EP3847284A1

EP3847284A1 - Hot-rolled flat steel product and method for the production thereof

Info

Publication number: EP3847284A1
Application number: EP18773945.3A
Authority: EP
Inventors: Nicholas WINZER; Richard G. THIESSEN; Rainer FECHTE-HEINEN
Original assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Current assignee: ThyssenKrupp Steel Europe AG; ThyssenKrupp AG
Priority date: 2018-09-06
Filing date: 2018-09-06
Publication date: 2021-07-14
Anticipated expiration: 2038-09-06
Also published as: EP3847284B1; EP3847284C0; WO2020048599A1

Abstract

The invention relates to a hot-rolled flat steel product having a tensile strength Rm of ≤ 550 MPa, a hole expansion ratio λ of ≤ 30%, and a microstructure consisting of ≤ 90 area % ferrite and, as the remainder, ≤ 10 area % perlite or cementite, with embedded carbonitride precipitates having an average diameter of ≤ 10 nm. The flat steel product consists of a steel containing (in wt%) 0.02-0.3% C, ≤ 0.7% Si, ≤ 1.0% AI, 0.2-2.5% Mn, 0.05-0.5% Cr, ≤ 0.02% P, ≤ 0.005% S, ≤ 0.01% N, ≤ 0.1% Cu and at least one element forming carbonitride precipitates from the group "Ti, Nb, V" and, as the remainder, Fe and unavoidable impurities, with the following provisos:- If V is present, the sum of the Ti_GEW and Nb_GEW contents is - 0.01 wt%; - if Ti or Nb are present, V_GEW is ≤ 0.01 wt%; - for X_GEW [wt%]=Ti_GEW+(V_GEW/1.06)+(Nb_GEW/1.94), the following applies: 0.02% ≤ X_GEW ≤ 0.5%; - for ratio = (X_AT+Cr_AT) / (C_AT+N_AT), the following applies: 0.5 ≤ ratio ≤ 2.0, where X_AT = Ti_AT+V_AT+Nb_AT; wherein Τi_ΑT, V_AT, Nb_AT, Cr_AT, C_AT and N_AT correspond to the Ti content Ti_GEW, the V content V_GEW, the Nb content Nb_GEW, the Cr content Cr_GEW, the C content C_GEW and Ν_ΑT the N content N_GEW respectively, each converted into atomic %, and the Ti_GEW, V_GEW, Nb_GEW, Cr_GEW, C_GEW and N_GEW contents are given in wt%.The invention also relates to a method for producing such a flat steel product.

Description

Hot rolled flat steel product and process for its manufacture

The invention relates to a hot-rolled flat steel product with an optimized combination of high tensile strength Rm and high

Hole expansion ratio l.

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

When we are talking about flat steel products, we mean rolled products such as steel strips or steel sheets, as well as blanks and blanks obtained from them, the thickness of which is considerably less than their length and width.

Information contained in this text on the levels of certain

Alloy elements relate to weight (in% by weight or YGEW, where Y is the element symbol). Likewise, the sums or total concentrations formed from this salary information relate to the weight (stated in% by weight or YGEW), unless expressly stated otherwise. Information given in “atom%” or “YAT” (where Y is the element symbol) about contents of alloy elements or summands formed therefrom relate to the number of atoms.

The usual formula is used to convert% by weight to atomic%, 100%

where m _{Y is} the atomic mass of element Y, I _G EW is the content (in% by weight) and m is the atomic mass of component i in the mixture with Z components.

The proportions of certain components in the structure of a flat steel product are given in the present text in relation to the area (expressed in% by area), unless expressly stated otherwise.

All information on the hole expansion properties of here

Treated steels known from the prior art or according to the invention and steel flat products produced therefrom are in accordance with the ISO 16630: 2017 standard of the International Organization for Standardization (see

https://www.iso.org/standard/69771.html).

The mechanical properties specified here, such as tensile strength Rm, yield strengths ReFI, ReL or elongation A50, have been determined based on DIN EN ISO 6892-1: 2017, unless otherwise stated.

The increasing demand for fuel-efficient cars is driving an increasing need for weight reduction through lightweight construction. The geometric complexity of such lightweight components calls for new steels with improved resistance for complex forming processes. The sensitivity of the steels to edge cracks, which can form when stamping sheet metal, is particularly important. In the laboratory, the edge crack sensitivity of a steel is assessed with the so-called "hole expansion test", in which one into one

Sheet metal punched hole is expanded with a mandrel until the first cracking.

Materials with very anisotropic or inhomogeneous microstructures are characterized by a relatively high sensitivity to edge cracking. These include dual-phase steels, which consist of hard (e.g. martensite) and soft (e.g. ferrite) phases and have a distinctive rolling texture. In contrast, flat steel products with an isotropic or homogeneous microstructure are characterized by a relatively low sensitivity to edge cracking. These steels include steels with a ferritic structure, in which very fine precipitates can be embedded to increase strength.

