EP3872194A1 - Method for producing hot-rolled flat steel product and flat steel product - Google Patents

Abstract

The invention relates to a method for producing a hot-rolled flat steel product with 5-40% by volume of tempered or freshly formed martensite and 50-95% by volume of ferrite and the remainder of up to 45% by volume of retained austenite, Bainite and / or cementite existing structure. For this purpose, a) a steel melt is poured into a preliminary product, which consists of, in% by 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% of at least one element of the group "Nb, Ti, 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; b) the intermediate product is heated through at 1150-1380 ° C; d) the intermediate product is pre-rolled at 1020 - 1150 ° C to a thickness of 30 - 50 mm, if it is a slab, e) the intermediate product into a 1.4 - 6.4 mm thick hot strip with a hot rolling end temperature ET (Ar <sub > 3 </sub> ≤ ET ≤ Ar ₃ + 200 ° C hot-rolled; f) the hot strip is cooled to a coiling temperature HT (martensite start temperature T _MS> HT ≥ room temperature), where the cooling rate dT1 in the temperature range ET to Tz (Tz = Ar ₁ -50 ° C) ≥ 20 K / s, after reaching Tz according to a first variant, the hot strip with a cooling rate dT2 'of 5-100 K / s is cooled to HT, or according to a second variant, the hot strip is cooled at a cooling rate dT2 "of 10-130 K / s first to a cooling stop temperature ZT1 amounting to 550-770 ° C and then cooled to HT; g) the hot strip is coiled into a coil; and h) optionally the hot-rolled flat steel product is cooled to room temperature in the coil or the hot-rolled flat steel product has been HAT at the coiling temperature held.

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 display and measurement of the retained austenite fringe present on the martensite islands of the structure in the flat steel products according to the invention, as explained below, was also carried out with the FE-SEM at a magnification of 20,000 times as well.

The strength and elongation properties mentioned here, such as tensile strength Rm, uniform elongation Ag, elongation at break A50 of flat steel products were determined in the tensile test according to DIN-EN 6892-1 specimen form 1 transverse 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 mm ² samples according to ISO 16630.

The qualitative C distribution in the structure of the flat steel products was determined by means of an FE microprobe, as described by H. Farivar et al. in the article "Experimental quantification of carbon gradients in martensite and its multiscale 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. 2 .

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 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 one Has a final hot rolling 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 baihit 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 *%.

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. Included 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 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 7.

Advantageous embodiments of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

• 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 auf die im Bereich zwischen der Martensitstarttemperatur T_MS des Stahls und Raumtemperatur liegende Haspeltemperatur HT gekühlt wird,
  oder
• gemäß einer zweiten Variante das Warmband mit einer Abkühlrate dT2" von 10 - 130 K/s zunächst auf eine Kühlstopptemperatur ZT1, 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 im Bereich zwischen Martensitstarttemperatur T_MS des Stahls und Raumtemperatur liegende Haspeltemperatur HT gekühlt wird;

g) Haspeln des auf die Haspeltemperatur HT abgekühlten Warmbands zu einem Coil;
h) optional Abkühlen des warmgewalzten Stahlflachprodukts im Coil auf Raumtemperatur oder Halten des warmgewalzten Stahlflachprodukts bei der Haspeltemperatur HT.In an inventive production of a hot-rolled flat steel product with a structure that consists of 5-40% by volume of tempered or freshly formed martensite and 50-95% by volume of ferrite and in which the portion not taken up by martensite and ferrite consists of If up to 45% by volume of retained austenite, bainite and / or cementite is filled, 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 preliminary 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,

• wherein the cooling rate dT1 in a temperature range from the hot rolling end temperature ET to an intermediate temperature Tz is sufficient, which 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 to the coiling temperature HT, which lies _{in the range between the martensite start temperature T MS of the steel and room temperature,}
  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 ZT1, 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 , and is then cooled to the coiling temperature HT lying in the range between the martensite start _{temperature T MS of the steel and room temperature;}

g) Coiling of the hot strip cooled to the coiling temperature HT into a coil;
h) optionally cooling the hot-rolled flat steel product in the coil to room temperature or keeping the hot-rolled flat steel product at the coiling temperature HT.

