EP3847284B1 - Hot-rolled flat steel product and method for the production thereof - Google Patents
Hot-rolled flat steel product and method for the production thereof Download PDFInfo
- Publication number
- EP3847284B1 EP3847284B1 EP18773945.3A EP18773945A EP3847284B1 EP 3847284 B1 EP3847284 B1 EP 3847284B1 EP 18773945 A EP18773945 A EP 18773945A EP 3847284 B1 EP3847284 B1 EP 3847284B1
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- flat steel
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- 229910000831 Steel Inorganic materials 0.000 title claims description 158
- 239000010959 steel Substances 0.000 title claims description 158
- 238000004519 manufacturing process Methods 0.000 title claims description 15
- 238000000034 method Methods 0.000 title claims description 7
- 239000000047 product Substances 0.000 claims description 87
- 239000002244 precipitate Substances 0.000 claims description 43
- 229910052719 titanium Inorganic materials 0.000 claims description 31
- 229910052758 niobium Inorganic materials 0.000 claims description 26
- 229910052720 vanadium Inorganic materials 0.000 claims description 19
- 229910052757 nitrogen Inorganic materials 0.000 claims description 18
- 229910052804 chromium Inorganic materials 0.000 claims description 17
- 229910052750 molybdenum Inorganic materials 0.000 claims description 16
- 238000001816 cooling Methods 0.000 claims description 15
- 229910052721 tungsten Inorganic materials 0.000 claims description 14
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 13
- 239000012535 impurity Substances 0.000 claims description 13
- 238000005098 hot rolling Methods 0.000 claims description 12
- 229910000859 α-Fe Inorganic materials 0.000 claims description 12
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- 229910052710 silicon Inorganic materials 0.000 claims description 10
- 229910052782 aluminium Inorganic materials 0.000 claims description 9
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- 238000010438 heat treatment Methods 0.000 claims description 8
- 229910052698 phosphorus Inorganic materials 0.000 claims description 8
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- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 claims description 6
- 229910052742 iron Inorganic materials 0.000 claims description 5
- 229910052785 arsenic Inorganic materials 0.000 claims description 4
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- IUVCFHHAEHNCFT-INIZCTEOSA-N 2-[(1s)-1-[4-amino-3-(3-fluoro-4-propan-2-yloxyphenyl)pyrazolo[3,4-d]pyrimidin-1-yl]ethyl]-6-fluoro-3-(3-fluorophenyl)chromen-4-one Chemical compound C1=C(F)C(OC(C)C)=CC=C1C(C1=C(N)N=CN=C11)=NN1[C@@H](C)C1=C(C=2C=C(F)C=CC=2)C(=O)C2=CC(F)=CC=C2O1 IUVCFHHAEHNCFT-INIZCTEOSA-N 0.000 description 1
- 229910000851 Alloy steel Inorganic materials 0.000 description 1
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- OYPRJOBELJOOCE-UHFFFAOYSA-N Calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 229910000599 Cr alloy Inorganic materials 0.000 description 1
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- ZOKXTWBITQBERF-UHFFFAOYSA-N Molybdenum Chemical compound [Mo] ZOKXTWBITQBERF-UHFFFAOYSA-N 0.000 description 1
- 238000001016 Ostwald ripening Methods 0.000 description 1
- OAICVXFJPJFONN-UHFFFAOYSA-N Phosphorus Chemical compound [P] OAICVXFJPJFONN-UHFFFAOYSA-N 0.000 description 1
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- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 238000003723 Smelting Methods 0.000 description 1
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- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- RQNWIZPPADIBDY-UHFFFAOYSA-N arsenic atom Chemical compound [As] RQNWIZPPADIBDY-UHFFFAOYSA-N 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
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- 229910017052 cobalt Inorganic materials 0.000 description 1
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 description 1
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- 230000001771 impaired effect Effects 0.000 description 1
- 239000003999 initiator Substances 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- FZLIPJUXYLNCLC-UHFFFAOYSA-N lanthanum atom Chemical compound [La] FZLIPJUXYLNCLC-UHFFFAOYSA-N 0.000 description 1
- 229910000734 martensite Inorganic materials 0.000 description 1
- 238000002493 microarray Methods 0.000 description 1
- 238000000386 microscopy Methods 0.000 description 1
- 239000011733 molybdenum Substances 0.000 description 1
- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
- 230000008520 organization Effects 0.000 description 1
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- 229910052761 rare earth metal Inorganic materials 0.000 description 1
- 238000003303 reheating Methods 0.000 description 1
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- 239000007858 starting material Substances 0.000 description 1
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- 238000005496 tempering Methods 0.000 description 1
- 230000000930 thermomechanical effect Effects 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical compound [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 description 1
- 239000013585 weight reducing agent Substances 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
- 229910052845 zircon Inorganic materials 0.000 description 1
- GFQYVLUOOAAOGM-UHFFFAOYSA-N zirconium(iv) silicate Chemical compound [Zr+4].[O-][Si]([O-])([O-])[O-] GFQYVLUOOAAOGM-UHFFFAOYSA-N 0.000 description 1
Images
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the invention relates to a hot-rolled flat steel product with an optimized combination of high tensile strength Rm and a high hole expansion ratio ⁇ .
- the invention also relates to a method for producing such a hot-rolled flat steel product.
- m Y is the atomic mass of element Y
- i EW is the content (in % by weight)
- m i is the atomic mass of component i in the mixture with Z components.
- 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 durability for complex forming processes.
- the sensitivity of the steel to edge cracks, which can form during the forming of stamped sheets, is particularly important.
- the edge crack sensitivity of a steel is evaluated using the so-called "hole expansion test", in which a hole punched in a sheet metal sample is expanded with a mandrel until the first cracks form.
- Materials with very anisotropic or inhomogeneous microstructures are characterized by a relatively high sensitivity to edge cracks. These include dual-phase steels, which consist of hard (e.g. martensite) and soft (e.g. ferrite) phases and have a pronounced rolling texture.
- dual-phase steels consist of hard (e.g. martensite) and soft (e.g. ferrite) phases and have a pronounced rolling texture.
- 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.
- Such a steel is from EP 1 338 665 A1 known.
- the steel described there has a tensile strength of at least 550 MPa, which is said to be present in combination with high elongation and excellent expansion flangeability. Because of this combination of properties, such a steel is particularly suitable for the production of complex-shaped automobile chassis parts.
- the EP 1 338 665 A1 mentions that a hot-rolled steel strip that meets these requirements (in % by weight) contains ⁇ 0.15% C, 0.02 - 0.35% Ti and 0.05 to 0.7% Mo and should have a microstructure that should essentially consist of ferrite with single-phase and fine precipitations, which are dispersed in the ferrite matrix with a grain size of less than 10 nm.
- EP 1 338 665 A1 specifically mentioned hot-rolled sheet steel 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% Al, ⁇ 0.006% N, 0.02 - 0.10% Ti, 0.05 - 0.6% Mo and the balance Fe.
- the steel sheet composed in this way should have precipitations in its single-phase structure consisting of ferrite, the grain size of which is ⁇ 10 nm and of which 5 ⁇ 10 4 / ⁇ m 3 per volume unit are present in the structure.
- the steel flat products that meet these requirements and are marketed under the name "Nanohiten” have a ferritic structure in which TiC precipitations with a size of ⁇ 5 nm are embedded, which ensure the high strength and low edge cracking sensitivity of the steel.
- the fineness of the TiC precipitations is made possible by the addition of Mo, which prevents the TiC precipitations from becoming coarser.
- ⁇ is composed in part of the strain energy due to the mismatch between the carbides and the surrounding ferrite matrix. Reducing the strain energy promotes a coherent interface between the carbides and the ferrite matrix and prevents coarsening of the carbides.
- Mo or W atoms are incorporated into the carbide and replace the Ti, Nb or V atoms there. In this connection, it is known that both the inclusion of the Mo and W atoms lead to the reduction of the strain energy.
- a high-strength hot-rolled steel sheet which contains (in % by weight) 0.035 - 0.12% C, ⁇ 0.1% Si, ⁇ 1.2% Mn, ⁇ 0.03% P, ⁇ 0.005% S, ⁇ 0.1% Al , ⁇ 0.01% N, 0.14 - 0.30% Ti, ⁇ 3.0% Cu and the balance Fe and unavoidable impurities.
- the Cr content of the steel is adjusted to ⁇ 0.08% in order to achieve optimum corrosion resistance of the steel sheet, with Cr contents which are as low as possible being found to be particularly advantageous in this regard.
- the structure of the hot-rolled steel sheet composed in this way consists of at least 95% by area of ferrite with an average grain diameter of ⁇ 7 ⁇ m, with carbide precipitations with an average diameter of less than 8 nm being present in the structure and the structure containing less than 0.1 vol. -% contains cementite.
- the steel sheet produced in this way should have a tensile strength of at least 780 MPa.
- the invention was based on the object of specifying a hot-rolled flat steel product that can be produced cost-effectively, which has high strength and at the same time is suitable for forming into complex shaped steel parts, in particular components for the body and chassis of vehicles suitable.
- Sheet metal blanks made of steel with a tensile strength of 550 MPa and more are increasingly being used for the construction of body and chassis components.
- the sheet metal blanks are often subjected to particularly high deformation at their cut edges, which can lie on the outer edge of the sheet metal blank and/or on holes and recesses. These deformations can occur when using materials with heterogeneous microstructures such. B. dual-phase steels, lead to failure of the edges.
- a flat steel product should be provided which, due to its homogeneous microstructure, is particularly suitable for the production of body and chassis components and, due to its low sensitivity to edge cracking, makes it possible to design these components in such a way that the load to be carried by them is safely exceeded by the strong deformed edges can be initiated.
- the invention has achieved this object in that such a flat steel product has at least the features specified in claim 1 .
- a method that achieves the above-mentioned object according to the invention is specified in claim 13 .
- 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 ⁇ of at least 30%, with its microstructure consisting of at least 90% by area of ferrite and the remainder being up to 10% by area. consists of pearlite or cementite and carbonitride precipitates with an average diameter of no more than 10 nm are embedded in the structure of the steel flat product.
- the Ti OUT , V OUT , Nb OUT and Cr OUT contents of the precipitations, which are given in atomic %, can be determined, for example, by energy-dispersive X-ray micro-area analysis ("EDX").
- EDX energy-dispersive X-ray micro-area analysis
- 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 regularly also achieves tensile strengths of at least 780 MPa or even at least 960 MPa.
- the hole expansion ratio ⁇ determined according to ISO 16630:2017 for a flat steel product according to the invention is in each case at least 30%, in particular at least 50%, with hole expansion ratios ⁇ of at least 60% also being able to be represented.
- a flat steel product according to the invention has a particularly favorable ratio of hole expansion capacity to strength.
- high hole expansion ratios ⁇ are achieved even with high strengths. This is reflected in high values for the product Rm ⁇ of tensile strength Rm and hole expansion ⁇ .
- 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 microstructure of a flat steel product according to the invention consists of at least 90% by area of ferrite, a microstructure that is completely ferritic in the technical sense with carbonitride precipitations being particularly advantageous.
- the appropriate alloy layer, cementite and pearlite can form in the structure, the proportions of which can be up to 10% by area. may amount.
- the mean grain size of the structure is typically 3-10 ⁇ m, in particular 3-8 ⁇ m.
- the Cr-containing carbonitride precipitates embedded in the structure of a flat steel product according to the invention based on the contents of Ti, Nb or V provided according to the invention are present with an average particle diameter of at most 10 nm.
- the fineness of the precipitates makes a decisive contribution to the desired combination of high strength and good hole expansion capacity.
- the mean particle diameter of the precipitates present in the flat steel product according to the invention is therefore 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 flat steel product according to the invention.
- the carbon (C) provided in the alloy steel of the present invention is mainly fixed in the precipitates.
- the concentration of the C dissolved in the mixed crystal is thereby minimized.
- a C content of more than 0.02% by weight, in particular more than 0.05% by weight, is necessary 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, in turn, would lead to the formation of larger pearlite contents in the structure, which would reduce ductility and increase sensitivity to edge cracking.
- the C content is therefore limited to a maximum of 0.3% by weight, in particular a maximum of 0.15% by weight, with the negative effects 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.