An example of such a steel is known from EP 1 338 665 A1. The steel described there has a tensile strength of at least 550 MPa when hot-rolled into a flat steel product, which should be in combination with a high elongation and excellent elongation flange capability. Due to this combination of properties, such a steel is particularly suitable for the production of complex shapes

Automotive chassis parts. In general, EP 1 338 665 A1 mentions that a hot-rolled steel strip (in

Wt .-%) <0.15% C, 0.02 - 0.35% Ti and 0.05 to 0.7% Mo and should have a structure that should essentially consist of ferrite with single-phase and fine precipitations which are dispersed in the ferrite matrix with a grain size smaller than 10 nm. A hot-rolled steel sheet corresponding to this requirement, specifically mentioned in EP 1 338 665 A1, should consist of (in% by weight) <0.06% C, <0.5% Si, 0.5 - 2.0% Mn, <0.06% P, <0.005 S, <0.1% AI, <0.006% N, 0.02 - 0.10% Ti, 0.05 - 0.6% Mo and the rest are Fe. At the same time, the steel sheet composed in this way should have precipitates in its single-phase structure consisting of ferrite, the grain size of which is <10 nm and of which 5 × 10 ⁴ / pm ³ are present in the structure per unit volume. The under the name "Nanohiten"

Market-known steel flat products which meet these requirements have a ferritic structure in which TiC precipitates with a size of <5 nm are embedded, which are of high strength and low

Ensure the edge crack sensitivity of the steel. The fineness of the TiC precipitates is made possible by the addition of Mo, which prevents coarsening of the TiC precipitates. The mechanism underlying the carbide coarsening can be described by the so-called "Ostwald ripening" according to equation (1):

In this equation, d ”and d” denote the precipitation quantities at the time 0 and t, V _m the molar volume of the precipitation, Xc and XD the concentration or diffusivity of the speed-limiting element (for example Ti in the case of TiC precipitations) and g the interfacial energy g is composed in part of the strain energy, which is due to the

Mismatch between the carbides and the surrounding ferrite matrix arises. Reducing the strain energy promotes a coherent interface between the carbides and the ferrite matrix and prevents the carbides from becoming coarser. Mo or W atoms are taken up in the carbide in the first stage of the precipitation process and replace the Ti, Nb or V atoms there. In this context, it is known that both the uptake of Mo and W atoms to reduce the

Deformation energy leads.

In addition to the prior art explained above, the

JP 2013-133498 A discloses a high-strength hot-rolled steel sheet which (in% by weight) 0.035-0.12% C, <0.1% Si, <1.2% Mn, <0.03% P, <0.005% S,

<0.1% AI, <0.01% N, 0.14 - 0.30% Ti, <3.0% Cu and the rest contains Fe and unavoidable impurities. The Cr content of the steel is set to <0.08% to ensure optimum corrosion resistance of the

To achieve steel sheet, Cr contents, which are as low as possible, are particularly advantageous in this regard. The structure of the hot-rolled steel sheet composed in this way consists of at least 95 area% ferrite with an average grain diameter of <7 pm, carbide precipitates with an average diameter of less than 8 nm are present in the structure and the structure less than 0.1 vol. -% Contains cementite. The steel sheet obtained in this way should have a tensile strength of at least 780 MPa.

Against the background of the prior art explained above, the invention was based on the object of producing an inexpensive one

hot-rolled steel flat product, which has a high strength and at the same time is too complex shaped for forming

Steel moldings, especially components for the body and chassis of vehicles are suitable. For the construction of bodywork and

Chassis components are increasingly being used to cut sheet metal from steel with a tensile strength of 550 MPa and more. The sheet metal blanks are often subjected to a particularly high degree of deformation at their cut edges, which can lie on the outer edge of the sheet metal blank and / or on holes and cutouts. These deformations can occur when using materials with heterogeneous microstructures, such as. B.

Dual phase steels, lead to edge failure. Therefore, a flat steel product should be provided, which due to its homogeneous microstructure is particularly suitable for the manufacture of bodywork and

Chassis components is suitable and because of its low

Edge crack sensitivity makes it possible to design these components so that the load to be borne by them can be safely introduced via the strongly deformed edges.

In addition, there should be a method of making one

hot-rolled steel flat product.

With regard to the flat steel product, the invention has achieved this object in that such a flat steel product has at least the features specified in claim 1.

A method according to the invention that achieves the above object is specified in claim 13. Advantageous refinements of the invention are specified in the dependent claims and are explained in detail below, like the general inventive concept.

A hot-rolled flat steel product according to the invention is therefore characterized in that it has a tensile strength Rm of at least 550 MPa and a hole expansion ratio l of at least 30%, its structure comprising at least 90 area% of ferrite and the remainder up to 10 area% consists of pearlite or cementite and in the structure of the

Flat steel product carbonitride precipitates with a medium

Diameters of at most 10 nm are embedded.

For this purpose, the flat steel product according to the invention consists of a steel which consists of (in% by weight)

0.02 - 0.3% C,

<0.7% Si,

<1.0% AI,

0.2 - 2.5% Mn,

0.05 - 0.5 Cr,

<0.02% P,

<0.005% S,

<0.01% N,

<0.1% Cu

and from at least one element forming the carbonitride precipitates from the group “Ti, Nb, V” and the remainder from iron and

unavoidable impurities, whereby the following requirements apply to the element forming the carbonitride precipitates:

If V is present, the sum of the contents TIQEW of Ti and Nbc _E w of Nb is limited to a maximum of 0.01% by weight. - In the event that Ti and / or Nb are present, the V content V _G EW is at most 0.01% by weight.

- For a total concentration formed from the respective Ti content Ti _GEW , the respective V content V _GEW and the respective Nb content Nb _{GEW of} the steel

XGEW [% by weight] = Ti _GE w ⁺ (V _GE w / 1, 06) + (Nb _GE w / 1, 94) applies

0.02% s- X _GE w - 0.5%.

