Classifications

• • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/84—Controlled slow cooling
• • 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
  • 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • 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
  • C21D2211/002—Bainite
• • 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
  • C21D2211/008—Martensite
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • 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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
• • 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
  • 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
  • C22C38/36—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.7% by weight of carbon
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel

Definitions

• the invention relates to a hot-rolled flat steel product with optimized suitability for welding and optimized mechanical properties in the region of the heat-affected zone of a weld.
• the invention also relates to a method for producing such a hot-rolled flat steel product.
• Flat steel products are understood here to mean rolled products, the length and width of which are each substantially greater than their thickness.
• a flat steel product or a “sheet metal product” refers to rolled products such as steel strips or sheets from which, for example, vehicle, crane or infrastructure components as well as supporting structures or shields are used - or underground construction, blanks or blanks are divided.
• Sheet metal parts” or “sheet metal components” are produced from such flat steel or sheet metal products, the terms “sheet metal part” and “sheet metal component” being used synonymously here.
• Infrastructure construction is understood to mean the production of structures, bridges, ships and aircraft.
• Vehicle construction refers in particular to the construction of commercial vehicles, buses and trailers.
• Crane construction here refers in particular to the construction of mobile cranes, especially to the construction of crane booms.
• alloy contents relate to the weight (information in% by weight), unless otherwise expressly stated. If element contents are given in formulas, the corresponding alloy content in% by weight is also meant here, unless otherwise stated. Information on the content of structural components relate to the area considered in the metallographic section (information in area%), unless otherwise stated.
• HSLA steels High Strength Low Alloy Steels
• HSLA steels are characterized by a combination of high strength and formability with relatively low alloy contents. They achieve their high strength through the addition of micro-alloying elements such as titanium, niobium or vanadium in conjunction with a controlled rolling and cooling process. Due to their low content of alloying elements, they also have excellent weldability and can be produced at low cost.
• metal sheets are usually first rolled and tempered in an additional production step. This tempering usually requires reheating, which is followed by quenching to set a required hardness and subsequent tempering.
• thermomechanical hot rolling leads to a significant increase in strength compared to conventional hot rolling, lower alloy contents can be used for the same strength level than with conventionally hot-rolled and tempered steels.
• the flat steel products obtained by thermomechanical rolling and direct hardening therefore typically have better weldability than conventionally produced flat steel products with comparable mechanical parameters, without the need for complex measures.
• thermomechanically rolled and directly hardened flat steel products are used in particular for highly stressed welded structures.
• the mechanical-technological properties of the flat steel products that meet in the weld seams are of particular importance.
• the flat steel products connected to one another by the weld seam must also have optimized properties in the area of the heat-affected zone surrounding the weld seam that was exposed to the heat introduced during welding.
• a flat steel product that achieves this object has at least the features specified in claim 1.
• a method that achieves this object comprises at least the method steps specified in claim 12, it being understood that the person skilled in the art will use those not mentioned in claim 12 but in the prior art in the production and processing of hot-rolled flat steel products regularly completed work steps independently supplemented if the need arises.
• the invention hereby provides a flat steel product, the alloy and manufacturing process of which is tailored so that a flat steel product according to the invention optimally meets the requirements profile explained above for such flat steel products in practice.
• a hot-rolled flat steel product according to the invention with optimized weldability and optimized properties in the area of the heat-affected zone of a weld consists of (in% by weight) 0.03-0.3% C, 0.4-3% Mn, 0.05- 0.2% Al, 0.005-0.1% Nb, 0.0005-0.005% B, optionally Cr and / or Mo with the proviso that the contents% Cr of Cr and / or% Mo of Mo meet the condition 0, 1% ⁇ % Cr + (3 x% Mo) ⁇ 3%, optionally Ti with the proviso that the contents% Ti of Ti and% N of N meet the condition% Ti /% N ⁇ 5 and also optionally in each case one or more elements from the group "Si, Ni, Cu, V, Ca, REM", where the following applies to the content of these elements, if any: Si: 0.01-0.5%, Ni: 0, 1 -1.5%, Cu: 0.1 -1.5%, V: 0.005-0.10%, Ca: 0.0005-0.005%,
• a flat steel product according to the invention is in the hot-rolled and hardened or tempered state and typically has a Thickness of 1.5-25 mm, in particular up to 20 mm. Due to the intended field of application, in particular thicknesses of at least 2.0 mm are provided for flat steel products according to the invention, with sheet metal thicknesses of at least 3.0 mm also being selected to increase the resistance to buckling.