• das aus, in Masse-%, C: 0,05 - 0,15 %, Si: < 0,5 %, Mn: 0,7 - 2,1 %, AI: < 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 5 - 40 Vol.-% aus angelassenem oder frisch gebildeten Martensit und zu 50 - 95 Vol.-% aus Ferrit besteht und in dem der nicht durch Martensit und Ferrit eingenommene Anteil aus bis zu 45 Vol. % Restaustenit, Bainit und/oder Zementit besteht, wobei die Martensit-Anteile des Gefüges inselförmig vorliegen und bei mindestens 70 % aller Martensitinseln ein das Zentrum der jeweiligen Martensitinsel abdeckender Mittenbereich vorhanden ist, der von einem an den Rand der jeweiligen Martensitinsel grenzenden Randbereich der betreffenden Martensitinsel umgrenzt ist,
• wobei der C-Gehalt des Randbereichs zumindest in einem Abschnitt höher ist als der C-Gehalt des Mittenbereichs.

A hot-rolled flat steel product according to the invention accordingly comprises a steel substrate,

• that from, in mass%, C: 0.05 - 0.15%, Si: <0.5%, Mn: 0.7 - 2.1%, AI: <0.1%, Cr: 0, 2 - 1%, at least one element from 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 % is, B: <0.0015%, Mo: <0.2%, Cu: <0.2%, Ni: <0.2%, P: <0.05%, N: <0.01% and the remainder of Fe and a total of 0.1 mass% or less of unavoidable impurities
  and
• the structure of which consists of 5 - 40 vol.% of tempered or freshly formed martensite and 50 - 95 vol / or cementite, the martensite fractions of the structure being in the form of islands and in at least 70% of all martensite islands 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,
• wherein the C content of the edge region is higher than the C content of the central region, at least in one section.

Typically, the section or sections of the edge region of the martensite islands in which or in which a higher C content is present than in the central region of the respective martensite island, in total, takes up at least 30-70% of the circumference of the martensite island in question FIG 2 evident.

It is possible to produce a hot-rolled flat steel product according to the invention using the method of manufacture in which most martensite islands with an average diameter of more than 3 μm have a higher C content in at least one section of their edge areas than in the middle area of the respective martensite island. Such a flat steel product is characterized by particularly good hole expansion properties.

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

With regard to the mechanical properties of a flat steel product according to the invention, it proves to be particularly advantageous if the martensite islands of the structure of a flat steel product according to the invention are at least over part of their circumference surrounded by a hem consisting of retained austenite. Its 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 200,000 MPa% ² , in particular at least 300,000 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 20% and the uniform elongation Ag regularly values of at least 5%, in particular at least 8%.

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 particularly certain reached when the C content is at least 0.065% by mass. By limiting the C content to at most 0.15% by mass, in particular less than 0.15% by mass, it is ensured 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 is in Can deform partial areas at all and thus shear stresses can 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 by limiting the Al content to less than 0.05% by 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 one of 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 limited to a maximum of 0.1 mass%, in particular a maximum of 0.05 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 in order to enable the formation of sufficient quantities 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 or up to 0.018% by mass % Mon

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 increase the strength through precipitation and mixed crystal strengthening to increase further. 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. The positive effect of the presence of P can be used safely with 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 here is that the hot rolling end temperature ET, 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 temperature of the steel, with hot rolling end temperatures of 820-900 ° C are particularly practical.

mit %C = C-Gehalt, %Mn = Mn-Gehalt, %Si = Si-Gehalt, des Stahls
in einer für die Erfindung ausreichenden Weise abgeschätzt werden. Die Warmwalzendtemperatur ist dabei erfindungsgemäß so gewählt, dass möglichst ausschließlich in einem Temperaturbereich warmgewalzt wird, bei dem im warmgewalzten Stahlflachprodukt ein austenitisches Gefüge vorliegt. Hierzu kann die Warmwalzendtemperatur auf mindestens 820 °C eingestellt werden. Gleichzeitig liegt die Warmwalzendtemperatur um höchstens 200 °C, insbesondere weniger als 200 °C, oberhalb der Ar₃-Temperatur, um die Ausprägung eines feinkörnigen Austenitgefüges zu unterstützen, in dem möglichst viele Keimstellen für die nachfolgende Ferritbildung vorliegen. Besonders geeignete Warmwalzendtemperaturen liegen demnach im Bereich von 820 - 900 °C.The Ar ₃ temperature of steels of the type processed according to the invention can be determined experimentally in a conventional manner or according to the method described 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) $${\mathrm{Ar}}_{\mathrm{3}}=\mathrm{Ar}\mathrm{3}=\mathrm{902}-\mathrm{527}*\mathrm{C.}-\mathrm{62}*\mathrm{Mn}+\mathrm{60}*\mathrm{Si}$$