- For a quantitative ratio

Verhl = (XAT ⁺ OGAT) / (CAT ⁺ NAT),

applies 0.5 <Verhl <2.0, with XAT = TT ⁺ VAT + NbAT,

where:

Cr _AT is Cr _GEW converted into atomic%,

CAT is C _GEW converted to atomic%,

NAT is N _GEW converted to atomic%, Tΐ _A t is Ti _GEW converted to

Atom-%,

VAT is V _GEW converted into atomic%,

Nb _AT is Nb _GE w converted into atomic%.

Optimal properties of a flat steel product according to the invention result here if for a quantitative ratio

Verh2 = XAUS / Cr _A us

applies

2 <Ver2 <20 with XAUS [atom%] = TIAUS ⁺ VAUS ⁺ NbAus

where:

Ti _A us is the Ti content of the precipitations in atomic% V _A us is the V content of the precipitations in atomic%

Nb _A us is the Nb content of the precipitations in atomic%

Cr _A us is the Cr content of the precipitates in atomic%.

The contents Ti _AU s _> VAUS, Nb _AU s and Cr _AU s of the excretions given in atomic% can be determined, for example, by energy-dispersive

Determine x-ray micro-area analysis ("EDX").

A flat steel product according to the invention has a high tensile strength Rm of at least 550 MPa, in particular at least 660 MPa, and it also regularly achieves tensile strengths of at least 780 MPa or even at least 960 MPa.

The hole expansion ratio l determined for a steel flat product according to the invention in accordance with ISO 16630: 2017 is in each case at least 30%, in particular at least 50%, wherein hole expansion ratios l of at least 60% can also be represented.

It has been found here that a flat steel product according to the invention has a particularly favorable ratio of hole expansion capacity to

Has strength. Thus, with a flat steel product according to the invention, high hole expansion ratios l are achieved even at high strengths. This manifests itself in high values for the product Rm-l from tensile strength Rm and hole expansion l. Here, a flat steel product according to the invention regularly achieves Rm ^ values of at least 30,000 MPa%, in particular

at least 40,000 MPa% or even at least 50,000 MPa%.

The structure of a flat steel product according to the invention exists

at least 90% by area of ferrite, one in the technical sense

completely ferritic structure with carbonitride precipitates is particularly advantageous. However, with an appropriate alloy layer, cementite and pearlite can be formed in the structure, the proportions of which can be up to 10% by area. may be. The average grain size of the structure is typically 3 to 10 pm, in particular 3 to 8 pm.

The crystalline carbonitride precipitates embedded in the structure of a flat steel product according to the invention based on the inventive

The proposed contents of Ti, Nb or V are medium

Particle diameter of at most 10 nm. The delicacy of

Excretions contribute decisively to the desired combination of high strength and good hole expansion capacity. The mean particle diameter is therefore that in the flat steel product according to the invention

existing excretions preferably at most 7 nm, in particular at most 5 nm.

The alloy selected according to the invention lays the foundation for the optimized properties of a hot-rolled one according to the invention

Steel flat product.

The carbon (C) provided in the steel alloy according to the invention is mainly set in the precipitates. The concentration of the C dissolved in the mixed crystal is minimized. A C content of more than 0.02% by weight, in particular more than 0.05% by weight, is required in order to achieve a high precipitation density and thus to achieve the required tensile strength of at least 550 MPa. Too high a C content would in turn lead to the formation of larger pearlite contents in the structure, which would reduce the ductility and increase the sensitivity to edge cracks. The C content is therefore limited to a maximum of 0.3% by weight, in particular a maximum of 0.15% by weight, negative influences of the presence of C being able to be avoided particularly reliably if the C content of the steel is at most Is 0.10% by weight.

It should be noted in each case that the C content must be set within the framework of the specifications made here so that the invention prescribed requirements for the ratio Verhl are observed.

Specifically, the contents of the elements determining the quantitative ratio Verhl are set within the content ranges specified according to the invention for these elements in such a way that 0.5 Verhl <2.0 applies to the quantitative ratio Verhl, with quantitative ratios Verhl of 0.7-1.5 or 0.8 - 1, 3 have been found to be particularly favorable with regard to the desired properties of a flat steel product according to the invention.

Manganese (Mn) is an element that contributes to the strength of steel through the formation of mixed crystals. Mn also suppresses the formation of pearlite and cementite and in this way promotes the formation of Cr-containing carbonitride precipitates based on the contents of Ti, Nb or V provided according to the invention. For this reason, the Mn content in the steel according to the invention is at least 0 , 2% by weight, in particular more than 0.3% by weight, preferably at least 0.5% by weight, particularly preferably 1.0% by weight or 1.3% by weight, is provided. Too high a concentration of Mn has a negative effect on weldability and increases the risk of occurrence more strongly

Segregations. The upper limit of the Mn content is therefore set to at most 2.5% by weight, with lower Mn contents of at most 2.0% by weight, in particular at most 1.7% by weight, of the possible negative effects of the presence Avoid particularly safe from Mn.

Silicon (Si) can optionally be added in amounts to suppress the formation of pearlite in the structure of a flat steel product according to the invention. In order to achieve this effect of Si, a content of at least 0.05% by weight of Si is required. The Si levels would be too high

Surface quality of the flat steel product according to the invention

affect. Therefore the Si content is max. 0.7% by weight, with Si contents of up to 0.25% by weight, in particular up to 0.1% by weight, have proven to be particularly favorable with regard to the avoidance of negative influences of the presence of Si and, moreover, enable the product according to the invention to be galvanized in pieces later. If there are special demands on the ability to galvanize bars, a Si alloy is particularly preferably dispensed with and a maximum Si content of 0.03% by weight is selected.