- the contents of the elements determining the ratio Verh1 are set within the content ranges specified according to the invention for each of these elements such that the ratio Verh1 is 0.5 ⁇ Verh1 ⁇ 2.0, with the ratios Verh1 being 0.7-1.5 or 0.8-1.3 have turned out 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 by forming solid solutions. Mn also suppresses the formation of pearlite and cementite and in this way promotes the formation of Cr-containing carbonitride precipitations based on the contents of Ti, Nb or V provided according to the invention. For this reason, the steel according to the invention has an Mn content of 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.
- the upper limit of the Mn content is 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, the possible negative effects of the presence of Mn avoid particularly safe.
- 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.
- a content of at least 0.05% by weight of Si is required. Excessively high levels of Si would impair the surface quality of the flat steel product according to the invention.
- the Si content is therefore limited to a maximum of 0.7% by weight, with Si contents of up to 0.25% by weight, in particular up to 0.1% by weight, prove to be particularly favorable with regard to avoiding the negative influences of the presence of Si and also enable subsequent batch galvanizing of the product according to the invention. If there are special requirements for batch galvanizing, it is particularly preferable to dispense with an Si alloy and select a maximum Si content of 0.03% by weight.
- Si With contents of up to 0.7% by weight, Si also contributes to solid solution strengthening, so that higher Si contents can definitely be useful if lower requirements are placed on the surface quality and/or batch galvanizing ability. With Si contents above 0.7% by weight, however, the rollability of the steels according to the invention is adversely affected too much and growths can occur on the rolls during rolling.
- Aluminum (Al) can also be added as an optional element for suppressing the generation 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 usual production of the steel from which a flat steel product according to the invention is made. However, too high an Al content can have a negative effect on 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, particularly at most 0.5% by weight.
- Cr chromium
- Cr can be used to prevent the coarsening of the precipitates.
- This requires a Cr content of at least 0.05% by weight, preferably at least 0.06% by weight 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 can be specifically set in the steelworks. On the other hand, lower Cr contents are regarded as ineffective.
- the effectiveness of the addition of Cr provided according to the invention is also demonstrated by the fact that a low atomic ratio Verh2 forms in the precipitates.
- the atomic ratio of Ti to Cr in (Ti,Cr)(C,N) precipitates which are about 10 nm in size, is more than 10.
- Previous investigations have shown that when Mo or W for the formation of carbonitrides in precipitates with a size of about 10 nm, the atomic ratio of Ti to the Mo or W present in each case is at most 4. This shows that by including Cr in the precipitates in a steel flat product according to the invention, the strain energy of the precipitate is reduced more than when Mo or W is included.
- the upper limit of the content of Cr 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 here that the Cr contents of a flat steel product according to the invention are adjusted in such a way that no pure Cr carbides are present in the flat steel product according to the invention.
- micro-alloying elements titanium (Ti), niobium (Nb) and vanadium (V) are essential for the formation of precipitations in the structure of the flat steel product according to the invention.
- V vanadium
- the precipitates formed by Ti, Nb or V are not present in the flat steel product according to the invention as pure carbides, but rather as carbonitrides if nitrogen "N" is present in the alloy. 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.
- Ti or Nb are only added in contents of a maximum total of 0 0.01% by weight, preferably at most 0.005% by weight, which are in the range of 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 since the formation temperatures at which Ti or Nb precipitates form are close enough to allow timely precipitation of both elements. Therefore, when Ti or Nb is added, V is 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.
- the value X WEIGHT 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 micro-alloying elements is required for the desired strength. 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 the formation of undesired large precipitations. For these reasons, the Nb, Ti or V contents of the steel of a steel flat product according to the invention are advantageously adjusted in such a way that the value X GEW is not higher than 0.25% by weight.
- Phosphorus (P) is unfavorable for the weldability of a steel flat product according to the invention.
- the P contents of a flat steel product according to the invention are therefore limited to a maximum of 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 favorable.
- the S content must be limited to at most 0.005% by weight, particularly 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 impurity that is unavoidable due to production.
- the precipitations embedded in the structure of a steel flat product according to the invention are present as carbonitrides in the form of (Ti,Cr)(C,N); (Nb,Cr)(C,N); (V,Cr)(C,N) or (Ti,Nb,Cr)(C,N).
- nitrogen "N" is present, Ti, Nb and V in the simultaneous presence of C form nitrides or carbonitrides preferentially with N. Therefore, in practice, under the technically and economically viable conditions, the uptake of N in the excretions is unavoidable.
- the lowest possible N content should be aimed for, since N-dominated carbonitrides are often very coarse and angular, which is why they do not contribute to hardening but act as crack initiators.
- the upper limit of the N content is therefore set at 0.01% by weight, preferably 0.005% by weight.
- the upper limit of the Ca content according to the invention to 0.01% by weight, in particular at most 0.005% by weight, preferably at most 0.002% by weight.
- the contents of Mo and W are limited to a maximum of 0.05% by weight, in particular a maximum of 0.04% by weight, preferably 0.03% by weight, since these elements are present in a flat steel product according to the invention for the reasons explained above are not needed.
- 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 being slowed down by B segregated on them and thus the formation of Ti, Nb and V carbides and carbonitrides.
- the allowable upper limit of the Cu content in the flat steel product according to the invention is 0.1% by weight, in particular less than 0.04% by weight or less than 0.02% by weight.
- Ni, Sn, As, Co, Zr and rare earths, in particular La and/or Ce are also not required as alloying elements in the flat steel product according to the invention and, if they are nevertheless detectable in the flat steel product according to the invention, are among the unavoidable impurities.
- the Ni content is at most 0.1 wt%
- the Sn content is at most 0.05 wt%
- the As content is at most 0.02 wt%
- 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
- the content of the rare earth elements, such as La and Ce to a maximum of 0.002% by weight in each case %, in particular a maximum of 0.0002% by weight.
- O is also undesirable in the flat steel product according to the invention, since a Oxide coating, which can result from the presence of higher O contents, would have a negative effect both on the mechanics and on the castability and rollability of the steel flat product.
- the maximum permissible O content is therefore set at 0.005% by weight, preferably at 0.002% by weight.
- H is very mobile in the interstitial spaces in the steel and can lead to cracking in the core, particularly in high-strength steels when cooling from hot rolling.
- the H content of a flat steel product according to the invention should therefore be as low as possible, but in any case not more than 0.001% by weight, in particular not more than 0.0006% by weight or not more than 0.0004% by weight, with H Contents of at most 0.0002% by weight are particularly desirable.
- the steel alloyed according to the invention in accordance with the explanations given above in connection with the composition of a flat steel product according to the invention is, after it has been melted, cast into a preliminary product which, in the case of the classic production route, will be a slab of conventional dimensions.
- the steel can also be made into a continuous cast by direct hot rolling a thin slab as a preliminary product in a casting-rolling plant or a cast strip as a preliminary product in a strip casting plant.
- the precursor is heated to at least 1150 °C or kept at this temperature.
- a high heating temperature is required in order to dissolve carbides and nitrides already present in the pre-product. If the heating temperatures are too low, the alloying elements remain bound in the precipitations, so that no new precipitations can be formed. For economic reasons, the heating temperature is limited to a maximum of 1350 °C.
- the preliminary product is hot-rolled in a conventional manner, with the final temperature of hot-rolling having to be at least 880 °C. If the final hot rolling 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. End temperatures above 980 °C are technically not feasible.
- the hot-rolled steel strip leaving the hot-rolling train is cooled at a cooling rate of 20 - 400 °C/s to a coiling temperature in the range of 560 - 690 °C.
- a cooling rate of at least 20 °Cls is required to avoid as much as possible the formation of perlite and cementite. Cooling rates of more than 400°C/s are technically not feasible.
- Coiling 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, which means that the target properties of the flat steel product according to the invention can no longer be achieved.
- a targeted formation of small Precipitations whose size is less than 7 nm, in particular less than 5 nm, can be brought about by coiling temperatures of 580-670.degree. C., in particular 590-650.degree. At coiling temperatures below 580 °C, carbonitrides would no longer be separated and their strength-increasing effect would be absent.
- the atomic ratio Verh2 is of particular importance.
- a Verh2 ratio of 2-20 in the precipitates is required in order to prevent coarsening of the precipitates during cooling in the coil after coiling when producing a flat steel product according to the invention.
- the maximum size of the carbonitrides of at most 10 nm specified according to the invention and, associated with this, the minimum strength and the desired high hole expansion ratio could not be achieved.
- the ratio Verh2 is less than 2
- the edges of the precipitations have a significantly higher Cr content than the core of the precipitations.
- the “core” is defined as that area of the excretions that makes up approximately 50% of the area of the excretion in microscopy and is closest to the centroid of the area.
- the remaining area of the excretion is defined as the edge.
- the Cr content at the edge is significantly higher than in the core area and Cr no longer has any effect in terms of preventing coarsening of the precipitations.
- 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 that deviate from the requirements of the invention are underlined in Table 1 (steels B, D, E, F, N, O).
- the tensile strength Rm, the upper yield point ReH, the lower yield point ReL and the elongation A50 were determined on the hot-rolled steel strips obtained in this way in accordance with DIN EN ISO 6892-1:2017.
- the hole expansion ⁇ was determined according to ISO 16630:2017, the product Rm ⁇ ⁇ was formed and the average particle size of the precipitations and the ratio Verh2 were determined.
- Verh2 of X OUT (see Table 1) to Cr OUT (in atomic %) in the precipitates is given in Table 2.
- Verh2 increases with increasing coil temperature and increasing precipitate size.
- the correlation between Verh2 and the precipitation diameter indicates that Cr prevents the coarsening of the carbides.
- Example A is a reference composition used to study the effect of coiling temperature (see Examples A1 to A7). With this composition, the optimal mechanical-technological properties were achieved at coiling temperatures in the range of 590 - 650 °C.
- Steels G and H are also based on example steel A, but the ratio Verh1 was varied here.
- the variation of Verh1 in the range from 0.8 to 1.2 has no negative impact on the mechanical-technological properties under similar production specifications (see examples G14, G15, H17 and H18).
- the tests based on steels G and H were also used to study the influences of the heating temperature EWT and the finish rolling temperature ET on the properties of the steel strip obtained. Here it was shown that too low a heating temperature EWT or final rolling temperature ET leads to the formation of coarse precipitations or an anisotropic structure and thus 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 coiling temperatures (see Examples 119 to K28).
- Examples I to L contain different contents of Si and Al.
- the optimum mechanical and technological properties were achieved at coiling temperatures of 590 - 650 °C.
- Steels M to O have a similar X GEW concentration to steel A but different combinations of Ti, Nb and V.
- the steel strip produced in example M30 based on steel M (Ti and Nb) has comparable mechanical and technological properties to those Steel strips that have been produced on the basis of steel A at similar coiling temperatures.
- Steel P has a very high ratio Verh1.
- the steel strips consisting of steel P were produced in tests P33 - P35 with different cooling rates. If the cooling rate was too low, too much pearlite was formed, with the result that the mechanical-technological properties were severely impaired (see example P33).
- Steel Q has a high Al content but is otherwise relatively low alloyed.
- the steel R is relatively highly alloyed.
- Steel strips were produced from the steels Q and R under optimal conditions. These tests show that very different strengths can be achieved with high Rm ⁇ values, provided the ratio Verh1 is within the target range.
- Steel S was produced as a high-purity laboratory melt and processed with increased heating and hot-rolling temperatures as well as a very high cooling rate in order to validate the influences of varying manufacturing parameters.
- TEM transmission electron micrographs
- All samples show fine (Ti,Cr)(C,N) precipitates (darker particles).
- the diameter of the precipitates increases with increasing coiling temperature, so that the diameters of the particles in Examples A3, A4 and A5 are on average below 5 nm, this being the case particularly clearly in Example A3.
- the precipitates are not recognizable as individual particles and are in the form of agglomerates.
- Example A1 contained no (Ti,Cr)(C,N) precipitates.
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Description
Die Erfindung betrifft ein warmgewalztes Stahlflachprodukt mit einer optimierten Kombination aus hoher Zugfestigkeit Rm und einem hohem Lochaufweitungsverhältnis λ.The invention relates to a hot-rolled flat steel product with an optimized combination of high tensile strength Rm and a high hole expansion ratio λ.