• the properties of flat steel products according to the invention and the lightweight construction potential based on them can be exploited particularly effectively at thicknesses of a maximum of 15 mm.
• the yield strength R e of a hot-rolled flat steel product according to the invention is at least 680 MPa.
• Yield point R e the upper yield point R eH in the case of a pronounced yield point , and in the case of a non-pronounced yield point the 0.2% yield point R p0.2 .
• the structure of a flat steel product hot-rolled according to the invention can be adjusted so that its yield point R e is at least 890 MPa.
• the tensile strength R m of a flat steel product according to the invention is typically 700-1700 MPa.
• a high level of component safety can be reliably ensured by the tensile strength of sheets according to the invention being regularly at least 930 MPa.
• the toughness of flat steel products according to the invention it proves to be favorable that the tensile strength is regularly at most up to 1550 MPa.
• the yield strength ratio R e / R m is at least 0.75. Strong solidification of flat steel products according to the invention and the associated high deformation forces are avoided in this way that at Flat steel products according to the invention regularly set yield strength ratios R e / R m of at least 0.80.
• the elongation at break A determined in accordance with DIN EN ISO 6892, proportional sample, is 5-25% for a flat steel product according to the invention.
• good deformability results from the fact that the elongation at break A of flat steel products according to the invention is regularly at least 8%.
• the notched impact strength (Charpy-V according to DIN EN ISO 148-1) of a flat steel product according to the invention is at least 50 J / cm 2 at a test temperature of -20 ° C and at least 35 J / cm 2 at a test temperature of -40 ° C.
• the mechanical properties of a flat steel product according to the invention can be adjusted in a targeted manner by a targeted setting of the structure.
• the structure of the flat steel product according to the invention consists predominantly, ie at least 50% by area, of bainite or bainitic ferrite, with the bainite portion of the structure also up to 100% by area, so the structure in this case is purely bainitic.
• the term "bainite” or “bainitic” always includes structural components of the bainitic ferrite and the dislocation-rich ferrite.
• bainite proportions of less than 100 area% the remainder of the structure is occupied by up to 50 area% ferrite and, if the sum of the bainite and ferrite proportion is less than 100 area%, by martensite Martensite proportion of the structure is in any case limited to a maximum of 10 area%, preferably to a maximum of 5 area%.
• the proportion of martensite which can also include tempered martensite, predominates in the structure.
• the martensite part of the structure is in this case at least 50% by area, whereby the martensite proportion can also be up to 100% by area, which means that a completely martensitic structure is then present.
• the remainder of the structure is taken up by bainite or bainitic ferrite and, if present, ferrite, but its proportion of the structure is also limited to a maximum of 10 area%, preferably to a maximum of 5 area- %, is limited.
• structures with a martensite content of at least 80 area%, in particular at least 90 area% or at least 95 area% are set.
• a flat steel product according to the invention has good tempering resistance and is distinguished by excellent notched impact strength in the heat-affected zone of weld seams.
• a flat steel product according to the invention is outstandingly suitable for bending and has good wear resistance due to the high surface hardness.
• the flat steel product according to the invention is particularly suitable for use in welded constructions for crane booms in telescopic crane construction, in vehicle construction Infrastructure construction and for equipment used in surface or underground mining, such as supporting structures for shields and the like.
• a hot-rolled flat steel product according to the invention can be provided for further processing in the unheated, pickled or blasted state.
• it can be covered with a metallic protective layer, the zinc-based protective layers known from the prior art being particularly suitable for this purpose.
• Such Zn-based coatings can be applied in a practical manner to a flat steel product according to the invention, in particular by electrolytic galvanizing.