At levels of up to 0.7% by weight, Si also contributes to solid solution strengthening, so that higher levels of Si may well be expedient if less demands are made on the surface quality and / or the ability to galvanize the piece. At Si contents which are above 0.7% by weight, the rollability of the steels according to the invention is, however, influenced too strongly negatively, and the rolls may grow up during the rolling process.

Aluminum (AI) can also be added as an optional element to suppress the formation of pearlite. Because Al is usually used to deoxidize the melt, an Al content of at least 0.01% by weight is unavoidable in the normal production of the steel from which a flat steel product according to the invention is made. A too high Ai content can have a negative impact on the castability. Therefore, the upper limit of the Al content is limited to at most 1.0% by weight, preferably at most 0.7% by weight, in particular at most 0.5% by weight.

The presence of chromium (Cr) in contents of 0.05-0.5% by weight is of crucial importance for the invention. So Cr after the

Findings of the invention can be used to coarsen the

To prevent excretions. For this, a salary of at least

0.05% by weight of Cr, preferably at least 0.06% by weight of Cr or, particularly preferably, more than 0.08% by weight or at least 0.10% by weight. The detection limit for Cr in steels of the type according to the invention is in the range of 0.03% by weight, whereas Cr contents of at least 0.05% by weight in

Steel mill can be set specifically. However, the underlying Cr levels are considered ineffective. Surprisingly, it has been shown that the influence of Cr on the carbide coarsening is stronger than that of Mo and W with the same proportion in atomic%. In this way, the same mechanical properties can be achieved when using Cr as when Mo and W are added, but lower Cr alloy contents are required for this. This applies in particular when converting the alloy contents into% by weight, since the atomic masses of Mo and W are higher than those of Cr.

The effectiveness of the addition of Cr provided according to the invention is also demonstrated by the fact that a low atomic ratio Verh2 in the

Forms excretions. The atomic ratio of Ti to Cr is in

(Ti, Cr) (C, N) precipitates, which have a size of approx. 10 nm,

For example, more than 10. Previous studies have shown that using Mo or W exclusively to form

Carbonitrides in precipitates with a size of approx. 10 nm

Atomic ratio of Ti to the Mo or W present is at most 4. This shows that the fact that in a

Flat steel product Cr into which excretions have been taken up, the energy of deformation of the excretions is reduced more than when Mo or W.

Conversely, this means that fewer Cr atoms are required for the desired effect than would be the case if Mo or W were used. However, an excess of Cr increases the risk of pronounced grain boundary oxidation. Therefore, the upper limit of the Cr content in a flat steel product according to the invention is set to at most 0.5% by weight, preferably at most 0.25% by weight or at most 0.15% by weight. It should be noted that the Cr contents of a flat steel product according to the invention are set such that no pure Cr carbides are present in the flat steel product according to the invention. By using Cr instead of the elements Mo or W used in the prior art, the temperature at which the precipitation-stabilizing effect occurs is also reduced. This increases the certainty that a sufficient number of excretions can first be formed before the obstruction of coarsening of the excretions begins.

This relationship is considered to be the cause of the above-described higher effectiveness of Cr in terms of the atomic proportions required.

The microalloying elements titanium (Ti), niobium (Nb) and vanadium (V) are for the formation of the precipitates in the structure of the invention

Steel flat product essential. By adjusting the V, Ti and Nb levels so that the total concentration

XGEW [% by weight] - TIGEW + (VGEW / 1 _> 06) + (NbGEw / 1 _> 94) is at least 0.02% by weight, in particular at least 0.05% by weight, the required density distribution of excretions achieved.

The precipitates formed by Ti, Nb or V are in the

Flat steel product according to the invention not as pure carbides, but, if nitrogen "N" is present in the alloy, regularly as carbonitrides.

It is known that carbides and nitrides formed with Ti, Nb and V have very different solubilities in austenite and ferrite. That is why they form at very different temperatures.

In order to nevertheless ensure a homogeneous distribution of the Cr added according to the invention in the carbides and thus effectively prevent the coarsening of the precipitate, in the event that effective V contents are added with regard to the properties of the steel, Ti or Nb are only used in total contents of at most 0 , 01% by weight, preferably at most 0.005% by weight, which are in the range of the unavoidable impurities in which neither Ti nor Nb have an effect on the properties of the steel. Ti and Nb can be added alone or together, however, because the

Formation temperatures at which Ti or Nb precipitates are formed are close enough to allow timely elimination of both

Allow elements. When Ti or Nb is added, V is therefore only tolerated as an unavoidable impurity, which can be present in contents of up to 0.01% by weight, preferably up to 0.005% by weight.

In order to avoid negative effects of excessive levels of microalloying elements, the XGEW value is limited to 0.5% by weight. In this way it is avoided that, for example, increased Nb contents lead to crack formation during continuous casting or during slab cooling or reheating. At the same time, only a certain content of microalloying elements is required for the desired strengths. If this is exceeded, there is only a slight further increase in strength. In addition, the average diffusion distances decrease, which increases the risk of undesired large precipitates. For these reasons, the Nb, Ti or V contents of the steel of a flat steel product according to the invention

advantageously set so that the value XGEW is not higher than 0.25% by weight.

Phosphorus (P) is unfavorable for the weldability of a flat steel product according to the invention. The P contents of a flat steel product according to the invention are therefore limited to at most 0.02% by weight, in particular less than 0.02% by weight, with P contents of at most 0.010% by weight, in particular less than 0.005% by weight. -%, are particularly cheap.

