WO2020201352A1

WO2020201352A1 - Hot-rolled flat steel product and method for the production thereof

Info

Publication number: WO2020201352A1
Application number: PCT/EP2020/059253
Authority: WO
Inventors: Rainer FECHTE-HEINEN; Christine ESCHER; Elena Schaffnit; Christian Mertin; Swen Prochotta; Tim Stötzel; Achim Peuster
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2019-04-01
Filing date: 2020-04-01
Publication date: 2020-10-08
Also published as: EP3719147A1; CN113661260A; CN113661260B

Abstract

The invention relates to a hot-rolled flat steel product with a particularly uniform distribution of its mechanical properties over its length and breadth, having, moreover, good deformability and low tendency to elastic recovery. To this end the flat steel product consists of (in % w/w) C: 0.02 - 0.1 %, Mn: 0.1 - 2.5 %, AI: 0.02 - 0.1 %, Nb: 0.02 - 0.12 %, and optionally of one or more elements of the group, Si, Ti, V, Cr, B, Ca, Mo, with the condition that the Si content is < 0.6 %, the Ti content is < 0.12 %, the V content is < 0.2 %, the Cr content is < 0.2 %, the B content is < 0.0025 %, the Ca content is < 0.01 % and the Mo content is < 0.3 % and the remainder consists of iron and unavoidable impurities, with the impurities including < 0.05 % P, < 0.03 % S, < 0.01 % N, < 0.2 % Ni, < 0.15 % Cu. The microstructure of the flat steel product here is > 60 area percentage ferrite and/or bainite and the remainder is perlite and carbide nitride or carbon nitride precipitations and < 2 area percentage of other microstructure components, and the microstructure has a grain aspect ratio of 0.2 - 0.7. The flat steel product likewise has a yield strength Re, for which Re > REJ3ER applies, where Re_BER = (400 + 2243 * %Nb) i (d0'15) and with %Nb being Nb content in % w/w and d being the particular thickness of the flat steel product in mm. The invention also relates to a method for producing a flat steel product of this kind.

Description

HOT-ROLLED STEEL FLAT PRODUCT AND

PROCEDURE FOR ITS MANUFACTURING

The invention relates to a hot-rolled flat steel product with a particularly high uniformity of the mechanical-technological properties over its length and width, excellent deformation and toughness properties and low springback.

The invention also relates to a method for producing such a hot-rolled flat steel product.

"Flat steel products" are understood here as rolled products, the length and width of which are each significantly greater than their thickness. These include, in particular, steel strips, steel sheets and blanks obtained from them, such as blanks and the like.

Unless explicitly stated otherwise, information on alloy components is always given in% by weight in the present text.

The proportions of certain components in the structure of a flat steel product, on the other hand, are given in area%, unless otherwise noted.

Flat steel products made from micro-alloyed steels have been manufactured for more than 40 years. They are used for applications where a combination of high strength and good formability is required. Due to their low content of alloying elements, such steels also have excellent weldability and can be compared can be produced inexpensively. The production of flat steel products from micro-alloyed steels is usually based on a melt analysis based on at least one of the micro-alloying elements niobium, titanium or vanadium. By processing the preliminary product cast from a steel melt alloyed in this way into a hot-rolled strip in a controlled thermomechanical rolling and cooling process, the resulting

hot-rolled flat steel product has a very fine-grain structure. This typically consists mainly of ferrite and / or bainite and small amounts of perlite or cementite (Fe ₃ C) as well as fine and finest

Precipitations that ensure the fine grain of the structure. The nice one

The grain structure and the high density of fine and extremely fine precipitates, together with the solid solution strengthening caused by the presence of manganese and silicon, lead to the high strength of such flat steel products.

In terms of materials, the fineness of the structure leads to an enormous

Obstruction of the migration of dislocations during the deformation, since substituted foreign atoms (e.g. manganese) bypassed and very many

Grain boundaries and large amounts of fine and extremely fine precipitates have to be overcome. Although the elongation is known to be reduced with increasing strength, it nevertheless remains at a high level in flat steel products made of micro-alloyed steels.

From EP 2 924 140 A1 a method for producing a

Flat steel product with a yield strength of at least 700 MPa and with a structure of at least 70% by volume bainitic structure, in which a steel melt is melted in the first step which (in% by weight) consists of C: 0.05-0.08 %, Si: 0.015 - 0.500%, Mn: 1.60 - 2.00%, P: up to 0.025%, S: up to 0.010%, AI: 0.020 - 0.050%, N: up to 0.006%, Cr: up to 0.40%, Nb: 0.060-0.070%, B: 0.0005-0.0025%, Ti: 0.090-0.130%, and the remainder consists of iron and technically unavoidable impurities, of which up to 0 , 12% Cu, up to 0.100% Ni, up to 0.010% V, up to 0.004% Mo and up to 0.004% Sb. The melt composed in this way becomes a slab which is then poured onto a

Reheating temperature of 1200 - 1300 ° C is heated. The slab is then pre-rolled at a roughing temperature of 950-1250 ° C with a total pass reduction of at least 50% achieved through roughing and then finished hot-rolled, the hot-rolling being terminated at a final hot-rolling temperature of 800-880 ° C. Within a maximum of 10 s after the finish hot rolling, the hot strip obtained is then intensively cooled at a cooling rate of at least 40 K / s to a coiling temperature of 550 - 620 ° C, at which it is finally wound into a coil. The alloying elements of the steel are matched to one another within narrow limits so that one is operationally safe

procedure to be carried out maximized mechanical properties and optimized surface properties can be achieved.

It is known that niobium, titanium and vanadium make different contributions to increasing strength. Niobium is the strongest

strength-increasing effect, followed by titanium and vanadium.