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 so that hot rolling is carried out as exclusively as possible in a temperature range in which im hot-rolled flat steel product has an austenitic structure. For this purpose, the hot rolling end temperature can be set to at least 820 ° C. At the same time, the final hot-rolling 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 in which as many nuclei as possible for the subsequent ferrite formation are present. 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 of decisive importance for the success of the invention. Above all, the cooling rate Td1 between the hot rolling end temperature and the intermediate temperature of Ar1 -100 ° C must be at least 20 K / s, so that a concentration profile of C in the austenite arises during the ferrite formation, which is later converted to martensite. Cooling speeds Td1 of at least 30 K / s are particularly suitable here. In practice, the upper cooling rate Td1 is limited to 90 K / s for reasons of efficiency. Through the cooling down to the intermediate temperature Tz controlled according to the invention, the cooling rate is controlled 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 made possible, through which the residual austenite, which later forms the fringing area of the martensite islands, with carbon is enriched. 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
to be estimated by 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.

In the case of a flat steel product according to the invention, 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 and this C concentration at the edge is so high that the retained austenite at 20,000 times magnification can be seen (see FIG. 1 ), a total of at least 30% of the circumference of the martensite island concerned.

The "higher C content" is defined in such a way that it is at least 0.05% by weight, with higher C contents of at least 0.1% by weight, in particular at least 0.15% by weight have proven to be particularly advantageous in practice. The concentration of the C content is determined with an FE microprobe within a range of 300 ^∗ 300 nm ² .

As a result of the cooling controlled according to the invention, island-like martensite is obtained in the structure of a flat steel product according to the invention, which mostly has an inhomogeneous distribution of the carbon content over its volume. In some cases, retained austenite remains on the martensite edges, which forms a border surrounding the respective martensite island. This typically has a width of 10 nm - 1 µm, whereby its width can also be up to 1/3 of the island diameter. The carbon concentration increasing towards the edge area extends in the case of a flat steel product according to the invention by at least 30% of the circumference of the martensite islands (see, for example, the martensite islands shown in FIG Fig. 1 shown) and is included at least 70% of all martensite islands are present. The profile of the C concentration generated according to the invention can be observed on all martensite islands which have a diameter ØMI of more than 3 μm determined according to the formula ØMI = half the shortest length of the martensite island + half the longest length of the martensite island (see Fig. 1 , Martensite island M).

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 martensite islands 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, the residual austenite present according to the invention between the ferrite matrix and the respective martensite island achieves smoother transitions between the soft ferrite matrix and the hard martensite islands 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. This results in good elongation in combination with good hole expansion values HER with high strength values.

The further cooling strategy promotes the advantageous product properties to a subordinate extent: 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 in such a way that the diffusion length of C in austenite remains as limited as possible.

mit %C = C-Gehalt, %Mn = Mn-Gehalt, %Si = Si-Gehalt, %Cr = Cr-Gehalt und %Mo = Mo-Gehalt des Stahls
abgeschätzt werden.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 section of cooling, starting from the intermediate temperature Tz, cooling is carried out at a cooling rate Td2 'of at least 5 K / s, in particular more than 5 K / s or at least 20 K / s, until the range between the martensite start temperature TMS and room temperature is reached so that the carbon diffusion does not homogenize all of the adjacent retained austenite with carbon. The cooling rate in this variant is limited to a maximum of 100 K / s in order to ensure that 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 Td2 'to a maximum of 70 K / s. It is particularly practical when the cooling rate Td2 'is 20 - 70 K / s.

According to the second variant of the second section of cooling, the cooling up to the martensite start temperature TMS is completed at a cooling rate Td2 ″ of 10-130 K / s. A cooling rate Td2 ″ of at least 10 K / s, in particular at least 30 K / s, is also limited 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 Td2 ″ 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 limited to a maximum of 80 K / s will. It is particularly practical when the cooling rate Td2 "is 30 - 80 K / s.

The third section of cooling, in which the hot-rolled flat steel product reaches the coiling temperature HT, 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 reel temperature HT is typically 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.

Fig. 1

zeigt einen Ausschnitt eines Gefüges eines erfindungsgemäßen Stahlflachprodukts in 20.000-facher Vergrößerung;

Fig. 2

zeigt einen vergrößerten Ausschnitt von Fig. 1;

Fig. 3

eine zeichnerische Darstellung des Ausschnitts gemäß Fig. 2.