Ebenso betrifft die Erfindung ein Verfahren zur Herstellung eines solchen warmgewalzten Stahlflachprodukts.The invention also relates to a method for producing such a hot-rolled flat steel product.
Wenn hier von Stahlflachprodukten die Rede ist, dann sind damit Walzprodukte, wie Stahlbänder oder Stahlbleche, sowie daraus gewonnene Zuschnitte und Platinen gemeint, deren Dicke wesentlich geringer ist als ihre Länge und Breite.If flat steel products are mentioned here, then this means rolled products such as steel strips or steel sheets, as well as blanks and blanks obtained from them, the thickness of which is significantly less than their length and width.
In diesem Text enthaltene Angaben zu den Gehalten von bestimmten Legierungselementen beziehen sich auf das Gewicht (Angabe in Gew.-% bzw. YGEW, wobei Y das Elementsymbol ist). Ebenso beziehen sich die aus diesen Gehaltsangaben gebildeten Summen oder Gesamtkonzentrationen auf das Gewicht (Angabe in Gew.-% bzw. YGEW), soweit nichts anderes ausdrücklich angegeben ist. In "Atom-%" bzw. "YAT" (wobei Y das Elementsymbol ist) gemachte Angaben zu Gehalten von Legierungselementen oder daraus gebildete Summanden beziehen sich jeweils auf die Atomanzahl.Information on the content of certain alloying elements contained in this text relates to the weight (in percent by weight or Y GEW , where Y is the element symbol). Likewise, the sums or total concentrations formed from these content details relate to the weight (in percent by weight or Y weight ), unless expressly stated otherwise. Statements made in "Atomic %" or "Y AT " (where Y is the element symbol) on the contents of alloying elements or summands formed therefrom relate in each case to the number of atoms.
Zur Umrechnung von Gew-% in Atom-% wird die übliche Formel verwendet,
Die Anteile, die bestimmte Bestandteile am Gefüge eines Stahlflachprodukts einnehmen, sind im vorliegenden Text bezogen auf die Fläche (Angabe in Flächen-%) angegeben, sofern nichts anderes ausdrücklich angegeben ist.The proportions that certain components make up in the structure of a flat steel product are given in the present text in relation to the area (specified in area %), unless expressly stated otherwise.
Sämtliche Angaben zu den Lochaufweitungseigenschaften der hier behandelten, aus dem Stand der Technik bekannten oder erfindungsgemäßen Stähle und daraus erzeugter Stahlflachprodukte sind gemäß der Norm ISO 16630:2017 der International Organization for Standardization (s. https://www.iso.org/standard/69771.html) ermittelt worden.All information on the hole expansion properties of the steels treated here, known from the prior art or according to the invention and steel flat products produced from them are in accordance with the ISO 16630:2017 standard of the International Organization for Standardization (see https://www.iso.org/standard/ 69771.html) has been determined.
Die hier angegebenen mechanischen Eigenschaften wie Zugfestigkeit Rm, Streckgrenzen ReH, ReL oder Dehnung A50, sind anhand der DIN EN ISO 6892-1 :2017 ermittelt worden, sofern nichts anderes angegeben ist.The mechanical properties specified here, such as tensile strength Rm, yield points ReH, ReL or elongation A50, have been determined using DIN EN ISO 6892-1:2017, unless otherwise stated.
Die steigende Nachfrage nach kraftstoffeffizienten Autos treibt einen steigenden Bedarf an Gewichtsreduzierung durch Leichtbau. Die geometrische Komplexität solcher Leichtbauteile fordert neue Stähle mit verbesserter Beständigkeit für komplexe Umformverfahren. Besonders wichtig ist die Empfindlichkeit der Stähle für Kantenrisse, die sich bei der Umformung von gestanzten Blechen bilden können. Im Labor wird die Kantenrissempfindlichkeit eines Stahls mit der sogenannten "Lochaufweitungsprüfung" bewertet, in welcher ein in eine Blechprobe gestanztes Loch mit einem Dorn bis zur ersten Rissbildung aufgeweitet wird.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 durability for complex forming processes. The sensitivity of the steel to edge cracks, which can form during the forming of stamped sheets, is particularly important. In the laboratory, the edge crack sensitivity of a steel is evaluated using the so-called "hole expansion test", in which a hole punched in a sheet metal sample is expanded with a mandrel until the first cracks form.
Materialien mit sehr anisotropen bzw. inhomogenen Mikrostrukturen zeichnen sich durch eine relative hohe Kantenrissempfindlichkeit aus. Dazu zählen Dualphasenstähle, die aus harten (z. B. Martensit) und weichen (z. B. Ferrit) Phasen bestehen und eine ausgeprägte Walztextur haben.Materials with very anisotropic or inhomogeneous microstructures are characterized by a relatively high sensitivity to edge cracks. These include dual-phase steels, which consist of hard (e.g. martensite) and soft (e.g. ferrite) phases and have a pronounced rolling texture.
Im Gegensatz dazu zeichnen sich Stahlflachprodukte mit einer isotropen bzw. homogenen Mikrostruktur durch eine relative niedrige Kantenrissempfindlichkeit aus. Zu diesen Stählen zählen Stähle mit ferritischem Gefüge, in das zur Festigkeitssteigerung sehr feine Ausscheidungen eingebettet sein können.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.
Ein Beispiel für einen solchen Stahl ist aus der
Der der Karbid-Vergröberung zu Grunde liegende Mechanismus lässt sich durch die sogenannte "Ostwald-Reifung" nach der Gleichung (1) beschreiben:
In dieser Gleichung bezeichnen
Neben dem voranstehend erläuterten Stand der Technik ist aus der
Vor dem Hintergrund des voranstehend erläuterten Standes der Technik lag der Erfindung die Aufgabe zu Grunde, ein kostengünstig herstellbares warmgewalztes Stahlflachprodukt anzugeben, das eine hohe Festigkeit aufweist und sich gleichzeitig für die Umformung zu komplex geformten Stahlformteilen, insbesondere Bauteilen für die Karosserie und das Fahrwerk von Fahrzeugen eignet. Für die Konstruktion von Karosserie- und Fahrwerksbauteilen werden immer häufiger Blechzuschnitte aus Stählen mit einer Zugfestigkeit von 550 MPa und mehr verwendet. Die Blechzuschnitte werden an ihren Schnittkanten, die an der Außenkante des Blechzuschnittes und / oder an Löchern und Aussparungen liegen können, häufig einer besonders hohen Verformung unterzogen. Diese Verformungen können bei Verwendung von Werkstoffen mit heterogenen Mikrostrukturen, wie z. B. Dualphasenstählen, zu einem Versagen der Kanten führen. Daher sollte ein Stahlflachprodukt bereitgestellt werden, das auf Grund seiner homogenen Mikrostruktur besonders für die Fertigung von Karosserie- und Fahrwerksbauteilen geeignet ist und es auf Grund seiner geringen Kantenrissempfindlichkeit ermöglicht, diese Bauteile so zu konstruieren, dass die von ihnen zu tragende Last sicher über die stark verformten Kanten eingeleitet werden kann.Against the background of the prior art explained above, the invention was based on the object of specifying a hot-rolled flat steel product that can be produced cost-effectively, which has high strength and at the same time is suitable for forming into complex shaped steel parts, in particular components for the body and chassis of vehicles suitable. Sheet metal blanks made of steel with a tensile strength of 550 MPa and more are increasingly being used for the construction of body and chassis components. The sheet metal blanks are often subjected to particularly high deformation at their cut edges, which can lie on the outer edge of the sheet metal blank and/or on holes and recesses. These deformations can occur when using materials with heterogeneous microstructures such. B. dual-phase steels, lead to failure of the edges. Therefore, a flat steel product should be provided which, due to its homogeneous microstructure, is particularly suitable for the production of body and chassis components and, due to its low sensitivity to edge cracking, makes it possible to design these components in such a way that the load to be carried by them is safely exceeded by the strong deformed edges can be initiated.
Darüber hinaus sollte ein Verfahren zur Herstellung eines solchen warmgewalzten Stahlflachprodukts genannt werden.In addition, a method for producing such a hot-rolled flat steel product should be mentioned.
In Bezug auf das Stahlflachprodukt hat die Erfindung diese Aufgabe dadurch gelöst, dass ein solches Stahlflachprodukt mindestens die in Anspruch 1 angegebenen Merkmale aufweist.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
Ein die voranstehend genannte Aufgabe erfindungsgemäß lösendes Verfahren ist in Anspruch 13 angegeben.A method that achieves the above-mentioned object according to the invention is specified in claim 13 .
Vorteilhafte Ausgestaltungen der Erfindung sind in den abhängigen Ansprüchen angegeben und werden nachfolgend wie der allgemeine Erfindungsgedanke im Einzelnen erläutert.Advantageous refinements of the invention are specified in the dependent claims and are explained in detail below, along with the general idea of the invention.
Ein erfindungsgemäßes warmgewalztes Stahlflachprodukt zeichnet sich demnach dadurch aus, dass es eine Zugfestigkeit Rm von mindestens 550 MPa und ein Lochaufweitungsverhältnis λ von mindestens 30 % aufweist, wobei sein Gefüge zu mindestens 90 Flächen-% aus Ferrit und als Rest zu bis zu 10 Flächen-% aus Perlit oder Zementit besteht und in das Gefüge des Stahlflachprodukts Karbonitrid-Ausscheidungen mit einem mittleren Durchmesser von höchstens 10 nm eingebettet sind.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 λ of at least 30%, with its microstructure consisting of at least 90% by area of ferrite and the remainder being up to 10% by area. consists of pearlite or cementite and carbonitride precipitates with an average diameter of no more than 10 nm are embedded in the structure of the steel flat product.
Dazu besteht das erfindungsgemäße Stahlflachprodukt aus einem Stahl, der aus (in Gew.-%)
- Im Fall, dass V vorhanden ist, ist die Summe der Gehalte TiGEW an Ti und NbGEW an Nb auf höchstens 0,01 Gew.-% beschränkt.
- Im Fall, dass Ti und/oder Nb vorhanden sind, beträgt der V-Gehalt VGEW höchstens 0,01 Gew.-%.
- Für eine aus dem jeweiligen Ti-Gehalt TiGEW, dem jeweiligen V-Gehalt VGEW und dem jeweiligen Nb-Gehalt NbGEW des Stahls gebildete Gesamtkonzentration
- Für ein
Mengenverhältnis - CrAT ist CrGEW umgerechnet in Atom-%,
- CAT ist CGEW umgerechnet in Atom-%,
- NAT ist NGEW umgerechnet in Atom-%, TiAT ist TiGEW umgerechnet in Atom-%,
- VAT ist VGEW umgerechnet in Atom-%,
- NbAT ist NbGEW umgerechnet in Atom-%.
- In the case where V is present, the sum of the contents Ti WT of Ti and Nb WT of Nb is limited to at most 0.01 wt%.
- In the case where Ti and/or Nb is present, the V content V GEW 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
- For a
ratio - Cr AT is Cr WEW converted to atomic %,
- C AT is C GEW converted to atomic %,
- N AT is N GEW converted to atomic %, Ti AT is Ti GEW converted to atomic %,
- V AT is V GEW converted to atomic %,
- Nb AT is Nb GEW converted to atomic %.
Optimale Eigenschaften eines erfindungsgemäßen Stahlflachprodukts ergeben sich hier, wenn für ein Mengenverhältnis
- TiAUS ist Ti-Gehalt der Ausscheidungen in Atom-%
- VAUS ist V-Gehalt der Ausscheidungen in Atom-%
- NbAUS ist Nb-Gehalt der Ausscheidungen in Atom-%
- CrAUS ist Cr-Gehalt der Ausscheidungen in Atom-%.
- Ti AUS is Ti content of precipitates in atomic %
- V OUT is V content of the precipitates in atomic %
- Nb AUS is Nb content of the precipitates in atomic %
- Cr OUT is Cr content of the precipitates in atomic %.
Die jeweils in Atom-% angegebenen Gehalte TiAUS, VAUS, NbAUS und CrAUS der Ausscheidungen lassen sich beispielsweise durch energiedispersive Röntgenmikrobereichsanalyse ("EDX") bestimmen.The Ti OUT , V OUT , Nb OUT and Cr OUT contents of the precipitations, which are given in atomic %, can be determined, for example, by energy-dispersive X-ray micro-area analysis ("EDX").