Alloy concepts based on titanium without niobium and vanadium are usually cheap because titanium is inexpensive. However, pure titanium micro-alloyed concepts only lead to high strengths with very high titanium contents. Due to its high affinity for nitrogen, titanium forms titanium nitrides, which also occur in

Temperatures> 1200 ° C remain stable, and during the forming of such alloyed flat steel products into a component due to their mostly

sharp-edged structure can have a detrimental effect.

Alloy concepts based on vanadium usually do not achieve the desired strengths, even with high vanadium contents, since the precipitation hardening by vanadium is significantly weaker than that of niobium and titanium. Furthermore, vanadium is very expensive, which is why only low contents are usually used. A combination of micro-alloy elements (e.g. titanium and vanadium) in order to achieve high strengths is possible and common practice. However, these concepts often tend to fluctuate in the mechanical properties over the length and width of the flat steel product made from the corresponding alloyed steel. The reason for this is that during the cooling in the coil wound from the hot-rolled steel strip obtained after hot rolling, the temperature is not constant over the entire length and width of the strip for technical reasons, so that the formation of precipitations of the micro-alloying elements over the length of the strip is dependent on the respective temperature and width varies to a greater or lesser extent with the result that the mechanical properties of the steel strip that are directly influenced by these precipitations show correspondingly strong fluctuations.

Against this background, the task was set to develop a hot-rolled flat steel product, which is characterized by a particularly uniform distribution of its mechanical properties over its length and width and at the same time good formability and low

Has a tendency to spring back, which makes it suitable for the production of complex-shaped components with a particularly wide range of requirements.

A flat steel product that achieves this object according to the invention has at least the features specified in claim 1.

In addition, a process should be specified that enables the targeted production of such a hot-rolled flat steel product.

A method that achieves this object comprises at least the method steps specified in claim 11. Advantageous embodiments of the invention are specified in the dependent claims and, like the general inventive concept, are explained in detail below.

A flat steel product according to the invention accordingly consists of (in% by weight) 0.02-0.1% C, 0.1-2.5% Mn, 0.02-0.1% Al, 0.02-0.12 % Nb as well as optionally from one element or more elements from the group "Si, Ti, V,

Cr, B, Ca, Mo "with the proviso that the Si content is not more than 0.6%, the Ti content is not more than 0.12%, the V content is not more than 0.2%, the Cr content is not more than 0, 2%, the B content is not more than 0.0025%, the Ca content is not more than 0.01% and the Mo content is not more than 0.3%, and the remainder is iron and unavoidable impurities, with the impurities up to 0 Count .05% P, up to 0.03% S, up to 0.01% N, up to 0.2% Ni and up to 0.15% Cu.

The structure of a flat steel product according to the invention consists of at least 60% by area of ferrite and / or bainite and the remainder of pearlite, of carbide or carbonitride precipitates and of up to 2% by area of other structural components. At the same time, the structure has a grain aspect ratio of 0.2-0.7.

It has proven to be particularly favorable if the structure of a

Flat steel product according to the invention also has a medium one

Ferrite grain size of at most 15 mm. Which can be reliably achieved in the structure in particular by the method of manufacture according to the invention

Komstreckungsbedingungen contribute significantly to the good

Toughness properties of a flat steel product according to the invention. The fine structure, characterized by small ferrite grain sizes, further improves the toughness properties.

Due to its special composition and structure, which is a result of the way it is manufactured, a flat steel product according to the invention has excellent forming properties, which are paired with optimized mechanical properties. The following applies to the yield point Re of flat steel products according to the invention Re> RE_BER, where:

Re_BER = (400 + 2243 *% Nb) / (d ⁰ ^'15 )

with% Nb: respective Nb content of the flat steel product in% by weight and

d: respective thickness of the flat steel product in mm

The yield strengths Re are typically more according to the invention

Flat steel products more than 350 MPa, especially more than 400 MPa, 450 MPa or more than 500 MPa. Furthermore, the

Yield strength Re below 800 MPa, in particular below 750 MPa or below 700 MPa.

At the same time, according to DIN EN ISO 148-1, notched bar impact tests carried out "longitudinally" in the test direction result in the case of the invention

Flat steel products each have an impact energy of> 100 J at -20 ° C and -30 ° C, whereby values for the impact energy of more than 125 J, in particular more than 150 J, have regularly been achieved. With the same

Test set-up, but at test temperatures of -60 ° C, the impact energy determined in the "longitudinal" test direction at -60 ° C was more than 27 J, with values of more than 80 J being regularly achieved for the impact energy. Even at a test temperature of -80 ° C., flat steel products according to the invention still achieved a notched impact energy of more than 27 J. Their correspondingly optimized notched impact strength and their higher notch impact strength in the “longitudinal” test direction

Brittle fracture resistance make flat steel products according to the invention particularly suitable for applications in which deeper

Temperatures, ie in particular temperatures of less than -40 ° C, prevail. The springback determined in accordance with DIN EN 10149-2 with a mandrel diameter of 8 mm is for hot-rolled according to the invention

Flat steel products less than 20%, with springback values of less than 17%, in particular less than 15%, regularly occurring in practice.

Their properties develop according to the invention hot-rolled

Flat steel products due to the presence of niobium as the sole mandatory micro-alloying element, without the need to add further micro-alloying elements, namely titanium and vanadium. These elements can, as explained below, in an inventive

Flat steel product optionally in effective contents to support the

Effect of the niobium content that is always present according to the invention. However, the alloy concept is based on one according to the invention

Flat steel product based on the presence of niobium, its contents

are matched according to the invention in such a way that basically no additional micro-elements are required to meet all the requirements placed on formability, strength, toughness, notch impact behavior and resilience of flat steel products according to the invention. So it is shown in the deformation that with the invention

Flat steel products due to their special microstructure (i.e. their

special grain stretching and fiber orientation) the risk of a

Crack formation is so delayed that even complex components such as B.

Strut mounts for chassis of motor vehicles can be easily manufactured.

