPa] [%] [MPa *% ² ] WV * E1 3.2 300 6th 788 875 0.90 8th 16.7 38 12th 266000 W1 E1 4th 150 8th 508 705 0.72 11.3 22.9 58 24 462057 W1 E2 4th 200 12th 518 699 0.74 11.8 24.1 58 24 478396 W1 E2 4th 400 10 557 649 0.86 9.3 22.7 102 68 615641 W2 E2 2 150 15th 480 709 0.68 13.1 22.1 53 15th 492259 W2 E2 2 250 13th 558 692 0.81 12.3 22.1 63 25th 536231 W2 E2 2 400 6th 573 658 0.87 10.7 22.1 103 65 725182 W2 E2 2 250 12th 565 734 0.81 11.7 18.4 52 14th 446566 W2 E2 2 400 9 589 721 0.87 10.5 18.1 98 60 741909 W6 E3 2.9 150 14th 439 651 0.67 12th 23.1 73 25th 570276 W6 E3 2.9 200 20th 494 640 0.77 13.4 23.5 76 28 651776 W6 E3 2.9 250 8th 516 628 0.82 13.1 25.4 84 36 691051.2 W6 E3 2.9 400 5 493 576 0.86 11.5 27.1 112 64 741888 W9 E3 2 300 11 832 1005 0.83 8.4 13.3 68 43 574056 W9 E4 2 400 7th 812 945 0.86 8.5 13.1 78 53 626535 W11 E4 2 300 12th 632 789 0.80 10.2 19.5 52 22nd 418485.6 *) not according to the invention

Claims (16)

Process for the production of a hot-rolled flat steel product with a structure consisting of 4-50% by volume of tempered, island-shaped martensite and 30-96% by volume of ferrite, the proportion of which is not occupied by martensite and ferrite Structure consists of up to 66% by volume of bainite and / or bainitic ferrite and less than 10% by volume of retained austenite, comprising the following work steps: a) Casting a steel melt to a preliminary product in the form of a slab, thin slab or a cast strip, the melt consisting of, in mass%, C: 0.05 - 0.15%, Si: < 0.5%, Mn: 0.7 - 2.1%, Al: < 0.1%, Cr: 0.2 - 1 %, at least one element from the group "Nb, Ti, V" with the proviso that the sum of the Nb, Ti and / or V contents is 0.01-0.1%, B: <0.0015%, Mon: <0.2%, Cu: <0.2%, Ni: <0.2% P: <0.05% N: <0.01% and the remainder consists of Fe and a total of at most 0.1% by mass of unavoidable impurities; b) heating through the intermediate product at a temperature of 1150-1380 ° C; c) optional: descaling of the preliminary product; d) in the case that the preliminary product is a slab: preliminary rolling of the preliminary product at temperatures of 1020 - 1150 ° C to a thickness of 30 - 50 mm e) Hot rolling of the optionally pre-rolled pre-product in one or more rolling passes to form a hot-rolled strip with a thickness of 1.4-6.4 mm, the hot-rolling end _{temperature ET, at which the hot-rolling is ended, at least equal to the Ar 3} temperature of the steel and is at most 200 ° C higher than the Ar ₃ temperature of the steel; f) cooling the hot strip obtained to a coiling temperature HT which is in a range which _{begins below the martensite start temperature T MS of} the steel and ends at room temperature, - where the cooling rate dT1 in a temperature range that extends from the hot rolling end temperature ET to an intermediate temperature Tz that is 50 ° C below the Ar ₁ temperature of the steel is at least 20 K / s, - where after reaching the intermediate temperature Tz - According to a first variant, the hot strip is cooled at a cooling rate dT2 'of 5-100 K / s until the martensite start temperature T _{MS of} the steel is reached,
or - According to a second variant, the hot strip with a cooling rate dT2 "of 10-130 K / s, initially to a cooling stop temperature Tsp, which is in the range 550-770 ° C and at which the hot strip can optionally be held for up to 5 s holding time tH is held, and is then _{cooled to the martensite start temperature T MS of} the steel,
and - the hot strip is then cooled to the coiling temperature,
and the hot strip cooled in this way is coiled into a coil, in the event that the coiling temperature HT is above room temperature, the flat steel product is finally cooled in the coil to room temperature; g) annealing the hot-rolled flat steel product through a tempering duration t _AN 4-16 h at a tempering temperature T _at 150-500 ° C under a reducing or inert atmosphere h) Cooling the tempered flat steel product at a cooling rate of 4-700 K / h to room temperature. Method according to Claim 1, characterized in that the final hot rolling temperature ET is 820-900 ° C and the cooling rate dT1 is at most 70 K / s. Method according to Claim 1 or 2, characterized in that the cooling rate dT1 is at least 30 K / s. Method according to one of the preceding claims, characterized in that during the cooling (step f)) taking place according to the first variant, the cooling rate dT2 '= 20-70 K / s. Method according to one of Claims 1 to 3, characterized in that in the case of the cooling (step f)) taking place according to the second variant, the cooling rate dT2 "= 30-80 K / s. Method according to one of the preceding claims, characterized in that the coiling temperature HT is in a range which extends from room temperature to 100 ° C. Method according to one of the preceding claims, characterized in that the tempering temperature is 150-400 ° C. Hot-rolled flat steel product with a hole expansion HER of at least 30%, the hot-rolled flat steel product having a steel substrate, - that from, in% by mass, C: 0.05 - 0.15%, Si: <0.5%, Mn: 0.7 - 2.1%, Al: <0.1%, Cr: 0 , 2 -1%, at least one element of the group "Nb, Ti, V" with the proviso that the sum of the contents of Nb, Ti and / or V is 0.01-0.1%, B: <0, 0015%, Mo: <0.2%, Cu: <0.2%, Ni: <0.2%, P: <0.05%, N: <0.01% and the remainder of Fe and in total there is a maximum of 0.1% by mass of unavoidable impurities
and - the structure of which consists of 4 - 50% by volume of tempered martensite present in the structure in the form of islands, in which at least 10% of the martensite islands are at least partially bordered on their circumference, and 30 - 96% by volume consists of ferrite , whereby the portion not taken up by martensite and ferrite consists of up to 66 vol .-% bainite or bainitic ferrite and less than 10 vol .-% residual austenite and the C content of the seam is at least in one section higher than the C- Content of the central area of the martensite island. Flat steel product according to Claim 8, characterized in that the edge, in which there is a higher C content than in the central area of the respective martensite island, in total takes up at least 50% of the circumference of the respective martensite island. Flat steel product according to one of Claims 8 or 9, characterized in that in all martensite islands with an average diameter of more than 3 µm a higher C content is present in at least one section of their edge areas, and in the middle area of the respective martensite island. Flat steel product according to Claims 8 to 10, characterized in that the seam comprises residual austenite and / or martensite. Flat steel product according to Claim 11, characterized in that the width of the seam is 10 nm to 1 µm. Flat steel product according to one of Claims 8-12, characterized in that the product Rm x HER x Ag formed from its tensile strength Rm, its hole expansion HER and its uniform elongation Ag is at least 300,000 MPa% ² . Flat steel product according to Claims 8-13, characterized in that its tensile strength Rm is at least 530 MPa. Flat steel product according to one of Claims 8-14, characterized in that its hole expansion HER is at least 30%. Flat steel product according to one of Claims 8 to 15, characterized in that its uniform elongation Ag is at least 5%.

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