Ein erfindungsgemäßes Stahlflachprodukt weist dabei eine hohe Zugfestigkeit Rm von mindestens 550 MPa, insbesondere mindestens 660 MPa auf, wobei es regelmäßig auch Zugfestigkeiten von mindestens 780 MPa oder sogar mindestens 960 MPa erreicht.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 regularly also achieves tensile strengths of at least 780 MPa or even at least 960 MPa.
Das für ein erfindungsgemäßes Stahlflachprodukt gemäß ISO 16630:2017 ermittelte Lochaufweitungsverhältnis λ beträgt dabei jeweils mindestens 30 %, insbesondere mindestens 50 %, wobei sich auch Lochaufweitungsverhältnisse λ von mindestens 60 % darstellen lassen.The hole expansion ratio λ determined according to ISO 16630:2017 for a flat steel product according to the invention is in each case at least 30%, in particular at least 50%, with hole expansion ratios λ of at least 60% also being able to be represented.
Hierbei hat sich herausgestellt, dass ein erfindungsgemäßes Stahlflachprodukt ein besonderes günstiges Verhältnis von Lochaufweitungsvermögen zu Festigkeit aufweist. So werden bei einem erfindungsgemäßen Stahlflachprodukt auch bei hohen Festigkeiten hohe Lochaufweitungsverhältnisse λ erzielt. Dies äußert sich in hohen Werten für das Produkt Rm·λ aus Zugfestigkeit Rm und Lochaufweitung λ. Hier erreicht ein erfindungsgemäßes Stahlflachprodukt regelmäßig Rm·λ-Werte von mindestens 30.000 MPa·%, insbesondere mindestens 40.000 MPa·% oder sogar mindestens 50.000 MPa·%.It has been found here that a flat steel product according to the invention has a particularly favorable ratio of hole expansion capacity to strength. Thus, in a steel flat product according to the invention, high hole expansion ratios λ are achieved even with high strengths. This is reflected in high values for the product Rm·λ of tensile strength Rm and hole expansion λ. Here, a flat steel product according to the invention regularly achieves Rm·λ values of at least 30,000 MPa·%, in particular at least 40,000 MPa·% or even at least 50,000 MPa·%.
Das Gefüge eines erfindungsgemäßen Stahlflachprodukts besteht zu mindestens 90 Flächen-% aus Ferrit, wobei ein im technischen Sinne vollständig ferritisches Gefüge mit Karbonitrid-Ausscheidungen besonders vorteilhaft ist. Jedoch kann es bei entsprechender Legierungslage zur Bildung von Zementit und Perlit im Gefüge kommen, deren Anteile bis zu 10 Flächen-% betragen dürfen. Die mittlere Korngröße des Gefüges liegt typischerweise bei 3 - 10 µm, insbesondere 3 - 8 µm.The microstructure of a flat steel product according to the invention consists of at least 90% by area of ferrite, a microstructure that is completely ferritic in the technical sense with carbonitride precipitations being particularly advantageous. However, with the appropriate alloy layer, cementite and pearlite can form in the structure, the proportions of which can be up to 10% by area. may amount. The mean grain size of the structure is typically 3-10 μm, in particular 3-8 μm.
Die im Gefüge eines erfindungsgemäßen Stahlflachprodukts eingebetteten Cr-haltigen Karbonitrid-Ausscheidungen auf Basis der erfindungsgemäß vorgesehenen Gehalte an Ti, Nb oder V liegen mit einem mittleren Partikeldurchmesser von höchstens 10 nm vor. Die Feinheit der Ausscheidungen trägt entscheidend zur angestrebten Kombination aus hoher Festigkeit und gutem Lochaufweitungsvermögen bei. Daher beträgt der mittlere Partikeldurchmesser der im erfindungsgemäßen Stahlflachprodukt vorhandenen Ausscheidungen bevorzugt höchstens 7 nm, insbesondere höchstens 5 nm.The Cr-containing carbonitride precipitates embedded in the structure of a flat steel product according to the invention based on the contents of Ti, Nb or V provided according to the invention are present with an average particle diameter of at most 10 nm. The fineness of the precipitates makes a decisive contribution to the desired combination of high strength and good hole expansion capacity. The mean particle diameter of the precipitates present in the flat steel product according to the invention is therefore preferably at most 7 nm, in particular at most 5 nm.
Die erfindungsgemäß ausgewählte Legierung legt den Grundstein für die optimierten Eigenschaften eines erfindungsgemäßen warmgewalzten Stahlflachprodukts.The alloy selected according to the invention lays the foundation for the optimized properties of a hot-rolled flat steel product according to the invention.
Der in der erfindungsgemäßen Stahllegierung vorgesehene Kohlenstoff (C) wird hauptsächlich in den Ausscheidungen abgebunden. Die Konzentration des im Mischkristall gelösten C wird dabei minimiert. Ein C-Gehalt von mehr als 0,02 Gew.-%, insbesondere mehr als 0,05 Gew.-%, ist erforderlich, um eine hohe Ausscheidungsdichte zu erreichen und so die geforderte Zugfestigkeit von mindestens 550 MPa zu erreichen. Ein zu hoher C-Gehalt wiederum würde zur Bildung von größeren Perlitgehalten im Gefüge führen, durch die die Duktilität verringert und die Kantenrissempfindlichkeit erhöht würde. Daher ist der C-Gehalt auf höchstens 0,3 Gew.-%, insbesondere höchstens 0,15 Gew.-%, beschränkt, wobei sich negative Einflüsse der Anwesenheit von C dadurch besonders sicher vermeiden lassen, wenn der C-Gehalt des Stahls höchstens 0,10 Gew.-% beträgt.The carbon (C) provided in the alloy steel of the present invention is mainly fixed in the precipitates. The concentration of the C dissolved in the mixed crystal is thereby minimized. A C content of more than 0.02% by weight, in particular more than 0.05% by weight, is necessary 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, in turn, would lead to the formation of larger pearlite contents in the structure, which would reduce ductility and increase sensitivity to edge cracking. The C content is therefore limited to a maximum of 0.3% by weight, in particular a maximum of 0.15% by weight, with the negative effects 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.
Zu beachten ist dabei jeweils, dass der C-Gehalt im Rahmen der hier gemachten Vorgaben so eingestellt werden muss, dass die erfindungsgemäß vorgeschriebenen Maßgaben für das Mengenverhältnis Verh1 eingehalten werden.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 according to the invention prescribed stipulations for the quantity ratio Verh1 are complied with.
Konkret werden die Gehalte der das Mengenverhältnis Verh1 bestimmenden Elemente innerhalb der jeweils zu diesen Elementen erfindungsgemäß vorgegebenen Gehaltsbereiche so eingestellt, dass für das Mengenverhältnis Verh1 gilt 0,5 ≤ Verh1 ≤ 2,0, wobei sich Mengenverhältnisse Verh1 von 0,7-1,5 oder 0,8 - 1,3 als besonders günstig im Hinblick auf die angestrebten Eigenschaften eines erfindungsgemäßen Stahlflachprodukts herausgestellt haben.Specifically, the contents of the elements determining the ratio Verh1 are set within the content ranges specified according to the invention for each of these elements such that the ratio Verh1 is 0.5≦Verh1≦2.0, with the ratios Verh1 being 0.7-1.5 or 0.8-1.3 have turned out to be particularly favorable with regard to the desired properties of a flat steel product according to the invention.
Mangan (Mn) ist ein Element, welches durch Mischkristallbildung zur Festigkeit des Stahls beiträgt. Mn unterdrückt zudem die Bildung von Perlit und Zementit und fördert auf diese Weise die Entstehung von Cr-haltigen Karbonitrid-Ausscheidungen auf Basis der erfindungsgemäß vorgesehenen Gehalte an Ti, Nb oder V. Aus diesem Grund ist im erfindungsgemäßen Stahl ein Mn-Gehalt von mindestens 0,2 Gew.-%, insbesondere mehr als 0,3 Gew.-%, bevorzugt mindestens 0,5 Gew.-%, besonders bevorzugt 1,0 Gew.-% oder 1,3 Gew.-%, vorgesehen. Eine zu hohe Mn-Konzentration wirkt sich allerdings negativ auf die Schweißbarkeit aus und erhöht das Risiko des Auftretens starker Seigerungen. Daher ist die Obergrenze des Mn-Gehalts auf höchstens 2,5 Gew.-% gesetzt, wobei geringere Mn-Gehalte von höchstens 2,0 Gew.-%, insbesondere höchstens 1,7 Gew.-%, die möglichen negativen Auswirkungen der Anwesenheit von Mn besonders sicher vermeiden.Manganese (Mn) is an element that contributes to the strength of steel by forming solid solutions. Mn also suppresses the formation of pearlite and cementite and in this way promotes the formation of Cr-containing carbonitride precipitations based on the contents of Ti, Nb or V provided according to the invention. For this reason, the steel according to the invention has an Mn content of 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. However, too high a Mn concentration has a negative effect on weldability and increases the risk of severe segregation occurring. Therefore, the upper limit of the Mn content is 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, the possible negative effects of the presence of Mn avoid particularly safe.
Silizium (Si) kann optional in Gehalten zur Unterdrückung der Entstehung von Perlit im Gefüge eines erfindungsgemäßen Stahlflachprodukts zugegeben werden. Um diese Wirkung von Si zu erzielen, ist ein Gehalt von mindestens 0,05 Gew.-% Si erforderlich. Zu hohe Gehalte an Si würden die Oberflächenqualität des erfindungsgemäßen Stahlflachprodukts beeinträchtigen. Daher ist der Si-Gehalt auf max. 0,7 Gew.-% beschränkt, wobei Si-Gehalte von bis zu 0,25 Gew.-%, insbesondere bis zu 0,1 Gew.-%, sich im Hinblick auf die Vermeidung negativer Einflüsse der Anwesenheit von Si als besonders günstig erweisen und zudem ein späteres Stückverzinken des erfindungsgemäßen Produktes ermöglichen. Wenn besondere Ansprüche an die Stückverzinkungsfähigkeit vorliegen, wird besonders bevorzugt auf eine Si-Legierung verzichtet und ein maximaler Si-Gehalt von 0,03 Gew.-% gewählt. Bei Gehalten bis zu 0,7 Gew.-% trägt Si auch zur Mischkristallverfestigung bei, so dass höhere Si-Gehalte durchaus zweckmäßig sein können, wenn geringere Anforderungen an die Oberflächenqualität und/oder Stückverzinkungsfähigkeit gestellt werden. Bei Si-Gehalten, die oberhalb von 0,7 Gew.-% liegen, wird die Walzbarkeit der erfindungsgemäßen Stähle jedoch zu stark negativ beeinflusst und es kann bei der Walzbearbeitung zu Aufwachsungen auf den Walzen kommen.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. Excessively high levels of Si would impair the surface quality of the flat steel product according to the invention. The Si content is therefore limited to a maximum of 0.7% by weight, with Si contents of up to 0.25% by weight, in particular up to 0.1% by weight, prove to be particularly favorable with regard to avoiding the negative influences of the presence of Si and also enable subsequent batch galvanizing of the product according to the invention. If there are special requirements for batch galvanizing, it is particularly preferable to dispense with an Si alloy and select a maximum Si content of 0.03% by weight. With contents of up to 0.7% by weight, Si also contributes to solid solution strengthening, so that higher Si contents can definitely be useful if lower requirements are placed on the surface quality and/or batch galvanizing ability. With Si contents above 0.7% by weight, however, the rollability of the steels according to the invention is adversely affected too much and growths can occur on the rolls during rolling.
Aluminium (Al) kann ebenfalls als optionales Element zur Unterdrückung der Entstehung von Perlit zugegeben. Weil Al üblicher Weise zur Desoxidation der Schmelze verwendet wird, ist bei üblicher Erzeugung des Stahls, aus dem ein erfindungsgemäßes Stahlflachprodukt besteht, ein Al-Gehalt von mindestens 0,01 Gew.-% unvermeidbar. Ein zu hoher Al-Gehalt kann sich jedoch negativ auf die Gießbarkeit auswirken. Daher wird die Obergrenze des Al-Gehalts auf höchstens 1,0 Gew.-%, bevorzugt höchstens 0,7 Gew.-%, insbesondere höchstens 0,5 Gew.-%, beschränkt.Aluminum (Al) can also be added as an optional element for suppressing the generation 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 usual production of the steel from which a flat steel product according to the invention is made. However, too high an Al content can have a negative effect on 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, particularly at most 0.5% by weight.