The method according to the invention for the production of a flat steel product according to the invention comprises the following work steps: a) Production of a steel melt which consists of (in% by weight) 0.02-0.1% C, 0.1-2.5% Mn, 0, 02 - 0.1% Al, 0.02 - 0.12% Nb and each optionally from one or more elements of the group "Si, Ti, V, Cr, B, Ca, Mo" with the proviso that the Si content is at most 0.6%, the Ti content is at most 0.12%, the V content is at most 0.2%, the Cr content is at most 0.2%, the B content is at most 0 , 0025%, the Ca content is at most 0.01% and the Mo content is at most 0.3%, and the remainder consists of Elsen and unavoidable impurities, with up to 0.05% P, up to 0 Count .03% S, up to 0.01% N, up to 0.2% Ni, and up to 0.15% Cu; b) Pouring the melt into a preliminary product, which is a

Slab with a thickness of 70 mm - 350 mm or a thin slab with a thickness of 30 - 70 mm; c) Austenitizing the preliminary product in such a way that the preliminary product on a

Austenitizing temperature is heated through from 1150 - 1320 ° C; d) in the case that the intermediate product is a slab:

Pre-rolling the austenitized intermediate product in two or more passes to a thickness of at least 30 mm and a maximum of 70 mm at a pre-rolling temperature that is at most equal to the austenitizing temperature, but at least 30 ° C above the recrystallization stop temperature T _NR , which is calculated as follows:

T _NR [° C] = 913 ° C + 910 ° C / wt .-% *% Nb (1) with% Nb = respective Nb content of the flat steel product; e) Finish hot rolling the intermediate product to a hot rolled one

Flat steel product in several rolling passes,

- Where a number n _W of rolling passes which is greater than or equal to a value nw 'rounded off to an integer, according to the formula

n _W '³ 7 * root (d _EW / (6 * Z)) + 2 (2) with d _EW : final rolling thickness of the flat steel product Z: Thickness of the preliminary product is calculated, if it is below that according to formula (1)

calculated recrystallization stop temperature T _NR

Temperature,

- Wherein overall achieved for one over the finish hot rolling

Deformation degree Dh _tot applies

Ie _tot = (d _inlet - d _outlet ) / d _inlet * 00%> 65% (3) with d _inlet : Thickness of the flat steel product when entering the

Finished hot rolls,

d _outlet : thickness of the flat steel product at the end of the

Finish hot rolling,

- where for the final pass of the finish hot rolling achieved

Deformation degree Dh _LG is true;

Ie _{LG st} > Ie _{LG min} (4) with

d _{Inlet LG} : Thickness of the _flat steel _{product as it} enters the

last roll stand,

d _{Outlet LG} : Thickness of the flat steel product at the outlet from the

last roll stand

Ie _{LG min} = 1.8 * 10 ^-5 * e ^{(0.0135 * (TEW + 11))} _{+ 3}

Ie _{LG min} : minimum degree of deformation in the last pass of the finish hot rolling

TEW: hot rolling end temperature in ° C,

and

- The hot rolling end temperature is 760-940 ° C; f) cooling the hot-rolled flat steel product to a coiling temperature of 520-650 ° C; g) Coiling the hot-rolled flat steel product into a coil and cooling the hot-rolled flat steel product in the coil to room temperature.

The alloy components of a flat steel product according to the invention and the contents of these components are selected according to the invention as follows:

In addition to iron, C, Mn, Al and Nb are mandatory elements of the alloy of a flat steel product according to the invention. All other elements explained below are optionally present in the flat steel product according to the invention in order to develop certain properties or the unavoidable ones

Assigning impurities, the presence of which is undesirable, but cannot be avoided for manufacturing reasons. The contents of these unavoidable accompanying elements are limited in a flat steel product according to the invention, so that they have no negative influence on the properties of the flat steel product.

Carbon “C” is present in a flat steel product according to the invention in contents of 0.02-0.1% by weight. At least 0.02% by weight is required so that a flat steel product according to the invention has the required properties

Strength properties achieved. This effect can be achieved particularly reliably with C contents of at least 0.04% by weight. At the same time, the C content is limited to a maximum of 0.1% by weight in order to avoid a negative influence on weldability and formability. Negative effects of the presence of C, such as a reduction in toughness, can be avoided particularly reliably by setting a C content of at most 0.08% by weight, in particular at most 0.7% by weight.

Silicon "Si" is optionally used in the production of the steel

Flat steel product according to the invention used as a deoxidizer and contributes to the improvement of the flat steel product according to the invention

Strength properties at. In order to be able to use this effect of Si reliably, the Si content can be at least 0.01% by weight. Si contents of more than 0.6 wt% would improve the surface finish and the

Impact toughness properties of the material according to the invention, in particular the toughness in the heat-affected zone of one at a time

Flat steel product according to the invention produced weld seam. In addition, excessively high Si contents could reduce the weldability of the invention

Steel flat products deteriorate. In order to safely avoid these negative influences and in particular to ensure an optimized surface quality

ensure, the Si content can be limited to 0.25 wt .-%.

Manganese “Mn” is present in a flat steel product according to the invention in contents of 0.1-2.5% by weight in order to ensure good mechanical properties, in particular high toughness, and S-bonding. In the case of contents of less than 0.1% by weight, these would not be sufficient

Strengths achieved. With Mn contents of more than 2.5% by weight, however, the weldability and the formability would be adversely affected. To the strength-increasing effect of Mn in the invention

To be able to safely use flat steel product, the Mn content can be increased to at least 0.5% by weight. In order not to adversely affect the segregation behavior and toughness, the Mn content can be limited to a maximum of 2.0% by weight.