At sufficiently high concentrations, sulfur (S) leads to the formation of MnS or (Mn, Fe) S, which has a negative effect on the elongation. The S content must therefore be limited to at most 0.005% by weight, in particular less than 0.003% by weight, preferably less than 0.0015% by weight. N is present in the flat steel product according to the invention as an unavoidable impurity due to the production. The structure of one

Precipitates embedded in the steel flat product according to the invention lie as carbonitrides in the form of (Ti, Cr) (C, N); (Nb, Cr) (C, N); (V, Cr) (C, N) or (Ti, Nb, Cr) (C, N). If nitrogen "N" is present, Ti, Nb and V preferably form nitrides or carbonitrides in the presence of C with N. That is why in practice it is technically and economically among them

representable conditions the inclusion of N in the excretions inevitable. In principle, however, the N contents are as low as possible

to strive for, since N-dominated carbonitrides are often very coarse and angular, which is why they do not contribute to solidification but act as crack initiators. In order to avoid the formation of N-dominated carbonitrides, the upper limit of the N content is therefore set at 0.01% by weight, preferably 0.005% by weight.

Calcium (Ca), Boron (B), Copper (Cu), Molybdenum (Mo), Tungsten (W), Nickel (Ni), Tin (Sn), Arsenic (As), Cobalt (Co), Zircon (Zr), Lanthanum (La) and / or cerium (Ce), oxygen (O) and hydrogen (H) are, like all other conceivable alloy elements not explicitly mentioned here, to be attributed to the production-related unavoidable impurities, which are components of the starting material from which the steel is produced, or get into the steel due to the process of steel melting and processing. The contents of these elements are to be kept so low that they have no technical effect with regard to the properties of the flat steel product according to the invention.

Ca is usually added to the melt during steel production both for deoxidation and desulphurization, as well as to improve the castability. Too high a concentration of Ca can, however, lead to the formation of undesired inclusions, which have a negative effect on the mechanics and the rollability. Therefore, the upper limit of the Ca content according to the invention limited to 0.01 wt .-%, in particular at most 0.005 wt .-%, preferably at most 0.002 wt .-%.

The Mo and W contents are limited to at most 0.05% by weight, in particular at most 0.04% by weight, preferably 0.03% by weight, since these

Elements in one for the reasons explained above

Flat steel product according to the invention are not required.

The contents of B must not exceed 0.002% by weight, in particular 0.001% by weight, preferably 0.0005% by weight, in order to prevent the movement of the phase boundaries from being slowed down by B segregated on them and thereby the formation of Carbides and carbonitrides of Ti, Nb and V is hindered.

Cu can separate out as coarse particles, which have a negative effect on the mechanical properties. Cu also has a negative impact on castability. In order to avoid any influence of Cu, the permissible upper limit of the Cu content in the invention is

Flat steel product 0.1% by weight, in particular less than 0.04% by weight or less than 0.02% by weight.

Ni, Sn, As, Co, Zr, as well as rare earths, in particular La and / or Ce, are also not required as alloy elements in the flat steel product according to the invention and count in the event that they are nevertheless used in the invention

Flat steel products are detectable to the inevitable impurities. Accordingly, the Ni content is at most 0.1% by weight, the Sn content is at most 0.05% by weight, the As content is at most 0.02% by weight, and the Co content is at most 0.02% by weight, the Zr content to a maximum of 0.002% by weight, in particular a maximum of 0.0002% by weight, and the content of elements attributable to the rare earths, such as La and Ce, to a maximum of 0.002% by weight in each case .-%, in particular a maximum of 0.0002 wt .-% limited. O is the same

undesirable in the flat steel product according to the invention, since there is a Oxide coating, which can result from the presence of higher O contents, both on the mechanics and on the pourability and rollability of the

Flat steel product would have a negative impact. The maximum permissible O content is therefore set to 0.005% by weight, preferably 0.002% by weight. As the smallest atom, H is very mobile in interstitial spaces in steel and, particularly in high-strength steels, can tear open the core when it is hot-rolled. The H content of an inventive

Flat steel products should therefore be as low as possible, but in any case at most 0.001% by weight, in particular at most 0.0006% by weight or at most 0.0004% by weight, with H contents of at most 0.0002 % By weight are particularly desirable.

The method according to the invention for producing a flat steel product according to the invention comprises the following steps: a) Melting a steel which consists of (in% by weight) 0.02-0.3% C, <0.7% Si, <

1.0% AI, 0.2-2.5% Mn, 0.05-0.5% Cr, <0.02% P, <0.005% S, <0.01% N, <0.1% Cu and at least one element forming the carbonitride precipitates from the group “Ti, Nb, V” and the rest of iron and unavoidable impurities with the following requirements:

- In the event that V is present, the sum of the contents Ti _GEW of Ti and Nbc _E w of Nb is limited to a maximum of 0.01% by weight.

- In the event that Ti and / or Nb are present, the V content V _G EW is at most 0.01% by weight.

- For a steel formed from the respective Ti content Ti _GEW , the respective V content V _GEW and the respective Nb content Nb _GE w

Total concentration

XGEW [% by weight] = TIQEW ⁺ (V _GE w / 1, 06) + (Nb _GE w / 1, 94) applies 0.02% <X _GEW ^ 0.5%.