Die Anwesenheit von Chrom (Cr) in Gehalten von 0,05 - 0,5 Gew.-% ist von entscheidender Bedeutung für die Erfindung. So kann Cr nach den Erkenntnissen der Erfindung genutzt werden, um die Vergröberung der Ausscheidungen zu verhindern. Hierfür wird ein Gehalt von mindestens 0,05 Gew.-% Cr, bevorzugt mindestens 0,06 Gew.-% Cr oder, besonders bevorzugt, mehr als 0,08 Gew.-% oder mindestens 0,10 Gew.-%, benötigt. Die Nachweisgrenze für Cr in Stählen der erfindungsgemäßen Art liegt im Bereich von 0,03 Gew.-%, wogegen Cr-Gehalte von mindestens 0,05 Gew.-% im Stahlwerk gezielt eingestellt werden können. Darunter liegende Cr-Gehalte werden hingegen als unwirksam angesehen.The presence of chromium (Cr) in amounts of 0.05-0.5% by weight is of crucial importance for the invention. Thus, according to the findings of the invention, Cr can be used to prevent the coarsening of the precipitates. This requires a Cr content of at least 0.05% by weight, preferably at least 0.06% by weight 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 can be specifically set in the steelworks. On the other hand, lower Cr contents are regarded as ineffective.
Überraschender Weise hat sich gezeigt, dass der Einfluss von Cr auf die Karbidvergröberung stärker ist als der von Mo und W bei gleichem Anteil in Atom-%. Auf diese Weise können bei Verwendung von Cr die gleichen mechanischen Eigenschaften wie bei Zugabe von Mo und W erzielt werden, jedoch sind dazu geringere Cr-Legierungsgehalte erforderlich. Dies gilt insbesondere bei Umrechnung der Legierungsgehalte in Gew.-%, da die Atommassen von Mo und W höher sind als die von Cr.Surprisingly, it has been shown that the influence of Cr on carbide coarsening is greater than that of Mo and W with the same atomic percentage. In this way, when using Cr, the same mechanical properties can be achieved as when adding Mo and W, but lower Cr alloy contents are required. This applies in particular when converting the alloy contents into wt.%, since the atomic masses of Mo and W are higher than those of Cr.
Die Effektivität der erfindungsgemäß vorgesehenen Zugabe an Cr wird auch dadurch belegt, dass sich ein niedriges Atomverhältnis Verh2 in den Ausscheidungen bildet. So beträgt das Atomverhältnis von Ti zu Cr in (Ti,Cr)(C,N)-Ausscheidungen, die eine Größe von ca. 10 nm haben, beispielsweise mehr als 10. So haben frühere Untersuchungen gezeigt, dass bei der ausschließlichen Verwendung von Mo oder W zur Bildung von Karbonitriden in Ausscheidungen mit einer Größe von ca. 10 nm das Atomverhältnis von Ti zum jeweils anwesenden Mo oder W höchstens 4 beträgt. Dies zeigt, dass dadurch, dass bei einem erfindungsgemäßen Stahlflachprodukt Cr in die Ausscheidungen aufgenommen worden ist, die Formänderungsenergie der Ausscheidung stärker reduziert wird als bei der Aufnahme von Mo oder W.The effectiveness of the addition of Cr provided according to the invention is also demonstrated by the fact that a low atomic ratio Verh2 forms in the precipitates. For example, the atomic ratio of Ti to Cr in (Ti,Cr)(C,N) precipitates, which are about 10 nm in size, is more than 10. Previous investigations have shown that when Mo or W for the formation of carbonitrides in precipitates with a size of about 10 nm, the atomic ratio of Ti to the Mo or W present in each case is at most 4. This shows that by including Cr in the precipitates in a steel flat product according to the invention, the strain energy of the precipitate is reduced more than when Mo or W is included.
Im Umkehrschluss bedeutet dies, dass weniger Cr-Atome für den gewünschten Effekt benötigt werden als dies bei der Verwendung von Mo oder W der Fall wäre. Ein Überschuss von Cr erhöht allerdings die Gefahr einer ausgeprägten Korngrenzenoxidation. Daher ist die Obergrenze der Gehalte an Cr bei einem erfindungsgemäßen Stahlflachprodukt auf höchstens 0,5 Gew.%, bevorzugt höchstens 0,25 Gew.-% oder höchstens 0,15 Gew.-%, gesetzt. Zu beachten ist dabei, dass die Cr-Gehalte eines erfindungsgemäßen Stahlflachprodukts so eingestellt sind, dass im erfindungsgemäßen Stahlflachprodukt keine reinen Cr-Karbide vorliegen.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 content of Cr 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 here that the Cr contents of a flat steel product according to the invention are adjusted in such a way that no pure Cr carbides are present in the flat steel product according to the invention.
Durch den Einsatz von Cr an Stelle der im Stand der Technik verwendeten Elemente Mo bzw. W wird zudem die Temperatur abgesenkt, bei der der ausscheidungsstabilisierende Effekt eintritt. Dies erhöht die Sicherheit, dass eine ausreichende Anzahl an Ausscheidungen zunächst gebildet werden kann, bevor die Behinderung einer Vergröberung der Ausscheidungen einsetzt. Dieser Zusammenhang wird als ursächlich für die oben beschriebene höhere Wirksamkeit von Cr in Bezug auf die benötigten Atomanteile gesehen.The use of Cr instead of the elements Mo or W used in the prior art also lowers the temperature at which the precipitation-stabilizing effect occurs. This increases the certainty that a sufficient number of precipitates can initially be formed before coarsening of the precipitates is prevented. This relationship is seen as the cause of the higher effectiveness of Cr described above in relation to the required atomic proportions.
Die Mikrolegierungselemente Titan (Ti), Niob (Nb) und Vanadium (V) sind für die Bildung der Ausscheidungen im Gefüge des erfindungsgemäßen Stahlflachprodukts wesentlich. Indem die Gehalte an V, Ti und Nb so eingestellt werden, dass die Gesamtkonzentration
Die von Ti, Nb oder V gebildeten Ausscheidungen liegen im erfindungsgemäßen Stahlflachprodukt nicht als reine Karbide, sondern, falls Stickstoff "N" in der Legierung vorhanden ist, regelmäßig als Karbonitride vor. Es ist bekannt, dass Karbide und Nitride, die mit Ti, Nb und V gebildet werden, sehr unterschiedliche Löslichkeiten im Austenit und im Ferrit haben. Deshalb bilden sie sich bei sehr unterschiedlichen Temperaturen.The precipitates formed by Ti, Nb or V are not present in the flat steel product according to the invention as pure carbides, but rather as carbonitrides if nitrogen "N" is present in the alloy. 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.
Um dennoch eine homogene Verteilung des erfindungsgemäß zugegebenen Cr in den Karbiden zu sichern und damit die Ausscheidungsvergröberung wirksam zu verhindern, werden im Fall, dass hinsichtlich der Eigenschaften des Stahls effektive Gehalte an V zugegeben werden, Ti oder Nb nur in Gehalten von in Summe höchstens 0,01 Gew.-%, bevorzugt höchstens 0,005 Gew-% zulegiert, die im Bereich der unvermeidbaren Verunreinigungen liegen, in denen weder Ti noch Nb einen Effekt auf die Eigenschaften des Stahls haben. Ti und Nb können hingegen alleine oder gemeinsam zugegeben werden, da die Bildungstemperaturen, bei denen sich Ti- oder Nb-Ausscheidungen bilden, nahe genug beieinander liegen, um eine zeitnahe Ausscheidung beider Elemente zu ermöglichen. Bei Zugabe von Ti oder Nb wird deshalb V nur als unvermeidbare Verunreinigung geduldet, die in Gehalten von bis zu 0,01 Gew.-%, bevorzugt bis zu 0,005 Gew-%, vorliegen kann.However, in order to ensure a homogeneous distribution of the Cr added according to the invention in the carbides and thus effectively prevent the coarsening of the precipitation, in the event that effective contents of V are added with regard to the properties of the steel, Ti or Nb are only added in contents of a maximum total of 0 0.01% by weight, preferably at most 0.005% by weight, which are in the range of unavoidable impurities in which neither Ti nor Nb have an effect on the properties of the steel. Ti and Nb on the other hand, can be added alone or together since the formation temperatures at which Ti or Nb precipitates form are close enough to allow timely precipitation of both elements. Therefore, when Ti or Nb is added, V is 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.
Um negative Auswirkungen zu hoher Gehalte an Mikrolegierungselementen zu vermeiden, ist der Wert XGEW auf 0,5 Gew.-% beschränkt. Auf diese Weise wird vermieden, dass beispielsweise erhöhte Nb-Gehalte zu Rissbildungen beim Stranggießen oder bei der Brammenabkühlung oder -wiedererwärmung führen. Gleichzeitig wird für die angestrebten Festigkeiten nur ein bestimmter Gehalt an Mikrolegierungselementen benötigt. Wird dieser überschritten, erfolgt nur noch eine geringfügige weitere Festigkeitssteigerung. Zudem sinken die mittleren Diffusionsabstände, wodurch die Gefahr der Bildung unerwünschter großer Ausscheidungen steigt. Aus diesen Gründen werden die Nb-, Ti- oder V-Gehalte des Stahls eines erfindungsgemäßen Stahlflachprodukts vorteilhafterweise so eingestellt, dass der Wert XGEW nicht höher als 0,25 Gew.-% liegt.In order to avoid negative effects of too high levels of micro-alloying elements, the value X WEIGHT 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 micro-alloying elements is required for the desired strength. 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 the formation of undesired large precipitations. For these reasons, the Nb, Ti or V contents of the steel of a steel flat product according to the invention are advantageously adjusted in such a way that the value X GEW is not higher than 0.25% by weight.
Phosphor (P) ist ungünstig für die Schweißbarkeit eines erfindungsgemäßen Stahlflachprodukts. Die P-Gehalte eines erfindungsgemäßen Stahlflachprodukts sind daher auf höchstens 0,02 Gew.-%, insbesondere weniger als 0,02 Gew.-%, beschränkt, wobei P-Gehalte von höchstens 0,010 Gew.-%, insbesondere weniger als 0,005 Gew.-%, besonders günstig sind.Phosphorus (P) is unfavorable for the weldability of a steel flat product according to the invention. The P contents of a flat steel product according to the invention are therefore limited to a maximum of 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 favorable.
Schwefel (S) führt bei ausreichend hohen Konzentrationen zur Bildung von MnS oder (Mn, Fe)S, welches sich negativ auf die Dehnung auswirkt. Daher muss der S-Gehalt auf höchstens 0,005 Gew.-%, insbesondere weniger als 0,003 Gew.-%, bevorzugt weniger als 0,0015 Gew.-%, beschränkt werden.At sufficiently high concentrations, sulfur (S) leads to the formation of MnS or (Mn,Fe)S, which has a negative effect on elongation. Therefore, the S content must be limited to at most 0.005% by weight, particularly less than 0.003% by weight, preferably less than 0.0015% by weight.
N ist im erfindungsgemäßen Stahlflachprodukt als herstellungsbedingt unvermeidbare Verunreinigung vorhanden. Die im Gefüge eines erfindungsgemäßen Stahlflachprodukts eingebetteten Ausscheidungen liegen als Karbonitride in Form von (Ti,Cr)(C,N); (Nb,Cr)(C,N); (V,Cr)(C, N) oder (Ti,Nb,Cr)(C, N) vor. Sofern Stickstoff "N" anwesend ist, bilden Ti, Nb und V bei gleichzeitiger Anwesenheit von C vorzugsweise mit N Nitride oder Karbonitride. Deswegen ist in der Praxis unter den dort technisch und wirtschaftlich darstellbaren Bedingungen die Aufnahme von N in den Ausscheidungen unvermeidbar. Grundsätzlich sind aber möglichst geringe N-Gehalte anzustreben, da N-dominierte Karbonitride oft sehr grob und eckig sind, weshalb sie nicht zur Verfestigung beitragen, sondern als Rissinitiatoren wirken. Um die Bildung von N-dominierten Karbonitriden zu vermeiden, ist deshalb die Obergrenze des N-Gehalts auf 0,01 Gew.-%, bevorzugt 0,005 Gew.-%, festgesetzt.N is present in the flat steel product according to the invention as an impurity that is unavoidable due to production. The precipitations embedded in the structure of a steel flat product according to the invention are present 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 in the simultaneous presence of C form nitrides or carbonitrides preferentially with N. Therefore, in practice, under the technically and economically viable conditions, the uptake of N in the excretions is unavoidable. In principle, however, the lowest possible N content should be aimed for, since N-dominated carbonitrides are often very coarse and angular, which is why they do not contribute to hardening 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.