Aluminum “AI” is present in the flat steel product according to the invention in contents of 0.02-0.1% by weight. It is used as a deoxidizing agent in the production of the steel of a flat steel product according to the invention and, through the formation of AlN precipitates, prevents the austenite grain from becoming coarser in the course of processing the steel into the

flat steel product according to the invention passed through heating

("Austenitize"). If the aluminum content is below 0.02% by weight, the deoxidation processes in steel production do not take place completely. Exceeds the However, if the Al content exceeds the upper limit of 0.1%, undesired Al ₂ O ₃ inclusions can form. These would have a negative effect on the degree of purity and the toughness properties of a flat steel product according to the invention. If, in the production of flat steel products according to the invention, restrictions on the castability of the steel melt are to be avoided particularly reliably, then the Al content of a steel melt according to the invention can be used

Flat steel product can be limited to a maximum of 0.05% by weight.

Niobium “Nb” is present in the flat steel product according to the invention in contents of 0.02-0.12% by weight in order to achieve optimized strength properties

To achieve precipitation hardening when reeling carried out according to the invention. If the Nb content is less than 0.02% by weight, the required strength properties would not be achieved. In order to reliably set the grain refinement of the austenite structure in temperature-controlled rolling according to the invention, Nb can be added in contents of at least 0.04% by weight. If Nb contents of more than 0.12% by weight were provided, this would reduce the weldability and toughness in the

Heat affected zone of a welded joint produced on a flat steel product according to the invention deteriorates. The effects achieved by the Nb contents provided according to the invention can be achieved particularly economically with Nb contents of a maximum of 0.08% by weight.

Even if the alloy concept according to the invention aims to use only Nb as a micro-alloy element, titanium “Ti” can be used in the

Flat steel product according to the invention may be present in contents of up to 0.12% by weight in order to support the strength properties by preventing grain growth during austenitizing and by precipitation hardening during coiling. These advantageous effects of the presence of Ti can be used particularly reliably by reducing the Ti content of a flat steel product according to the invention to at least 0.005% by weight.

is set. If the Ti content is above 0.12% by weight, there is a risk that the formability, weldability and toughness of the Flat steel product deteriorated due to the formation of coarse Ti precipitates. This risk can be minimized by limiting the Ti content to a maximum of 0.10% by weight. To ensure the most uniform strength properties possible over length and width, a Ti alloy is in particular completely dispensed with or the content is preferably limited to at most 0.010% by weight, particularly preferably to at most 0.006% by weight.

Vanadium “V” can be added to a flat steel product according to the invention in contents of up to 0.2% by weight in order to support the strength through the formation of carbonitrides. If this effect is to be used specifically in a flat steel product according to the invention, a content of.

at least 0.005% by weight V can be provided. At contents of more than 0.2% by weight, there is no further increase in the positive effects of the optional presence of V in a flat steel product according to the invention.

The strength-increasing effect of V can be optimally used if up to 0.15% by weight of V is present in a flat steel product according to the invention. In principle, however, the addition of V is to be considered purely optional, i.e. the addition of V can be omitted completely because the alloy concept according to the invention is primarily based on the presence of the micro-element Nb. In order to ensure the most uniform strength properties possible over length and width, the V content is combined in a particularly practical variant of the invention

Flat steel product according to the invention reduced to technically ineffective levels. For this purpose, the V content can be limited to a maximum of 0.010% by weight, in particular a maximum of 0.006% by weight.

Chromium “Cr” is optionally present in the flat steel product according to the invention in contents of up to 0.2% by weight. The strength properties of a flat steel product according to the invention can also be improved by adding Cr. If the chromium content is too high, however, the

Weldability and toughness in the heat affected zone one at one Flat steel product according to the invention made welding negatively influenced. The positive influences of the optional presence of Cr in a flat steel product according to the invention can be used reliably if its Cr content is at least 0.02% by weight.

Boron “B” is optionally present in the flat steel product according to the invention in contents of up to 0.0025% by weight. B has a beneficial effect on the

Strength properties and the hardenability of the steel from which a steel flat product according to the invention is made. This beneficial effect of B can be used for an inventive

Flat steel product B contents of at least 0.0005% by weight B, in particular at least 0.0015% by weight B, can be provided. However, B contents of more than 0.0025% by weight would deteriorate the toughness properties.

Calcium “Ca” can optionally be present in the steel of a flat steel product according to the invention in order to form non-metallic inclusions in the structure of the flat steel product so that the toughness is improved. If the Ca content is above 0.01% by weight, however, this can have a negative effect on the degree of purity of the melt and, when casting the steel from which a steel flat product according to the invention is produced, it can lead to defects in the shell of the respective cast intermediate product to lead. The positive influences of the optional presence of Ca in a flat steel product according to the invention can be used reliably if its Ca content is at least 0.0005% by weight.

Molybdenum “Mo” can optionally be present in the steel of a flat steel product according to the invention in order to achieve the higher strength properties. The positive effects of the optional presence of Mo can be achieved from a Mo content of at least 0.02%. If the Mo content is too high, the elongation at break and the formability of the material are negatively affected. For this purpose, the Mo content must be at most 0.3% by weight, in particular to at most 0.25% by weight or, particularly advantageously, to at most 0.1% by weight.

A flat steel product according to the invention has an optimized combination of strength and toughness without the need for expensive alloying elements such as nickel “Ni” and copper “Cu”. However, these can also

Elements due to production, for example through the use of scrap in steel production, get into the steel as unavoidable impurities. In any case, the contents of Ni and Cu in a flat steel product according to the invention are kept so low that they have no influence on the properties of a flat steel product according to the invention. For the same reasons, the Ni content of a flat steel product according to the invention is at most 0.2% by weight, its Cu content at most 0.15% by weight.

Phosphorus “P” and sulfur “S” are also impurities that are undesirable in the flat steel product according to the invention because they affect its mechanical properties, in particular the impact energy and the

Formability, deteriorate. In order to avoid any influence of these accompanying elements, which are unavoidable due to production, the invention sets an upper limit for the P content of 0.05% by weight, in particular of at most 0.025% by weight or, particularly preferably, of at most 0.015% by weight , and for the S content an upper limit of at most 0.03% by weight fixed,

in particular of at most 0.01% by weight or, particularly preferably, of at most 0.003% by weight.