- For a ratio Verhl = (X _AT + Cr _AT ) / (C _A + N _AT ), applies 0.5 <Verh1 <2.0, with XAT ⁼ TT ⁺ VAT ⁺ NÖAT

where:

Cr _AT is CT _G EW converted into atomic%,

C _A T is CGEW converted into atomic%,

N _A T is NQEW converted into atomic%,

Ti _AT is TIGEW converted into atomic%,

_A V T is VGEW converted, in atomic%

NbAT is Nb _G Ew converted into atomic%. b) Casting the steel into a preliminary product, such as a slab

Thin slab or a cast tape;

c) Tempering the preliminary product to a temperature of 1150 - 1350 ° C

Temperature;

d) hot rolling the preliminary product to the hot-rolled flat steel product with a hot-rolling end temperature of 880-980 ° C .;

e) cooling the hot-rolled flat steel product obtained at a cooling rate of 20-400 ° C./s to a reel temperature of 560-690 ° C.;

f) coiling the hot strip cooled to the coiling temperature into a coil.

According to the invention in accordance with the above in connection with the composition of a flat steel product according to the invention

According to the explanations given, alloyed steel is cast into a preliminary product after it melts, which is the classic

Production route will be a standard size slab. However, the steel can also be obtained by direct hot rolling of a continuous casting in a casting or rolling mill as a preliminary product a thin slab or in a

Belt casting machine as a preliminary product, a cast belt can be produced.

The preliminary product is heated to or at at least 1150 ° C

Temperature maintained. Such a high heating temperature is necessary in order to dissolve carbides and nitrides already present in the preliminary product. If the heating temperatures are too low, the alloy elements remain bound in the precipitates, so that no new precipitates can be formed. For economic reasons, the heating temperature is max. Limited to 1350 ° C.

The hot rolling of the preliminary product takes place in a conventional manner, whereby the final temperature of the hot rolling must be at least 880 ° C. If the hot rolling end temperatures are too low, the rolling forces increase disproportionately and the desired isotropy of the material is lost due to the effects of thermomechanical rolling. Over 980 ° C

Final temperatures are not technically feasible.

The hot-rolled steel strip leaving the hot rolling stack is cooled at a cooling rate of 20-400 ° C / s to a coiling temperature which is in the range of 560-690 ° C.

A cooling rate of at least 20 ° C / s is required to avoid the formation of pearlite and cementite as far as possible. Cooling speeds above 400 ° C / s are not technically feasible.

Coiler temperatures of 560 - 690 ° C cover the temperature range in which precipitates with an average size of less than 10 nm, in particular less than 5 nm, are formed. At higher

Temperatures, the average size of the carbonitrides is more than 10 nm, with which the target properties of the flat steel product according to the invention can no longer be achieved. A targeted formation of small ones Excretions, the size of which is less than 7 nm, in particular less than 5 nm, can be caused by reel temperatures of 580-670 ° C., in particular 590-650 ° C. At reel temperatures below 580 ° C, carbonitrides would no longer be excreted and their strength-increasing effect would not be achieved.

The atomic ratio Verh2 is of particular importance with regard to the precipitation processes during cooling in the coil. A ratio of 2 - 20 in the precipitates is necessary in order to avoid a coarsening of the steel flat product according to the invention

Prevent excretions during cooling in the coil after reeling. In the case of larger ratios Verh2, the maximum size of the carbonitrides specified according to the invention of at most 10 nm and the associated minimum strength and the desired high level

Hole expansion ratio cannot be achieved. However, if the ratio Verh2 is less than 2, the risk increases that undesired crystal structures are formed if the carbides or carbonitrides are too strongly dominated by Cr, or even pure Cr carbides are formed.

The effects of the formation of Cr-containing precipitates used according to the invention can be brought about particularly reliably if the following applies to the ratio Verh2: 2.3 <Verh2 ^ 15.

In the event that precipitates with an average particle size of not more than 5 nm are to be produced, this can be achieved in addition to the selection of a suitable reel temperature explained above in a reliable manner by setting the ratio Verh2 to 2.5-5.

If, on the other hand, precipitations with an average particle size of more than 5 nm (i.e.> 5 nm) but not more than 10 nm (i.e. <10 nm) are to be produced, this can be done, as explained above, on the one hand by adjusting the reel temperature in the upper part of the reel temperature range specified according to the invention be effected, and on the other hand, be supported by the ratio Verh2 being set to 5-15, in particular 5-10.

As a high-resolution transmission electron microscopy shows, is present in the structure of a steel flat product according to the invention

Precipitates that have an average particle size of less than 5 nm present a homogeneous Cr distribution. In the case of precipitates in the size range from approx. 5 to 10 nm, however, it was shown that the edges of the precipitates have significantly higher Cr contents than the core of the precipitates. The "core" is defined as the area of the excretions that makes up approx. 50% of the area of the excretions in microscopy and is closest to the area center. The remaining area of the excretion is defined as the border. With precipitates of more than 10 nm, the Cr content at the edge is significantly higher than in the core area and Cr has no effect in terms of avoiding coarsening of the precipitates.

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

Figure 1 Optical micrographs of the structure of steel strips obtained in tests;

Figure 2 Transmission electron microscope (TEM) images of

Structure of steel strips obtained in tests.

The melts A - S alloyed according to the compositions given in Table 1 were produced and cast into slabs. The melts not according to the invention and their contents of certain alloying elements which deviate from the specifications of the invention are highlighted in Table 1 by underlining (steels B, D, E, F, N, O).