Kalzium (Ca), Bor (B), Kupfer (Cu), Molybdän (Mo), Wolfram (W), Nickel (Ni), Zinn (Sn), Arsen (As), Kobalt (Co), Zirkon (Zr), Lanthan (La) und/oder Cer (Ce), Sauerstoff (O) und Wasserstoff (H) sind wie alle anderen hier nicht explizit angeführten denkbaren Legierungselemente den herstellungstechnisch unvermeidbaren Verunreinigungen zuzurechnen, die als Bestandteile des Ausgangsmaterials, aus dem der Stahl erzeugt wird, oder prozessbedingt bei der Stahlerschmelzung und Verarbeitung in den Stahl gelangen. Die Gehalte an diesen Elementen sind so gering zu halten, dass sie keine technische Wirkung in Bezug auf die Eigenschaften des erfindungsgemäßen Stahlflachprodukts haben.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), like all other conceivable alloying elements not explicitly listed here, are to be attributed to the production-related unavoidable impurities which, as components of the starting material from which the steel is produced, or get into the steel during steel smelting and processing. The contents of these elements should be kept so low that they have no technical effect on the properties of the flat steel product according to the invention.
Ca wird bei der Stahlerzeugung üblicherweise der Schmelze sowohl zur Desoxidation und Entschwefelung, als auch zur Verbesserung der Gießbarkeit hinzugegeben. Eine zu hohe Konzentration von Ca kann jedoch zur Bildung von unerwünschten Einschlüsse führen, welche sich negativ auf die Mechanik und die Walzbarkeit auswirken. Daher ist die Obergrenze des Ca-Gehalts erfindungsgemäß auf 0,01 Gew.-%, insbesondere höchstens 0,005 Gew.-%, bevorzugt höchstens 0,002 Gew.-%, eingeschränkt.In steel production, Ca is usually added to the melt for deoxidation and desulfurization as well as to improve castability. However, too high a concentration of Ca can lead to the formation of undesirable inclusions, which have a negative effect on the mechanics and rollability. Therefore, the upper limit of the Ca content according to the invention to 0.01% by weight, in particular at most 0.005% by weight, preferably at most 0.002% by weight.
Die Gehalte an Mo und W sind auf höchstens 0,05 Gew.-%, insbesondere höchstens 0,04 Gew.-%, bevorzugt 0,03 Gew.-%, beschränkt, da diese Elemente aus den voranstehend erläuterten Gründen in einem erfindungsgemäßen Stahlflachprodukt nicht benötigt werden.The contents of Mo and W are limited to a maximum of 0.05% by weight, in particular a maximum of 0.04% by weight, preferably 0.03% by weight, since these elements are present in a flat steel product according to the invention for the reasons explained above are not needed.
Die Gehalte an B dürfen 0,002 Gew.-%, insbesondere 0,001 Gew.-%, bevorzugt 0,0005 Gew.-%, nicht überschreiten, um zu verhindern, dass die Bewegung der Phasengrenzen durch an ihnen segregiertes B gebremst und dadurch die Bildung von Karbiden und Karbonitriden von Ti, Nb und V behindert wird.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 being slowed down by B segregated on them and thus the formation of Ti, Nb and V carbides and carbonitrides.
Cu kann sich als grobe Partikel ausscheiden, welche sich negativ auf die mechanischen Eigenschaften auswirken. Zudem hat Cu einen negativen Einfluss auf die Gießbarkeit. Um jeden Einfluss von Cu zu vermeiden, beträgt die zulässige Obergrenze des Cu-Gehalts im erfindungsgemäßen Stahlflachprodukt 0,1 Gew.-%, insbesondere weniger als 0,04 Gew.-% oder weniger als 0,02 Gew.-%.Cu can separate out as coarse particles, which have a negative effect on the mechanical properties. In addition, Cu has a negative impact on castability. In order to avoid any influence of Cu, the allowable upper limit of the Cu content in the flat steel product according to the invention is 0.1% by weight, in particular less than 0.04% by weight or less than 0.02% by weight.
Auch Ni, Sn, As, Co, Zr, sowie Seltene Erden, insbesondere La und/oder Ce, werden im erfindungsgemäßen Stahlflachprodukt nicht als Legierungselemente benötigt und zählen im Fall, dass sie dennoch im erfindungsgemäßen Stahlflachprodukt nachweisbar sind, zu den unvermeidbaren Verunreinigungen. Dementsprechend ist der Ni-Gehalt auf maximal 0,1 Gew.-%, der Sn-Gehalt auf maximal 0,05 Gew.-%, der As-Gehalt auf maximal 0,02 Gew.-%, der Co-Gehalt auf maximal 0,02 Gew.-%, der Zr-Gehalt auf maximal 0,002 Gew.-%, insbesondere maximal 0,0002 Gew.-%, und der Gehalt an den Seltenen Erden zuzurechnenden Elementen, wie La und Ce, auf jeweils maximal 0,002 Gew.-%, insbesondere maximal 0,0002 Gew.-%, beschränkt. O ist ebenso unerwünscht im erfindungsgemäßen Stahlflachprodukt, da sich eine Oxidbelegung, die aus der Anwesenheit höherer O-Gehalte resultieren kann, sowohl auf die Mechanik, als auch auf die Gieß- und Walzbarkeit des Stahlflachprodukts negativ auswirken würde. Der höchstens zulässige O-Gehalt wird daher auf 0,005 Gew.-%, bevorzugt auf 0,002 Gew.-%, festgesetzt. H ist als kleinstes Atom auf Zwischengitterplätzen im Stahl sehr beweglich und kann insbesondere in höchstfesten Stählen beim Abkühlen von der Warmwalzung zu Aufreißungen im Kern führen. Der H-Gehalt eines erfindungsgemäßen Stahlflachprodukts sollte daher so gering wie möglich sein, in jedem Fall aber höchstens 0,001 Gew.-%, insbesondere maximal 0,0006 Gew.-% oder maximal 0,0004 Gew.-%, betragen, wobei H-Gehalte von höchstens 0,0002 Gew.-% besonders angestrebt werden.Ni, Sn, As, Co, Zr and rare earths, in particular La and/or Ce, are also not required as alloying elements in the flat steel product according to the invention and, if they are nevertheless detectable in the flat steel product according to the invention, are among the unavoidable impurities. Accordingly, the Ni content is at most 0.1 wt%, the Sn content is at most 0.05 wt%, the As content is at most 0.02 wt%, 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 the rare earth elements, such as La and Ce, to a maximum of 0.002% by weight in each case %, in particular a maximum of 0.0002% by weight. O is also undesirable in the flat steel product according to the invention, since a Oxide coating, which can result from the presence of higher O contents, would have a negative effect both on the mechanics and on the castability and rollability of the steel flat product. The maximum permissible O content is therefore set at 0.005% by weight, preferably at 0.002% by weight. As the smallest atom, H is very mobile in the interstitial spaces in the steel and can lead to cracking in the core, particularly in high-strength steels when cooling from hot rolling. The H content of a flat steel product according to the invention should therefore be as low as possible, but in any case not more than 0.001% by weight, in particular not more than 0.0006% by weight or not more than 0.0004% by weight, with H Contents of at most 0.0002% by weight are particularly desirable.
Das erfindungsgemäße Verfahren zur Herstellung eines erfindungsgemäßen Stahlflachprodukts umfasst folgende Arbeitsschritte:
- a) Erschmelzen eines Stahls, der aus (in Gew.-%) 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 sowie aus mindestens einem die Karbonitrid-Ausscheidungen bildenden Element aus der Gruppe "Ti, Nb, V" und als Rest aus Eisen und unvermeidbaren Verunreinigungen mit folgenden Maßgaben besteht:
- Im Fall, dass V vorhanden ist, ist die Summe der Gehalte TiGEW an Ti und NbGEW an Nb auf höchstens 0,01 Gew.-% beschränkt.
- Im Fall, dass Ti und/oder Nb vorhanden sind, beträgt der V-Gehalt VGEW höchstens 0,01 Gew.-%.
- Für eine aus dem jeweiligen Ti-Gehalt TiGEW, dem jeweiligen V-Gehalt VGEW und dem jeweiligen Nb-Gehalt NbGEW des Stahls gebildete Gesamtkonzentration
- Für ein Mengenverhältnis Verh1 = (XAT+CrAT) / (CAT+NAT), gilt
- CrAT ist CrGEW umgerechnet in Atom-%,
- CAT ist CGEW umgerechnet in Atom-%,
- NAT ist NGEW umgerechnet in Atom-%,
- TiAT ist TiGEW umgerechnet in Atom-%,
- VAT ist VGEW umgerechnet in Atom-%,
- NbAT ist NbGEW umgerechnet in Atom-%.
- b) Vergießen des Stahls zu einem Vorprodukt, wie einer Bramme, einer Dünnbramme oder ein gegossenes Band;
- c) Temperieren des Vorprodukts auf eine 1150 - 1350 °C betragende Temperatur;
- d) Warmwalzen des Vorprodukts zu dem warmgewalzten Stahlflachprodukt mit einer Warmwalzendtemperatur von 880 - 980 °C;
- e) Abkühlen des erhaltenen warmgewalzten Stahlflachprodukts mit einer 20 - 400 °C/s betragenden Abkühlgeschwindigkeit auf eine 560 - 690 °C betragende Haspeltemperatur;
- f) Haspeln des auf die Haspeltemperatur abgekühlten Warmbands zu einem Coil.
- 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 0.05 - 0.5% Cr, ≤ 0.02% P, ≤ 0.005% S, ≤ 0.01% N, ≤ 0.1% Cu and at least one element from the group "Ti, Nb, V" and the remainder consists of iron and unavoidable impurities with the following proviso:
- In the case where V is present, the sum of the contents Ti WT of Ti and Nb WT of Nb is limited to at most 0.01 wt%.
- In the case where Ti and/or Nb is present, the V content V GEW 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
- For a ratio Verh1 = (X AT +Cr AT ) / (C AT +N AT ), applies
- Cr AT is Cr WEW converted to atomic %,
- C AT is C GEW converted to atomic %,
- N AT is N GEW converted to atomic %,
- Ti AT is Ti GEW converted to atomic %,
- V AT is V GEW converted to atomic %,
- Nb AT is Nb GEW converted to atomic %.
- 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 of the pre-product to form the hot-rolled flat steel product with a final hot-rolling temperature of 880-980 °C;
- e) cooling the obtained hot-rolled flat steel product at a cooling rate of 20-400°C/s to a coiling temperature of 560-690°C;
- f) Coiling the hot strip, which has been cooled to the coiling temperature, into a coil.
Der erfindungsgemäß entsprechend den voranstehend im Zusammenhang mit der Zusammensetzung eines erfindungsgemäßen Stahlflachprodukts gegebenen Erläuterungen legierte Stahl wird nach seiner Erschmelzung zu einem Vorprodukt vergossen, bei dem es sich beim klassischen Produktionsweg um eine Bramme üblicher Abmessung handeln wird. Jedoch kann aus dem Stahl auch durch direktes Warmwalzen eines Stranggusses in einer Gießwalzanlage als Vorprodukt eine Dünnbramme oder in einer Bandgießanlage als Vorprodukt ein gegossenes Band erzeugt werden.The steel alloyed according to the invention in accordance with the explanations given above in connection with the composition of a flat steel product according to the invention is, after it has been melted, cast into a preliminary product which, in the case of the classic production route, will be a slab of conventional dimensions. However, the steel can also be made into a continuous cast by direct hot rolling a thin slab as a preliminary product in a casting-rolling plant or a cast strip as a preliminary product in a strip casting plant.