Nitrogen “N” is also an unavoidable one for manufacturing reasons

Impurity which, if the content is too high, worsens the toughness properties of a flat steel product according to the invention. The N content of a flat steel product according to the invention is therefore limited to at most 0.01% by weight, in particular 0.008% by weight or, particularly preferably, at most 0.006% by weight. Typical N contents of an inventive

Flat steel product are at least 0.004% by weight. Through this im flat steel product according to the invention typically at least present contents of N, the formation of AlN and, if Ti is present, of TiN precipitates, which, as already explained above, have a positive effect on the

Effect fineness of the structure of a flat steel product according to the invention.

The structure of a flat steel product according to the invention consists predominantly of ferrite and / or bainite. Thus, the ferrite and / or bainite proportion of the structure of a flat steel product according to the invention is typically at least 60 area% ferrite and / or bainite, the ferrite being the

polygonal ferrite and / or quasi-polygonal ferrite and / or strong

dislocation hardened ferrite may be present. To ensure the best possible formability, a proportion of at least 80% by area is preferred, and at least at least to increase the toughness

90 area%, particularly preferably at least 95 area% ferrite and / or bainite are set. The remainder of the structure of an inventive

Flat steel product is replaced by perlite as well as those already mentioned

Precipitations in the form of carbides or carbonitrides as well as a maximum of 2 surface% of other structural components, including martensite or retained austenite, are taken.

The structure of a flat steel product according to the invention is further characterized in that the grain aspect ratio is between 0.2 and 0.7. The setting of a grain stretching ratio in this range ensures good toughness properties. If the grain aspect ratio is above 0.7, the elongation is negatively influenced. If the ratio is below 0.2, the influence on the strength is too small.

In addition, the structure of the inventive

Flat steel product in that the proportions of the a-

Fiber <110> is parallel to the rolling direction in a proportion of at most 30% and the proportion of the g-fiber <111> present in the ferrite is parallel to the sheet normal in a proportion of at most 20%. This characteristic ensures good flow behavior during forming as well as good

Elongation at break properties, making an overall excellent

Forming behavior of flat steel products according to the invention is achieved.

The working steps of the method according to the invention and the method parameters set according to the invention are coordinated in a special way with the alloy concept according to the invention in such a way that the special combination of properties of flat steel products according to the invention is achieved in a reliable manner.

Due to the alloy concept based on the use of niobium as the preferably sole micro-alloying element according to the invention, high strengths can be achieved even with comparably low Nb contents. Due to the fact that niobium greatly delays the recrystallization, the production according to the invention results in inventive

Flat steel products in the course of hot rolling to a strong

Austenite grain elongation, which results in a very fine-grained, dislocated ferrite or bainite structure after reeling. Compared to titanium and vanadium, niobium delays recrystallization significantly more when the element contents are the same and also makes a particularly effective contribution to the grain refinement of the structure after reeling.

This results in the formation of fine Nb precipitates in the coil to which the flat steel product is wound after hot rolling in step g), the formation of which is stable and insensitive to a non-uniform temperature distribution over the entire length and width of the

Flat steel product expires. As a result, a flat steel product according to the invention is characterized by slight fluctuations in the mechanical

Properties across its length and width.

It goes without saying that when carrying out the method according to the invention, the person skilled in the art will not only use those mentioned in the claims and here explained procedural steps, but also carries out all other steps and activities involved in the practical implementation of such

Prior art methods are carried out regularly if the need arises.

For the production of flat steel products according to the invention

accordingly, in step a) in a manner known per se with regard to the procedure to be used, a steel melt composed according to the invention according to the above explanations is generated.

This molten steel is then cast under conditions which are also known per se to form a preliminary product, which is a

Slab or a thin slab. The

The casting temperature of the melt during continuous casting should be more than 1500 ° C to ensure that the steel does not solidify in the transport ladle. This applies in particular in the event that a thin slab is produced as a preliminary product.

The casting of the melt to form a slab can take place in any manner known for this purpose from the prior art.

In practice, for example, the "CSP process" (CSP = Compact-Strip-Production, see https://www.sms- group.com/de/anlagen/alle-anlagen/csp-technologie) is available for the production of thin slabs /) to disposal. In this process, the melt is poured into a strand in a continuous process, from which the thin slabs are then separated. For this purpose, the liquid steel is fed via a distribution channel into a mold, from which in practice two strands emerge parallel to one another, each of which begins to solidify through the formation of a so-called outer "strand shell". After exiting the mold, the solidification proceeds from the strand shell of the strand center (core) on and on until the

Core area is solidified. The strand thus completely solidified has a thickness of at least 30 mm, typically from 35-70 mm, and a temperature which is typically more than 600 ° C. After solidification, the thin slabs are separated from the strand.

The respective intermediate products (slab or thin slab) are heated for the so-called "austenitizing" (step c)) in a preheating or equalizing furnace to an austenitizing temperature above 1150 ° G, at which they have a completely austenitic structure. The high austenitizing temperature is important so that the coarse precipitates that have formed during the solidification of the respective preliminary product are dissolved in the course of austenitizing. The upper limit of the

According to the invention, the range provided for the austenitizing temperature is 1320.degree. C. in order to coarsen the austenite grain and increase it

To prevent scale formation.

In the event that the pre-product is a thin slab, this can be fed directly to the finish hot rolling (step e)). This, like the austenitizing in the equalizing furnace and the previous production of the thin slab, can be carried out in a manner known per se in a continuously running work sequence.