After heating in a pusher furnace to a

Heating temperature ("EWT") are those generated from steels A - S Slabs have been hot-rolled in a conventional manner to form a hot-rolled steel strip. The hot-rolled steel strip obtained in each case left the hot rolling stage with a hot rolling end temperature (“ET”) and was then cooled at a cooling rate (“dT”) to a coiling temperature (“HT”) at which they were each coiled into a coil. The steel strips in the coil were then cooled to room temperature. The production specifications "EWT", "ET", "dT" and "HT" are listed in Table 2.

The tensile strength Rm, the upper yield strength ReH, the lower yield strength ReL and the elongation A50 were determined on the hot-rolled steel strips thus obtained in accordance with DIN EN ISO 6892-1: 2017. In addition, the hole expansion l was determined in accordance with ISO 16630: 2017, the product Rm x l was formed and the average particle size of the excretions and the ratio Verh2 were determined.

The ratio Verh2 of X _A us (see Table 1) to Cr _A us (in atomic%) in the precipitates is given in Table 2. Verh2 increases with increasing reel temperature and excretion size. The

Relationship between Verh2 and the precipitation diameter indicates that Cr prevents the carbides from becoming coarser.

Example A is a reference composition which was used to investigate the influence of the reel temperature (see Examples A1 to A7). With this composition, the optimal mechanical-technological properties were achieved at reel temperatures in the range of 590 - 650 ° C.

The influences of C, Si, Mn and Cr were investigated separately on the basis of the tests based on steels B to E. In steel F it was investigated what influence the lack or insufficient addition of the micro alloy elements Ti, V and Nb has. The concentrations of the others Alloying elements in Examples B through F were otherwise similar to the tests based on Steel A. The lack of C, Mn, Cr or Ti, V and / or Nb leads to mechanical-technological properties outside the target range (see tests B8 and D10 to F12). The lack of Si has no significant impact on mechanics among similar ones

Production specifications (see experiment C9).

Steels G and H are also based on example steel A, but the ratio Verhl was varied here. The variation of Verhl in the range 0.8 to 1, 2 has no negative influence on the mechanical-technological

Properties under similar production specifications (see examples G14, G15, H17 and H18). The tests based on the steels G and H were also used to determine the effects of the heating temperature EWT and the final rolling temperature ET on the properties of the respectively obtained

Examine steel strips. Here it turned out that it was too low

Heating temperature EWT or finish rolling temperature ET leads to the formation of coarse deposits or an anisotropic structure and therefore poor mechanical-technological properties (see examples G13 and H16).

Steels I and J are Nb concepts and steels K and L are V concepts, from which steel strips were produced with different reel temperatures (see examples 119 to K28). Examples I to L contain different levels of Si and Al. As with those on steel A

based tests, the optimum mechanical-technological properties were achieved at reel temperatures of 590 - 650 ° C.

The steels M to O have a similar X _GE w concentration as the steel A but different combinations of Ti, Nb and V. The steel strip produced in the example M30 based on the steel M (Ti and Nb) has comparable ones

mechanical-technological properties like the steel strips that were produced on the basis of steel A at similar reel temperatures. The Tests N31 (Ti and V) and 032 (Nb and V), on the other hand, have similar steel strip with similar mechanical reel temperatures compared to the tests based on steel A, despite similar reel temperatures

Properties.

The steel P has a very high ratio. The steel P

Existing steel strips were tested in tests P33 - P35

different cooling rates. If the cooling rate was too low, too much pearlite was formed, which greatly deteriorated the mechanical-technological properties (see example P33).

The steel Q has a high Al content, but is otherwise relatively low alloyed. In contrast, the steel R is relatively high alloy. Steel strips were produced from steels Q and R under optimal conditions. These tests show that very different strengths can be achieved with high Rm ^ values, provided the ratio Verhl is in the target range.

Steel S was produced as a high-purity laboratory melt and with an increased heating and hot rolling temperature and a very high cooling rate

processed to validate the influences of different manufacturing parameters.

Light microscopic images of the structure of the steel strips obtained in tests A1, A3, A4, A6 and A7 are shown in Figure 1. A main phase of ferrite (gray / white) with traces of pearlite (black) can be seen. The pearlite content of the structure increases with increasing reel temperature.

The structure of the steel strip obtained in test A1 (HT = 550 ° C) contains no pearlite. The (Ti, Cr) (C, N) excretions are not recognizable at a magnification of 1000 times.

The transmission electron microscope (TEM) images of the structure of the steel strips obtained in tests A3, A4, A6 and A7 are shown in the figure 2 shown. All samples have fine (Ti, Cr) (C, N) deposits (darker particles). The diameter of the precipitates increases with increasing reel temperature, so that the diameters of the particles in A3, A4 and A5 in each case are below 5 nm on average, which is particularly clearly the case in A3. In Examples A3 and A4, these are

Excretions cannot be recognized as individual particles and are in the form of agglomerates. Example A1 contained no (Ti, Cr) (C, N) precipitates.

The distributions of Cr and Ti in the structure of the steel strip obtained in experiment A3 were measured by means of energy-dispersive X-ray micro-area analysis (EDX). It was found that the structure contained carbonitrides with a very high proportion of Cr and Ti.