The invention is explained below on the basis of exemplary embodiments.

Fig. 1

shows a section of a structure of a flat steel product according to the invention, magnified 20,000 times;

Fig. 2

shows an enlarged section of Fig. 1 ;

Fig. 3

a graphic representation of the section according to Fig. 2 .

To test the invention, four melts E1-E4 composed in accordance with the requirements of the invention and a comparative melt V1 not composed according to the invention, the compositions of which are given in Table 1, were melted.

_{In addition, Table 1 shows the martensite start temperatures Tmst, Ar 3} temperatures and Ar ₁ temperatures estimated in accordance with the above-explained formulas (1) - (3) for the melts E1-E4 and V1.

The melts E1-E4 and V1 have been cast in the conventional manner to form slabs, which were each heated through at 1150-1380 ° for a period of 60-240 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") W1-W11 and WV with a thickness Dw. When leaving the last pass of hot rolling, the hot strips W1-W11, WV had a hot-rolling end temperature Twe which was above the Ar ₃ temperature of the steel E1-E4 and V1 of which the hot strips W1-W11, WV each consisted.

After hot rolling, the hot strips W1-W11, WV obtained were cooled, starting from their respective final hot-rolling temperature Twe, at a cooling rate dT1 to an intermediate temperature Tz which was 50 ° C. below the Ar ₁ temperature of the steel E1-E4 and V1 from which the hot strips W1 - W11, WV each passed.

After the intermediate temperature Tz has been reached, the hot strips W1-W11, WV have been cooled at a cooling rate dT2 'down to the martensite start temperature Tmst of the steel.

Starting from the martensite start _{temperature T MS} , the hot strips W1-W11, WV have been cooled down to the respective coiling temperature HT at a cooling rate dT3 at which they have been coiled into a coil. Finally, cooling to room temperature took place in the coil.

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

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 strips W1 - W11, WV. The relevant properties as well as the product Rm x HER x Ag and the ratio Re / Rm are listed in Table 3 for the hot strips W1 - W11, WV.

The martensite, ferrite, bainite, pearlite and retained austenite components of the structure have been determined for the hot strips W1 - W11, WV. 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 (= martensite grains) in the structure of the hot strips W1 - W11, WV have been determined for some of the hot strips W1 - W11. The relevant values are listed in Table 4.

It turns out that the hot strips W1-W3 and W6-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 values for the product Rm x HER x Lead ag. In contrast, the hot strip WV produced from the comparative steel V1 not alloyed according to the invention in a manner not according to the invention does not reach the minimum limit specified according to the invention for the product Rm x HER x Ag. The same applies to the hot strips W4 (too high cooling rate dT1) and W5 (too low cooling rate dT1) that are alloyed according to the invention but not produced according to the invention.

In Fig. 1 the illustration of a martensite island M which is present in the structure of the hot strip W1 and is embedded in a ferritic structure F is shown. The one around the martensite island M is clearly visible Residual austenite border RAS, by which the martensite island M is separated from the surrounding ferrite F.