In the event that the intermediate product is a slab, on the other hand, its thickness must be reduced before the finish hot rolling. For this purpose, after austenitizing, the respective slab is reversely rolled in more than one pass to form a pre-strip with a thickness of at least 30 mm, but not more than 70 mm, in one or more reversing roll stands available in the prior art. The temperature of the slab at the start of roughing is at most equal to that

Austenitizing temperature and at least 1100 ° C. The temperature of the pre-rolled strip from the slab after completion of the pre-rolling is referred to as the pre-rolling temperature TVW. The roughing temperature is at most equal to the austenitizing temperature. The roughing temperature is preferred below 1150 ° C, in particular below 1120 ° C. At a roughing temperature above this limit value, on the one hand, a coarser austenite grain would form as a result of the grain growth after recrystallization. On the other hand, there would be an increased amount of discontinuities, ie

Defects, set on the surface of the finished hot rolled product. However, the roughing temperature must not be lower than a temperature TVW min which is at least 30 ° C. above the recrystallization stop temperature T _NR calculated in the manner indicated above as a function of the Nb content% Nb. This ensures that the recrystallization processes take place completely, thus maintaining a fine austenite grain and limiting subsequent grain growth. This has a positive effect on the toughness and on the

Elongation at break of flat steel products according to the invention. The elongation at break is typically at least 14%. If the roughing temperature were to fall below the TVW min, this would lead to an undesirable mixed structure as a result of the recrystallization that would then not take place completely, which would worsen the toughness and elongation at break properties of the flat steel product and, moreover, could result in a greater scatter in the mechanical properties.

In the case of the finish hot rolling according to the invention (step e)), the pre-rolled strip or the thin slab produced as a pre-product is hot-rolled into a fully hot-rolled flat steel product in a multi-stand, typically five, six or seven roll stands comprising, if the respective pre-product was a slab, which is typically a hot rolled strip. The parameters of the hot rolling are set in such a way that the flat steel product obtained has the respectively required thickness and the completion of the method according to the invention has the structural features already explained. With a “multi-stand” rolling process according to the invention meant at least three successive stitch decreases.

The number of pass reductions actually required in each case is selected depending on the thickness required for the finished hot-rolled flat steel product. There must be a certain number n _W of rolling passes in the

Finishing hot rolling mill can be carried out at a temperature below the calculated in the manner explained above

Recrystallization stop temperature T _NR (equation (1)). This number n _W is determined as the result n _W 'of the above-mentioned equation (2) rounded off to an integer.

According to the invention, the finish hot rolling is carried out in the manner of what is known as “thermo-mechanical rolling”. Also due to the degree of deformation, which is set in a targeted manner according to the invention and is maintained over the entire finish hot rolling process and the individual hot rolling passes

"Stitch decreases" called, and a precise temperature control is the inventive orientation of the fibers of the structure and its

Grain stretching achieved. This has a positive effect not only on the

Strength properties, but also toughness, elongation at break and the resulting deformation properties, in particular springback, compared to typical micro-alloy concepts based on two

Micro-alloy elements.

According to the invention over the entire finish hot rolling process

That is achieved cumulative reduction _ges, and which _is about the last rolling pass to be achieved deformation That _LG are tuned to a strong elongated austenite in the microstructure according to the invention that a

Steel flat product. The finish rolling is ended below a final rolling temperature of 940 ° C., in particular less than 920 ° C. or, particularly preferably, less than 890 ° C. Through the

according to the invention low hot rolling end temperature, the effect of the thermo- Reinforced mechanical rolling, so that in the structure of the flat steel product according to the invention at the end of hot rolling there is dislocation-rich austenite. Since the hot rolling end temperature is at most 940 ° C. according to the invention, it is ensured that no recrystallization processes take place even locally after the end of hot rolling. In this way it is ensured that the requirements according to the invention

Grain stretching is achieved and a favorable alignment for the deformation of the portions of the a-fiber <1 10> present in the ferrite parallel to the rolling direction and g-fiber <1 11> parallel to the sheet normal is obtained. At the same time, however, the finish hot rolling is ended at a temperature above 760 ° C. in order to ensure that there is no phase conversion during finish hot rolling, in particular in the area of the surface.

According to the invention, the total degree of deformation Dh _tot achieved in the finish hot rolling _{mill should be} at least 65%. In the last stand _, however, the degree of deformation Dh _LG should at least meet the condition according to equation (4). The degree of deformation Dh _{LG should} preferably be at least 4%. Are the provisions that the invention with respect to the

Deformation degrees Dh _tot and Dh _{LG is} established, is not observed, the grain stretching according to the invention and the inventive orientation of the proportions of a-fiber <1 10> present in the ferrite parallel to the rolling direction and g-fiber <1 11> parallel to the sheet normal are not reached.

After hot rolling, the hot rolled flat steel product obtained is cooled to a coiling temperature of 520-650 ° C. in step f). The cooling can take place in a manner known per se. Typical cooling rates suitable for the purposes according to the invention are in the range from 10-300 K / s. The cooling should start within a maximum of 20 s after the end of hot rolling. By adhering to the range prescribed according to the invention

Coil temperatures will be an optimal conversion and

Precipitation hardening achieved. The range of coiling temperatures is chosen so that the number of carbonitride precipitations reaches a maximum. Too low a reel temperature would lead to the

Elimination potential frozen and therefore that of one

flat steel product according to the invention would not reach the required minimum yield strength. Too high a coiling temperature, on the other hand, would lead to undesired precipitation and / or grain growth, which in turn could lead to a loss of toughness and yield strength.

Due to their combination of properties, hot-rolled flat steel products according to the invention are suitable for forming into complex components of all kinds, in particular for components used in the field of passenger or truck construction, which are typically chassis or chassis parts, such as strut mounts, axle supports, cross members or

Side members or around automobile seat parts, e.g. Seat rails, acts.

It is also suitable for painted components, which are made up, for example, using laser cutting.

The components of the structure mentioned in the present text can be examined using a light microscope, scanning electron microscope and electron back

Determine scattered diffraction "EBSD".