Claims

PATENT CLAIMS

1. Hot rolled flat steel product with a tensile strength Rm of

at least 550 MPa, a hole expansion ratio l of at least 30% and a structure consisting of at least 90 area% ferrite and the remainder up to 10 area% pearlite or cementite, into the carbonitride precipitates with an average diameter of at most 10 nm are embedded, consisting of a steel consisting of (in% by weight) 0.02 - 0.3% C, <0.7% Si, <1, 0% AI, 0.2 - 2.5% Mn,

0.05 - 0.5% Cr, <0.02% P, <0.005% S, <0.01% N, <0.1% Cu and from at least one element from the group “Ti , Nb, V ”and the remainder consists of iron and unavoidable impurities with the following requirements:

- If V is present, the sum of the contents is T! _GEW on Ti and Nb _GE w on Nb limited to at most 0.01% by weight;

- in the event that Ti and / or Nb are present, the V content V _G EW is at most 0.01% by weight;

- for a total concentration

XGEW [% by weight] = TIGEW ⁺ (VQEW / 1, 06) + (NÖ _G EW / 1, 94) applies 0.02% ^ X _G E _W - 0.5%.

- for a quantitative ratio

Verhl = (XAT ⁺ Cr _A -r) / (CAT ⁺ NAT)

applies 0.5 <Verh1 <2.0, with XAT = TΪAT ⁺ VAT + NÖAT, where Tί _A t is the Ti content converted into atomic% Ti _GEW , V _{AT is} the V content converted to atomic% VGEW, Nb _A r is the atomic%

converted Nb content Nb _GEV v, Cr _AT the Cr content converted to atomic% Cr _GEW , C _AT the C content converted to atomic% C _GE w and N _AT the N content N converted to atomic% _GEW corresponds and the contents Ti _GE w, V _G w, Nb _GE w, Cr _GE w, C _GE w and N _GE w are each given in% by weight.

2. Flat steel product according to claim 1, dad u rch I marked net that applies to a quantitative ratio Verh2 = X _A us / Cr _A us

2 <Ver2 <20 with X _A us [atom%] = Ti _A us ⁺ V _A us ⁺ Nb _A us

and

Ti _AUS : Ti content of the precipitations in atomic%

V _A us: V content of the precipitations in atomic%

Nb _A us: Nb content of the precipitations in atomic%

Cr _AU s: Cr content of the precipitates in atomic%.

3. Flat steel product according to claim 1, d a d u r c h

ge characterized net that its Cr content is more than 0.08 wt .-%.

4. Flat steel product according to one of the preceding claims, characterized by the fact that the average grain size of its ferritic structure is 3 to 10 pm.

5. Flat steel product according to one of the preceding claims, characterized in that its Mn content is at least 1.3% by weight.

6. Flat steel product according to one of the preceding claims, characterized in that its Si content is at least 0.05% by weight.

7. Flat steel product according to one of the preceding claims, characterized in that the value XGEW is 0.05-0.25% by weight.

8. Flat steel product according to one of the preceding claims, characterized in that Verhl applies to the quantitative ratio

0.7 <ratio <1.5.

9. Flat steel product according to one of the preceding claims, characterized in that applies to the quantitative ratio Verh2

2.3 <Beh2 <15.

10. Flat steel product according to one of the preceding claims, characterized in that the average diameter of the

Excretions <5 nm and that for the quantitative ratio Verh2 applies 2.5 <Verh2 <5.

11. Flat steel product according to one of claims 1 to 10, d a d u r c h

characterized that the average diameter of the Excretions> 5 nm and that for the quantitative ratio Verh2 applies 5 <Verh2 <15.

12. Flat steel product according to one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t, that a s s its hole expansion ratio l

is at least 50%.

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

Flat steel product claims comprising the following

Working steps:

a) melting of a steel consisting of (in% by weight) 0.02-0.3% C, <0.7% Si, <1.0% Al, 0.2-2.5% Mn, 0 , 05 - 0.5% Cr, <0.02% P, <0.005%

S, <0.01% N, <0.1% Cu and at least one element forming the carbonitride precipitates from the group "Ti, Nb, V" and the rest of iron and unavoidable impurities with the following requirements:

- If V is present, the sum of the contents TI _G EW in Ti and Nb _GE w in Nb is limited to a maximum of 0.01% by weight;

- in the event that Ti and / or Nb are present, the V content VGEW is at most 0.01% by weight;

- for a total concentration formed from the respective Ti content TIGEW, the respective V content VGEW and the respective Nb content NÖGEW of the steel

XGEW [% by weight] = TIGEW ⁺ (VQEW / 1 _> 06) + (Nb _GE w / 1 _> 94) applies 0.02% ^ XGEW— 0.5%;

- for a quantitative ratio

Verhl - (CAT + ^ AT) I (CAT ⁺ NAT) applies 0.5 <Verhl <2.0, with XAT = TI _AT ⁺ VAT + N0AT!

where TT is the Ti content converted into atomic% TIGEW, V _A T is the V content converted into atomic% VGEW, Nb _{AT is} the atomic%

converted Nb content Nb _GE w, Cr _AT the Cr content converted into atomic% Cr _G Ew- C _A T the C content converted into atomic% CQEW and N _A T the N content converted into atomic% N _G EW corresponds and where the contents are TIGEW · VQEW _I NbGEw _> CTQEW. CQEW and NQEW are each given in% by weight;

b) casting the steel into a preliminary product, such as a slab, a thin slab or a cast strip;

c) tempering the preliminary product to a temperature of 1150-1350 ° C;

d) hot rolling the intermediate to the hot rolled

Flat steel product at a hot rolling end temperature of 880 - 980 ° C; e) cooling the hot-rolled flat steel product obtained at a cooling rate of 20-400 ° C./s to a

560 - 690 ° C reel temperature;

f) coiling the hot strip cooled to the coiling temperature into a coil.

14. The method according to claim 13, characterized in that the reel temperature is 590 - 650 ° C.

15. Component, in particular body component or chassis component, characterized in that it is formed from a

Flat steel product according to one of claims 1-12.