As with the in Fig. 2 reproduced in Fig. 3 schematically and graphically abstracted enlargement of section A of Fig. 1 can be easily understood, the central area MMB of the martensite island M is delimited by an edge area MRB, around which the retained austenite border RAS in turn runs. Table 1 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 V1 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 E1 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 E2 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 E3 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 E4 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 stole thickness Oven temperature Lay time Opening stitch Sliver thickness ET HT dT1 dT2 ' dT2 " ZT1 tH mm [min] [° C] [mm] [° C] [K / s] [K / s] [° C] [s] V1 3.2 1240 130 1030 41 820 520 30th 2 E1 4th 1260 140 1080 40 860 60 40 50 690 2 E1 2 1280 240 1140 41 850 50 35 30th E1 1.5 1260 130 1130 42 850 30th 30th 40 E1 2 1230 150 1050 46 910 50 135 50 E2 1.6 1180 160 1040 47 850 30th 2 30th E2 1.6 1290 120 1140 44 860 50 51 60 E2 2 1260 145 1130 43 855 90 62 40 E2 3 1230 165 1110 42 900 30th 30th 40 E3 1.5 1260 125 1130 41 860 40 80 70 675 3 E3 2 1250 135 1125 42 850 80 31 30th E4 3.4 1270 140 1120 38 860 40 42 30th Hot strip stole thickness re Rm Re / Rm Aq A050 A80 HER Rm * HER * Ag According to the invention? mm MPa % MPa *% *% YES WV V1 3.2 762 856 0.89 7.9 15.5 14.1 26th 175822 NO W1 E1 4th 501 702 0.71 12.6 20.7 18.8 34 300737 YES W2 E1 2 474 711 0.67 12.4 19.4 17.6 38 335023 YES W3 E1 1.5 562 752 0.75 11.8 18.5 16.8 31 275082 YES W4 E1 2 750 934 0.80 12.4 15.2 13.8 15th 173724 NO W5 E2 1.6 390 590 0.66 12th 20.5 18.6 41 290280 NO W6 E2 1.6 438 652 0.67 11.8 20.0 18.2 48 369293 YES W7 E2 2 401 612 0.66 12.6 20.6 18.7 56 431827 YES W8 E2 3 480 687 0.70 11.5 18.4 16.7 45 355523 YES W9 E3 1.5 750 1014 0.74 8.5 13.4 12.2 25th 215475 YES W10 E3 2 689 975 0.71 9.1 13.9 12.6 27 239558 YES W11 E4 3.4 508 804 0.63 10.3 20.8 20.8 30th 248436 YES Hot strip stole Martensite ferrite Bainite Perlite RA KG Bras [Area-%] [µm] WV V1 5 10 75 5 2 - - W1 E1 15th 80 5 - 2 2 0.3 W2 E1 20th 75 5 - 2 - - W3 E1 25th 75 0 - 2 2 0.3 W4 E1 35 62 5 - 3 2 nb W5 E2 5 80 - 15th 0 - - W6 E2 15th 80 5-10 <1 1.5 4th 0.4 W7 E2 10 85 5 - 1.5 - - W8 E2 15th 85 - - 1.5 - - W9 E3 35 60 5 - 1 3 0.9 W10 E3 35 65 - - <1 - - W11 E3 30th 70 5 - 1.0 - - "-": Not determined

Claims (15)

Process for the production of a hot-rolled flat steel product with a structure which consists of 5-40% by volume of tempered or freshly formed martensite and 50-95% by volume of ferrite and in which the portion not taken up by martensite and ferrite consists of up to is filled to 45% by volume of retained austenite, bainite and / or cementite, 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 is 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 to the coiling temperature HT, which lies _{in the range between the martensite start temperature T MS of the steel and room temperature,}
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 ZT1, 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, and is then cooled to the coiling temperature HT lying in the range between the martensite start _{temperature T MS of the steel and room temperature;} g) Coiling of the hot strip cooled to the coiling temperature HT into a coil; h) optionally cooling the hot-rolled flat steel product in the coil to room temperature or holding the hot-rolled flat steel product at the coiling temperature HT. 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 s during the cooling (step f) taking place according to the first variant, the cooling rate is 20-70 K / s. Method according to one of Claims 1 to 3, characterized in that during the cooling (step f) taking place according to the second variant, the cooling rate is 30-80 K / s and the holding time is 1-3 s. Method according to one of the preceding claims, characterized in that the coiling temperature HT is in a temperature range ranging from room temperature to 100 ° C. Hot rolled flat steel product with 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 5 - 40% by volume of tempered or freshly formed martensite and 50 - 95% by volume of ferrite and in which the portion not taken up by martensite and ferrite consists of up to 45% by volume of retained austenite, bainite and / or cementite, the martensite fractions of the structure being in the form of islands and in at least 70% of all martensite islands a central area covering the center of the respective martensite island, which extends from one to the edge the edge area of the respective martensite island bordering the respective martensite island is delimited, - the C content of the edge region being higher than the C content of the central region, at least in one section. Flat steel product according to claim 7, characterized in that 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 and this C concentration at the edge is so high that the retained austenite can be seen at 20,000 times magnification, occupies or occupies in total at least 30% of the circumference of the martensite island in question. Flat steel product according to one of Claims 7 or 8, 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 than in the middle area of the respective martensite island. Flat steel product according to Claim 7 or 8, characterized in that the martensite islands are delimited at least over part of their circumference by a border consisting of retained austenite. Flat steel product according to Claim 10, characterized in that the width of the retained austenite border is 10 nm to 1 µm. Flat steel product according to one of Claims 7-11, 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 200,000 MPa% 2. Flat steel product according to claims 7-12, characterized in that its tensile strength Rm is at least 530 MPa. Flat steel product according to one of Claims 7-13, characterized in that its hole expansion HER is at least 20%. Flat steel product according to one of Claims 7-14, characterized in that its uniform elongation Ag is at least 5%.

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