For the determination of the structural components, samples were taken from a quarter of the width of the flat steel product at a third of the sheet thickness, prepared as a longitudinal section and treated with alcoholic nitric acid, which contains a nitric acid content of 3% by volume, (in technical language also as "nital" known) or etched sodium disulfite. The respective shares of the

Structural components were then by means of light or

Scanning electron microscopy determined in a known manner by means of area analysis. The grain aspect ratio and the proportions of the a-fiber <110> present in the ferrite parallel to the rolling direction and the g-fiber <11 1> parallel to the

Sheet metal standards can be determined using EBSD on a longitudinal section. For this purpose, a measuring field of 800 x 800 mm is positioned in a 1/3 position across the sheet thickness and scanned with a step width of 0.9 μm.

Twin grain boundaries are not considered as grain boundaries.

A misorientation of up to 5 between adjacent measuring points is permitted within the grains. A number of at least ten contiguous measuring points has been selected as the minimum grain size. In order to determine the grain stretching ratio, the grains are approximated as ellipses so that the grain stretching can then be specified as the length ratio of the semi-axes of the ellipse. A value of 1 therefore corresponds to a circular grain and the smaller the value, the more elongated the grain. In order to quantify the proportions of texture components from the a-fiber and the g-fiber, an orientation tolerance of 10 ° is assumed - i.e. Orientations are counted for the corresponding fiber if they deviate by up to 10 ° from the ideal fiber orientation. The

Measurements can be carried out, for example, on a LEO 1530 field emission scanning electron microscope from Carl Zeiss Microscopy GmbH with an EBSD system from EDAX Inc. with the Digiview camera. The data evaluation and creation of the

Grain size distributions as well as the quantification of the proportions of a-fiber <1 10> parallel to the rolling direction and g-fiber <1 1 1> parallel to the sheet metal normals can be carried out, for example, with the software OIM Analysis V 8 from EDAX Inc.

The tensile tests to determine the yield point (Re) were carried out in accordance with DIN EN ISO 6892-1 on longitudinal samples of the hot strip.

The notched impact tests to determine the notched impact work Av at -20 ° C, -30 ° C, -60 ° C and -80 ° C were carried out on longitudinal specimens according to DIN EN ISO 148-1 carried out. The results described here are always full samples.

The springback was determined in a 180 ° bending fold test according to DIN EN 10149-2, with a bending mandrel diameter of 8 mm.

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

For the experiments 1 - 32 explained below, alloyed steel melts A - M were produced in accordance with the invention

Compositions are given in Table 1. Table 1 also shows the recrystallization stop temperature T _{NR of} the respective steel determined according to the formula T _NR [° C] = 913 ° C + 910 ° C / wt% *% Nb as a function of its Nb content% Nb.

Some of these melts A - M have been cast into slabs (variant "A"). The other part of the melts A - M was processed in a CSP plant in a continuous workflow, initially into thin slabs and then directly into a hot-rolled flat steel product in the form of hot strip (variant "B").

The respective cast slabs have been austenitized at an austenitizing temperature TA. With variant A (= processing of slabs as a preliminary product), the slabs are then pre-rolled in several passes. At the beginning of the roughing process, the slabs each had a temperature that was approx. 30 ° C below the austenitizing temperature TA. After completing rough rolling at a specified in the table

Roughing temperature TVW, roughing strips 30-70 mm thick were then finish-hot rolled. With variant B (= processing of

Thin slabs as preliminary product) are the 30 - 70 mm thick thin slabs on the other hand directly, ie without intermediate pre-rolling, into the

Hot rolling mill has been directed.

In the hot rolling mill, the respective pre-strip or the respective slab has been rolled out in 5 to 7 hot rolling passes a to form a hot-rolled flat steel product with a thickness d. A total degree of deformation Dh tot was _calculated for all hot rolling passes and des for the last pass

A degree of deformation Dh _{LG is} achieved during hot rolling.

The hot rolling of the flat steel products was ended in each case with a hot rolling end temperature TEW.

The flat steel products exiting the hot rolling mill and exhibiting the final hot rolling temperature TEW have each been cooled by water cooling at cooling rates of 10 to 300 K / s to a coiling temperature HT, at which they have been wound into a coil. Finally, the coil was cooled to room temperature.

In Table 2, the steel used in each case, the variant passed through, the thickness "d" of the finished hot-rolled flat steel product, the austenitizing temperature "TA", the respective

Vorwalztemperatur "TVW", the respective hot-rolling "TEW", the "Dh _ges" cumulative reduction and scored on the last pass of the hot rolling deformation "means that _{LG is"} as well as according to the equation (4) calculated deformation "Dh _{LG min"} is entered.

The tests 3 not according to the invention (too low degree of deformation "Ie _{LG is} "),

7 (too high pre-rolling temperature and too high hot-rolling end temperature), 18 (too high austenitizing temperature) and 28 (too low coiling temperature) are highlighted by a subsequent "*". Table 3 shows the yield strength "Re", notched impact energy "Av" at -20 ° C., -30 ° C., -60 ° C. and -80 ° C., the grain stretching ratio, the position of the im Ferrite existing proportions of the a-fiber <110> parallel to the rolling direction and the g-fiber <111> parallel to the sheet normal, elongation at break A and the springback

specified, whereby it is also recorded in which sample position the relevant characteristic values were determined. For the information on

Elongation at break A applies that for specimens made from sheet metal with a thickness of greater than or equal to 3 mm the elongation at break A5 was determined in accordance with DIN EN ISO 6892-1 (December 2009) and that for specimens made from sheet metal with a thickness of less than 3 mm the elongation at break A80 according to DIN EN ISO 6892-1

(December 2009) was determined.