Das Vorprodukt wird auf mindestens 1150 °C erwärmt oder bei dieser Temperatur gehalten. Eine derart hohe Erwärmungstemperatur ist erforderlich, um im Vorprodukt schon vorhandene Karbide und Nitride aufzulösen. Bei zu niedrigen Erwärmungstemperaturen bleiben die Legierungselemente in den Ausscheidungen gebunden, so dass keine neuen Ausscheidungen gebildet werden können. Aus wirtschaftlichen Gründen ist die Erwärmungstemperatur auf max. 1350 °C begrenzt.The precursor is heated to at least 1150 °C or kept at this temperature. Such a high heating temperature is required in order to dissolve carbides and nitrides already present in the pre-product. If the heating temperatures are too low, the alloying elements remain bound in the precipitations, so that no new precipitations can be formed. For economic reasons, the heating temperature is limited to a maximum of 1350 °C.
Das Warmwalzen des Vorprodukts erfolgt in konventioneller Weise, wobei die Endtemperatur des Warmwalzens mindestens 880 °C betragen muss. Bei zu niedrigen Warmwalzendtemperaturen steigen die Walzkräfte unverhältnismäßig an und die angestrebte Isotropie des Materials geht durch Effekte des thermomechanischen Walzens verloren. Über 980 °C liegende Endtemperaturen sind technisch nicht realisierbar.The preliminary product is hot-rolled in a conventional manner, with the final temperature of hot-rolling having to be at least 880 °C. If the final hot rolling 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. End temperatures above 980 °C are technically not feasible.
Das die Warmwalzstaffel verlassende warmgewalzte Stahlband wird mit einer Abkühlgeschwindigkeit von 20 - 400 °C/s auf eine Haspeltemperatur abgekühlt, die im Bereich von 560 - 690 °C liegt.The hot-rolled steel strip leaving the hot-rolling train is cooled at a cooling rate of 20 - 400 °C/s to a coiling temperature in the range of 560 - 690 °C.
Eine Abkühlrate von mindestens 20 °Cls ist erforderlich, um die Bildung von Perlit und Zementit weitestgehend zu vermeiden. Über 400°C/s liegende Abkühlungsgeschwindigkeiten sind technisch nicht realisierbar.A cooling rate of at least 20 °Cls is required to avoid as much as possible the formation of perlite and cementite. Cooling rates of more than 400°C/s are technically not feasible.
Haspeltemperaturen von 560 - 690 °C decken den Temperaturbereich ab, in dem Ausscheidungen mit einer durchschnittlichen Größe von weniger als 10 nm, insbesondere weniger als 5 nm, gebildet werden. Bei höheren Temperaturen beträgt die durchschnittliche Größe der Karbonitride mehr als 10 nm, womit die Zieleigenschaften des erfindungsgemäßen Stahlflachprodukts nicht mehr erreicht werden können. Eine gezielte Bildung von kleinen Ausscheidungen, deren Größe weniger als 7 nm, insbesondere weniger als 5 nm, beträgt, lässt sich durch Haspeltemperaturen von 580 - 670 °C, insbesondere 590 - 650 °C, bewirken. Bei unterhalb von 580 °C liegenden Haspeltemperaturen würden keine Karbonitride mehr ausgeschieden und deren festigkeitssteigernde Wirkung würde ausbleiben.Coiling 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, which means that the target properties of the flat steel product according to the invention can no longer be achieved. A targeted formation of small Precipitations whose size is less than 7 nm, in particular less than 5 nm, can be brought about by coiling temperatures of 580-670.degree. C., in particular 590-650.degree. At coiling temperatures below 580 °C, carbonitrides would no longer be separated and their strength-increasing effect would be absent.
Im Hinblick auf die Ausscheidungsvorgänge bei der Abkühlung im Coil von besonderer Bedeutung ist das Atomverhältnis Verh2. Ein 2 - 20 betragendes Verhältnis Verh2 in den Ausscheidungen ist erforderlich, um bei der Erzeugung eines erfindungsgemäßen Stahlflachprodukts eine Vergröberung der Ausscheidungen während der Abkühlung im Coil nach dem Haspeln zu verhindern. Bei größeren Verhältnissen Verh2 könnte die erfindungsgemäß vorgegebene Maximalgröße der Karbonitride von höchstens 10 nm und damit einhergehend die Mindestfestigkeit und das gewünschte hohe Lochaufweitungsverhältnis nicht erreicht werden. Wenn jedoch das Verhältnis Verh2 weniger als 2 beträgt, so steigt die Gefahr, dass sich unerwünschte Kristallstrukturen bilden, wenn die Karbide oder Karbonitride zu stark von Cr dominiert werden, oder sich sogar reine Cr-Karbide bilden.With regard to the precipitation processes during cooling in the coil, the atomic ratio Verh2 is of particular importance. A Verh2 ratio of 2-20 in the precipitates is required in order to prevent coarsening of the precipitates during cooling in the coil after coiling when producing a flat steel product according to the invention. With larger ratios Verh2, the maximum size of the carbonitrides of at most 10 nm specified according to the invention and, associated with this, the minimum strength and the desired high hole expansion ratio could not be achieved. However, if the ratio Verh2 is less than 2, there is an increased risk that undesired crystal structures will form if the carbides or carbonitrides are too heavily dominated by Cr, or that pure Cr carbides will even form.
Besonders sicher lassen sich die erfindungsgemäß genutzten Effekte der Bildung von Cr-haltigen Ausscheidungen dann herbeiführen, wenn für das Verhältnis Verh2 gilt: 2,3 ≤ Verh2 ≤ 15.The effects of the formation of Cr-containing precipitations used according to the invention can be brought about particularly reliably if the following applies to the ratio Verh2: 2.3≦Verh2≦15.
Im Fall, dass Ausscheidungen mit einer mittleren Partikelgröße von nicht mehr als 5 nm erzeugt werden sollen, kann dies neben der oben erläuterten Wahl einer geeigneten Haspeltemperatur dadurch betriebssicher erreicht werden, dass das Verhältnis Verh2 auf 2,5 - 5 eingestellt wird.If precipitates with an average particle size of no more than 5 nm are to be produced, this can be achieved in an operationally reliable manner, in addition to the selection of a suitable coiling temperature explained above, by setting the ratio Verh2 to 2.5-5.
Sollen dagegen Ausscheidungen mit einer mittleren Partikelgröße von mehr als 5 nm (also >5 nm), aber nicht mehr als 10 nm (also ≤10 nm) entstehen, kann dies, wie oben erläutert, zum einen durch Einstellung der Haspeltemperatur im oberen Teil des erfindungsgemäß vorgegebenen Haspeltemperaturbereichs bewirkt werden, und zum anderen dadurch unterstützt werden, dass das Verhältnis Verh2 auf 5 - 15, insbesondere 5 - 10, eingestellt wird.If, on the other hand, precipitates with an average particle size of more than 5 nm (i.e. >5 nm) but no more than 10 nm (i.e. ≤10 nm) are to occur, this can, as explained above, be achieved by setting the coiling temperature in the upper part of the according to the invention specified coiler temperature range be effected, and on the other hand supported by the fact that the ratio Verh2 is set to 5-15, in particular 5-10.
Wie eine hochauflösende Transmissionselektronenmikroskopie zeigt, liegt bei im Gefüge eines erfindungsgemäßen Stahlflachprodukts vorhandenen Ausscheidungen, die eine mittlere Partikelgröße von weniger als 5 nm besitzen, eine homogene Cr-Verteilung vor. Bei Ausscheidungen im Größenbereich von ca. 5 bis 10 nm wurde dagegen gezeigt, dass die Ränder der Ausscheidungen wesentlich höhere Cr-Gehalte aufweisen als der Kern der Ausscheidungen. Hierbei wird als "Kern" derjenige Bereich der Ausscheidungen definiert, der ca. 50 % der Fläche der Ausscheidung in der Mikroskopie ausmacht und dem Flächenschwerpunkt am nächsten ist. Als Rand wird die verbleibende Fläche der Ausscheidung definiert. Bei Ausscheidungen von mehr als 10 nm ist der Cr-Gehalt am Rand deutlich höher als im Kernbereich und Cr hat keine Wirkung mehr in Bezug auf die Vermeidung einer Vergröberung der Ausscheidungen.As a high-resolution transmission electron microscopy shows, there is a homogeneous Cr distribution in the structure of a steel flat product according to the invention present precipitations which have an average particle size of less than 5 nm. In the case of precipitations in the size range of approx. 5 to 10 nm, on the other hand, it was shown that the edges of the precipitations have a significantly higher Cr content than the core of the precipitations. Here, the “core” is defined as that area of the excretions that makes up approximately 50% of the area of the excretion in microscopy and is closest to the centroid of the area. The remaining area of the excretion is defined as the edge. In the case of precipitations larger than 10 nm, the Cr content at the edge is significantly higher than in the core area and Cr no longer has any effect in terms of preventing coarsening of the precipitations.
Nachfolgend wird die Erfindung anhand von Ausführungsbeispielen näher erläutert. Es zeigen:
Abbildung 1- lichtmikroskopische Aufnahmen der Gefüge von bei Versuchen erhaltenen Stahlbändern;
Abbildung 2- transmissionselektronmikroskopische (TEM) Aufnahmen von Gefügen von bei Versuchen erhaltenen Stahlbändern.
-
illustration 1 - light micrographs of the structure of steel strips obtained in tests;
- Figure 2
- Transmission electron microscopic (TEM) images of microstructures of steel strips obtained in tests.
Es sind die entsprechend den in Tabelle 1 angegebenen Zusammensetzungen legierten Schmelzen A - S erzeugt und zu Brammen vergossen worden. Die nicht erfindungsgemäßen Schmelzen und ihre von den Vorgaben der Erfindung abweichenden Gehalte an bestimmten Legierungselementen sind in Tabelle 1 durch Unterstreichungen hervorgehoben (Stähle B, D, E, F, N, O).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 that deviate from the requirements of the invention are underlined in Table 1 (steels B, D, E, F, N, O).
Nach einer in einem Stoßofen erfolgenden Erwärmung auf eine Erwärmungstemperatur ("EWT") sind die aus den Stählen A - S erzeugten Brammen in konventioneller Weise zu einem warmgewalzten Stahlband warmgewalzt worden. Das jeweils erhaltene warmgewalzte Stahlband hat die Warmwalzstaffel mit einer Warmwalzendtemperatur ("ET") verlassen und ist anschließend mit einer Abkühlrate ("dT") auf eine Haspeltemperatur ("HT") abgekühlt worden, bei der sie jeweils zu einem Coil gehaspelt worden sind. Anschließend sind die Stahlbänder im Coil auf Raumtemperatur abgekühlt worden. Die Produktionsvorgaben "EWT", "ET", "dT" und "HT" sind in Tabelle 2 aufgeführt.After being heated to a heating temperature ("EWT") in a pusher furnace, those produced from Steels A - S are Slabs have been hot-rolled in a conventional manner into a hot-rolled steel strip. The hot-rolled steel strip obtained in each case left the hot-rolling train with a final hot-rolling 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.
An den so erhaltenen warmgewalzten Stahlbändern sind gemäß DIN EN ISO 6892-1:2017 die Zugfestigkeit Rm, die obere Streckgrenze ReH, die untere Streckgrenze ReL und die Dehnung A50 bestimmt worden. Zusätzlich ist gemäß ISO 16630:2017 die Lochaufweitung λ bestimmt, das Produkt Rm × λ gebildet und die durchschnittliche Partikelgröße der Ausscheidungen sowie das Verhältnis Verh2 ermittelt worden.The tensile strength Rm, the upper yield point ReH, the lower yield point ReL and the elongation A50 were determined on the hot-rolled steel strips obtained in this way in accordance with DIN EN ISO 6892-1:2017. In addition, the hole expansion λ was determined according to ISO 16630:2017, the product Rm × λ was formed and the average particle size of the precipitations and the ratio Verh2 were determined.
Das Verhältnis Verh2 von XAUS (s. Tabelle 1) zu CrAUS (in Atom-%) in den Ausscheidungen ist in Tabelle 2 angegeben. Verh2 nimmt mit zunehmender Haspeltemperatur und zunehmender Ausscheidungsgröße zu. Der Zusammenhang zwischen Verh2 und dem Ausscheidungsdurchmesser weist darauf hin, dass Cr die Vergröberung der Karbide verhindert.The ratio Verh2 of X OUT (see Table 1) to Cr OUT (in atomic %) in the precipitates is given in Table 2. Verh2 increases with increasing coil temperature and increasing precipitate size. The correlation between Verh2 and the precipitation diameter indicates that Cr prevents the coarsening of the carbides.