It turns out that the flat steel products obtained in the tests, obtained and produced according to the invention, have a combination of high yield strength, high impact energy, i.e. high toughness, pronounced grain elongation, good alignment of the proportions of the α-fiber present in the ferrite parallel to the rolling direction and g-fiber <111> parallel to the normal to the sheet metal, as well as good springback behavior, which result in an optimal

Ensure forming behavior with optimized usage properties at the same time.

all contents in% by weight, remainder Elsen and unavoidable impurities

Claims

PATENT CLAIMS

1. Flat steel product made from (in% by weight)

C: 0.02-0.1%,

Mn: 0.1-2.5%,

AI: 0.02 - 0.1%,

Nb: 0.02 - 0.12%,

as well as optionally from one or more elements of the group "Si, Ti, V, Cr, B, Ca, Mo" with the proviso that the Si content is at most 0.6%, the Ti content at most 0.12% , the V content at most 0.2%, the Cr content at most 0.2%, the B content at most 0.0025%, the Ca content at most 0.01% and the Mo content at most 0.3% amounts

and the remainder consists of iron and unavoidable impurities, with impurities up to 0.05% P, up to 0.03% S, up to 0.01% N, up to 0.2% Ni, up to 0, 15% Cu count,

the structure of the flat steel product

- consists of at least 60% by area of ferrite and / or bainite and the remainder of pearlite as well as carbide or carbonitride precipitates and a maximum of 2% by area of other structural components,

and

- has a grain aspect ratio of 0.2-0.7,

and wherein the steel flat product

- has a yield point Re for which Re> RE_BER applies, where

Re_BER = (400 + 2243 *% Nb) / (d ⁰ ^'15 )

with% Nb: respective Nb content of the flat steel product in% by weight and d: respective thickness of the flat steel product in mm.

2. Flat steel product according to claim 1, characterized in that the structure has an average ferrite grain size of at most 15 mm.

3. Flat steel product according to one of the preceding claims, characterized in that it has an impact energy of more than 27 J in the “longitudinal” test direction at -60 ° C.

4. Flat steel product according to one of the preceding claims, characterized in that it was determined in a 180 ° folding test according to DIN EN 10149-2 with a bending mandrel diameter of 8 mm

Has springback of less than 20%.

5. Flat steel product according to one of the preceding claims,

characterized in that it contains at least 0.005 wt% Ti.

6. Flat steel product according to one of the preceding claims, characterized in that it contains at least 0.005 wt.

7. Flat steel product according to one of the preceding claims, characterized in that it contains at most 0.25% by weight of Si.

8. Flat steel product according to one of the preceding claims, characterized in that it contains at most 2.0% by weight of Mn.

9. Flat steel product according to one of the preceding claims, characterized in that it contains at least 0.5% by weight of Mn.

10. Flat steel product according to one of the preceding claims, characterized in that in the structure of the flat steel product at most 30% of the proportions of a-fiber <110> present in the ferrite parallel to the rolling direction and at most 20% of the proportions of g-fiber <111 present in the ferrite > are aligned parallel to the sheet normal.

11. A method for producing one according to one of the preceding

Claims made flat steel product comprising the following

Work steps:

a) Production of a steel melt, which from (in wt .-%) 0.02-0.1% C, 0.1

- 2.5% Mn, 0.02 - 0.1% Al, 0.02 - 0.12% Nb and optionally in each case from one or more elements from the group "Si, Ti, V, Cr, B, Ca, Mo "with the proviso that the Si content is at most 0.6%, the Ti content at most 0.12%, the V content at most 0.2%, the Cr content at most 0.2%, the B Content is not more than 0.0025%, the Ca content is not more than 0.01% and the Mo content is not more than 0.3% and the remainder consists of iron and unavoidable impurities, with up to 0.05% P to the impurities include 0.03% S, up to 0.01% N, up to 0.2% Ni, up to 0.15% Cu;

b) pouring the melt into a preliminary product, which is a slab with a thickness of 70-350 mm or a thin slab with a thickness of 30-70 mm;

c) Austenitizing the preliminary product in such a way that the preliminary product is heated through to an austenitising temperature of 1150-1320 ° C; d) in the case that the intermediate product is a slab: Pre-rolling the austenitized precursor in two or more

Roll passes to a thickness of at least 30 mm and a maximum of 70 mm at a roughing temperature which is at most equal to

Austenitizing temperature, but at least 30 ° C above the recrystallization stop temperature T _NR , which is calculated as follows:

Flat steel product in several rolling passes,

- where a number n _W of rolling passes, which is equal to a value n _W 'rounded off to an integer, which according to the formula n _W ' = 7 * root (d _EW / (6 * Z)) + 2 (2) with d _EW : final rolled thickness of the steel flat product

Z: Thickness of the pre-product is calculated, if it is below that according to formula (1)

calculated recrystallization stop temperature T _NR

Temperature,

- Wherein overall achieved for one over the finish hot rolling

Deformation degree Dh _tot applies

Finished hot rolls,

d _outlet : thickness of the steel flat product at the end of the

Finish hot rolling,

- where the degree of deformation Dh _LG achieved in the last pass of the finish hot rolling is:

Ie _{LG is} > Dh _{LG min} (4) With

Ie _LG = (d _{inlet LG} - d _{outlet LG} ) / d _{inlet LG} d _{inlet LG} : Thickness of the flat steel product when entering the last one

Roll stand,

d _{Exit LG} Thickness of the flat steel product at the exit from the last roll stand

Ie _{LG min} = 1.8 * 10 ^-5 * e ^{(0.0135 * (TEW + 11))} + 3

Ie _{LG min} : minimum degree of deformation in the last rolling pass of the

Finish hot rolling

TEW: hot rolling end temperature in ° C, and

- The hot rolling end temperature is 760-940 ° C ;, f) cooling the hot-rolled flat steel product to a

Coiling temperature of 520 - 650 ° C; g) Coiling the hot-rolled flat steel product into a coil and cooling the hot-rolled flat steel product in the coil to room temperature.

12. The method according to claim 11, characterized in that the roughing temperature in step d) is at most 1150 ° C.

13. The method according to any one of claims 11-12, d a d u r c h

characterized in that the hot rolling end temperature in step e) does not exceed 920 ° C.

14. The method according to claim 13, characterized in that the hot rolling end temperature in step e) is at most 890 ° C.

15. Use of a flat steel product according to claims 1-10 for

Manufacturing a component