Das Beispiel A ist eine Referenzzusammensetzung, welche zur Untersuchung des Einflusses der Haspeltemperatur verwendet wurde (siehe Beispiele A1 bis A7). Mit dieser Zusammensetzung wurden die optimalen mechanisch-technologischen Eigenschaften bei Haspeltemperaturen im Bereich von 590 - 650 °C erreicht.Example A is a reference composition used to study the effect of coiling temperature (see Examples A1 to A7). With this composition, the optimal mechanical-technological properties were achieved at coiling temperatures in the range of 590 - 650 °C.
Anhand der auf den Stählen B bis E aufbauenden Versuche wurden die Einflüsse von C, Si, Mn und Cr getrennt untersucht. In Stahl F wurde untersucht, welchen Einfluss das Fehlen bzw. die zu geringe Zugabe der Mikrolegierungselemente Ti, V und Nb hat. Die Konzentrationen der anderen Legierungselemente in den Beispielen B bis F waren ansonsten ähnlich wie bei den auf dem Stahl A basierenden Versuchen. Der Mangel an C, Mn, Cr oder Ti, V und/oder Nb führt zu mechanisch-technologischen Eigenschaften außerhalb des Zielbereichs (siehe Versuche B8 und D10 bis F12). Der Mangel an Si hat keinen wesentlichen Einfluss auf die Mechanik unter ähnlichen Produktionsvorgaben (siehe Versuch C9).Based on the tests based on steels B to E, the influences of C, Si, Mn and Cr were examined separately. In steel F, the influence of the absence or insufficient addition of the micro-alloying elements Ti, V and Nb was examined. The concentrations of others Alloying elements in Examples B through F were otherwise similar to the Steel A based experiments. 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 the mechanics under similar production conditions (see experiment C9).
Die Stähle G und H basieren ebenfalls auf dem Beispielstahl A, jedoch wurde hier das Verhältnis Verh1 variiert. Die Variation von Verh1 im Bereich 0,8 bis 1,2 hat keinen negativen Einfluss auf die mechanisch-technologischen Eigenschaften unter ähnlichen Produktionsvorgaben (siehe Beispiele G14, G15, H17 und H18). Die auf den Stählen G und H basierenden Versuche wurden ebenfalls benutzt, um die Einflüsse der Erwärmungstemperatur EWT und der Endwalztemperatur ET auf die Eigenschaften des jeweils erhaltenen Stahlbands zu untersuchen. Hier zeigte sich, dass eine zu niedrige Erwärmungstemperatur EWT oder Endwalztemperatur ET zur Bildung von groben Ausscheidungen oder einem anisotropen Gefüge und dadurch schlechten mechanisch-technologischen Eigenschaften führt (siehe Beispiele G13 und H16).Steels G and H are also based on example steel A, but the ratio Verh1 was varied here. The variation of Verh1 in the range from 0.8 to 1.2 has no negative impact on the mechanical-technological properties under similar production specifications (see examples G14, G15, H17 and H18). The tests based on steels G and H were also used to study the influences of the heating temperature EWT and the finish rolling temperature ET on the properties of the steel strip obtained. Here it was shown that too low a heating temperature EWT or final rolling temperature ET leads to the formation of coarse precipitations or an anisotropic structure and thus poor mechanical-technological properties (see examples G13 and H16).
Die Stähle I und J sind Nb-Konzepte und die Stähle K und L sind V-Konzepte, aus denen mit unterschiedlichen Haspeltemperaturen jeweils Stahlbänder erzeugt wurden (siehe Beispiele 119 bis K28). Die Beispiele I bis L enthalten unterschiedliche Gehalte an Si und Al. Wie bei den auf dem Stahl A basierenden Versuchen wurden die optimalen mechanisch-technologischen Eigenschaften bei Haspeltemperaturen von 590 - 650 °C erreicht.Steels I and J are Nb concepts and steels K and L are V concepts, from which steel strips were produced with different coiling temperatures (see Examples 119 to K28). Examples I to L contain different contents of Si and Al. As in the tests based on steel A, the optimum mechanical and technological properties were achieved at coiling temperatures of 590 - 650 °C.
Die Stähle M bis O haben eine ähnliche XGEW-Konzentration wie der Stahl A aber verschiedene Kombinationen von Ti, Nb und V. Das beim Beispiel M30 auf Basis des Stahls M (Ti und Nb) erzeugte Stahlband hat vergleichbare mechanisch-technologischen Eigenschaften wie die Stahlbänder, die auf Basis des Stahls A bei ähnlichen Haspeltemperaturen erzeugt worden sind. Die Versuche N31 (Ti und V) und O32 (Nb und V) haben dagegen trotz ähnlicher Haspeltemperaturen Stahlbänder mit im Vergleich zu den auf dem Stahl A aufbauenden Versuchen schlechteren mechanisch-technologischen Eigenschaften ergeben.Steels M to O have a similar X GEW concentration to steel A but different combinations of Ti, Nb and V. The steel strip produced in example M30 based on steel M (Ti and Nb) has comparable mechanical and technological properties to those Steel strips that have been produced on the basis of steel A at similar coiling temperatures. The Tests N31 (Ti and V) and O32 (Nb and V), on the other hand, produced steel strips with poorer mechanical-technological properties compared to the tests based on steel A, despite similar coiling temperatures.
Der Stahl P hat ein sehr hohes Verhältnis Verh1. Die aus dem Stahl P bestehenden Stahlbänder wurden in den Versuchen P33 - P35 mit unterschiedlichen Abkühlraten erzeugt. Bei einer zu niedrigen Abkühlrate wurde zu viel Perlit gebildet, wodurch die mechanisch-technologischen Eigenschaften stark verschlechtert wurden (siehe Beispiel P33).Steel P has a very high ratio Verh1. The steel strips consisting of steel P were produced in tests P33 - P35 with different cooling rates. If the cooling rate was too low, too much pearlite was formed, with the result that the mechanical-technological properties were severely impaired (see example P33).
Der Stahl Q hat einen hohen Al-Gehalt, ist aber ansonsten relativ niedrig legiert. Im Gegensatz dazu ist der Stahl R relativ hochlegiert. Aus den Stählen Q und R wurden Stahlbänder unter optimalen Bedingungen erzeugt. Diese Versuche zeigen, dass sehr unterschiedliche Festigkeiten mit hohen Rm·λ-Werten erreicht werden können, sofern das Verhältnis Verh1 im Sollbereich liegt.Steel Q has a high Al content but is otherwise relatively low alloyed. In contrast, the steel R is relatively highly alloyed. Steel strips were produced from the steels Q and R under optimal conditions. These tests show that very different strengths can be achieved with high Rm·λ values, provided the ratio Verh1 is within the target range.
Der Stahl S wurde als hochreine Laborschmelze erzeugt und mit erhöhter Erwärmungs- und Warmwalztemperatur sowie sehr hoher Abkühlrate prozessiert, um die Einflüsse entsprechend variierter Herstellparameter zu validieren.Steel S was produced as a high-purity laboratory melt and processed with increased heating and hot-rolling temperatures as well as a very high cooling rate in order to validate the influences of varying manufacturing parameters.
Lichtmikroskopische Aufnahmen der Gefüge der bei den Versuchen A1, A3, A4, A6 und A7 erhaltenen Stahlbänder sind in
Die transmissionselektronmikroskopischen (TEM) Aufnahmen der Gefüge der bei den Versuchen A3, A4, A6 und A7 erhaltenen Stahlbänder sind in
Die Verteilungen von Cr und Ti im Gefüge des beim Versuch A3 erhaltenen Stahlbands wurden mittels energiedispersiver Röntgenmikrobereichsanalyse (EDX) gemessen. Es zeigte sich, dass im Gefüge Karbonitride enthalten waren, die einen sehr hohen Anteil von Cr und Ti aufweisen.
Claims (15)
- Hot-rolled flat steel product having a tensile strength Rm of at least 550 MPa, determined according to DIN EN ISO 6892-1:2017, a hole expansion ratio λ of at least 30 %, determined according to standard ISO 16650:2017, and a structure consisting to an extent of at least 90 area% of ferrite and as the balance up to 10 area% of perlite or cementite, in which carbonitride precipitates having a mean diameter of at most 10 nm are embedded, consisting of a steel which in weight% consists of 0.02 - 0.5 % 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 also of at least one element that forms the carbonitride precipitates and is from the group of Ti, Nb and V, and, as the balance, consists of iron and unavoidable impurities, with provisos as follows:- If V is present, the sum of the contents TiGEW of Ti and NbGEW of Nb is limited to at most 0.01 weight%;- if Ti and/or Nb are present, the V content VGEW is at most 0.01 weight%;- for a total concentration XGEW in weight% = TiGEW + (VGEW/1.06)+(NbGEW/1.94) it is the case that 0.02 % ≤ XGEW ≤ 0.5 %,- for a proportion
- Flat steel product according to Claim 1, characterized in that for a proportion Prop2 = XAUS/CrAUS it is the case thatTiAUS: Ti content of the precipitates in atom%VAUS: V content of the precipitates in atom%NbAUS: Nb content of the precipitates in atom%CrAUS: Cr content of the precipitates in atom%.
- Flat steel product according to Claim 1, characterized in that its Cr content is more than 0.08 weight%.
- Flat steel product according to any of the preceding claims, characterized in that the mean grain size of its ferritic structure is 5 - 10 µm.
- Flat steel product according to any of the preceding claims, characterized in that its Mn content is at least 1.5 weight%.
- Flat steel product according to any of the preceding claims, characterized in that its Si content is at least 0.05 weight%.
- Flat steel product according to any of the preceding claims, characterized in that the value XGEW is 0.05 - 0.25 weight%.
- Flat steel product according to any of the preceding claims, characterized in that for the proportion Prop1 it is the case that 0.7 ≤ Prop1 ≤ 1.5.
- Flat steel product according to any of the preceding claims, characterized in that for the proportion Prop2 it is the case that 2.5 ≤ Prop2 ≤ 15.
- Flat steel product according to any of the preceding claims, characterized in that the mean diameter of the precipitates is ≤ 5 nm and in that for the proportion Prop2 it is the case that 2.5 ≤ Prop2 < 5.
- Flat steel product according to any of Claims 1 to 9, characterized in that the mean diameter of the precipitates is > 5 nm and in that for the proportion Prop2 it is the case that 5 ≤ Prop2 ≤ 15.
- Flat steel product according to any of the preceding claims, characterized in that its hole expansion ratio λ is at least 50 %.
- Method for producing a flat steel product configured according to any of the preceding claims, comprising the following operations:a) melting a steel which in weight% consists of 0.02 - 0.5 % 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 also of at least one element that forms the carbonitride precipitates and is from the group of Ti, Nb and V, and, as the balance, consists of iron and unavoidable impurities, with provisos as follows:- if V is present, the sum of the contents TiGEW of Ti and NbGEW of Nb is limited to at most 0.01 weight%;- if Ti and/or Nb are present, the V content VGEW is at most 0.01 weight%;- for a total concentration XGEW, formed from the respective Ti content TiGEW, the respective V content VGEW and the respective Nb content NbGEW of the steel, in weight% = TiGEW + (VGEW/1.06)+(NbGEW/1.94) it is the case that 0.02 % ≤ XGEW ≤ 0.5 %,- for a proportionb) casting the steel to form a preliminary product, such as a slab, a thin slab or a cast strip;c) heating the preliminary product to a temperature of 1150 - 1350 °C;d) hot-rolling the preliminary product to form the hot-rolled flat steel product at a final hot-rolling temperature of 880 - 980 °C;e) cooling the resulting hot-rolled flat steel product at a cooling rate of 20 - 400 °Cls to a coiling temperature of 560 - 690 °C;f) coiling the hot strip, cooled to the coiling temperature, to form a coil.
- Method according to Claim 15, characterized in that the winding temperature is 590 - 650 °C.
- Component, more particularly body work component or chassis component, characterized in that it is produced by forming from a flat steel product according to any of Claims 1 - 12.
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