• the alloy components of a flat steel product according to the invention have been selected as follows: Carbon (C) is primarily present in the steel substrate of a steel flat product according to the invention in a content of 0.03-0.3% by weight in order to increase the tensile strength and yield point. C develops its effect through different mechanisms. Thus, up to a certain proportion, C can be present as an interstitial solution in both the body-centered and face-centered cubic iron lattices and in this way cause an increase in strength. In the alloy concept according to the invention, however, the primary task of carbon is to enable a martensitic structural transformation when the flat steel product is quenched, which results in a significant increase in strength.
• the martensitic structural transformation is ensured by the widely differing solubility of the carbon in the car and krz lattice in conjunction with a sufficiently high cooling rate.
• the austenite-stabilizing effect of carbon ensures that the required cooling rate for martensite formation is reduced and the strength of the resulting martensite is increased.
• carbon also brings about a lowering of the martensite start temperature, ie the Temperature from which martensite is formed during cooling, so that lower temperatures must be set for martensite formation.
• a minimum carbon content of 0.03% by weight is required, with the steel of a flat steel product according to the invention preferably containing at least 0.04% by weight of C so that the positive effect of C is certain entry.
• An increasing carbon content causes an increase in strength and yield strength.
• the C content has the greatest influence on the value of the carbon equivalent CE, which according to the information given in the article " Determination of Suitable Minimum Preheating Temperature for the Cold-Crack-Free Welding of Steels, UWER, D., & HOHNE, H., IIW Document IX-1631-91, 1991 , outlined relationships is determined. High C contents lead to high CE values here.
• the C content of the steel of a flat steel product according to the invention is limited to at most 0.3% by weight, with contents of at most 0.2% by weight ensuring particularly good weldability leaves.
• the presence of C in the steel of a flat steel product according to the invention can thus be used optimally if a flat steel product according to the invention has C contents of 0.04-0.2% by weight.
• Manganese (Mn) is present in the steel of a flat steel product according to the invention in contents of 0.4-3% by weight in order to fulfill three essential tasks.
• Manganese forms a substitution mixed crystal with iron, which increases the strength.
• manganese has an austenite-stabilizing effect and thus enables a martensitic transformation during quenching, even at lower cooling rates.
• manganese has a high affinity for sulfur (S) and binds it to MnS. In this way, the formation of embrittling phases such as FeS can be avoided.
• S sulfur
• a minimum level of manganese is required 0.4% by weight is required, the positive effect of Mn particularly occurring with contents of at least 0.6% by weight, in particular at least 0.8% by weight.
• the manganese content is limited to at most 3% by weight, with Mn contents of at most 2.0% by weight, in particular at most 1.7% by weight, particularly reliably excluding negative influences of Mn .
• the presence of Mn in the steel of a flat steel product according to the invention can thus be used optimally if a flat steel product according to the invention has Mn contents of 0.6-2.0% by weight, in particular 0.8-1.7% by weight.
• micro-alloying element niobium as well as optionally further micro-alloying elements such as titanium or vanadium, combined according to the invention, is of particular importance with regard to setting the properties of a flat steel product according to the invention.
• B Boron
• B Boron
• B is present in the flat steel product according to the invention in contents of 0.0005-0.005% by weight in order to achieve an optimally high hardness.
• B segregates at the austenite grain boundaries and suppresses the nucleation of ferrite there. In this way, the ferritic-pearlitic transformation is shifted to longer cooling times and a martensitic transformation can be achieved at lower cooling rates.
• a B content in the flat steel product according to the invention of at least 0.0005% by weight is required, the beneficial effect of B being particularly reliable when the B content of a flat steel product according to the invention is at least 0.0010% by weight .-%, in particular at least 0.0015% by weight.
• B is particularly effective with B contents of the flat steel product according to the invention of up to 0.004% by weight, in particular up to 0.0035% by weight.
• B contents of the flat steel product according to the invention can thus be optimally used if a flat steel product according to the invention has B contents of 0.0010-0.004% by weight, in particular 0.0015-0.0035% by weight.
• the invention provides for the combined addition of aluminum (Al) and niobium (Nb), which are also strong nitride and carbide or carbonitride formers.
• the flat steel product according to the invention contains 0.05-0.2% by weight Al.
• the positive effects of Al in the flat steel product according to the invention are particularly reliable with Al contents of at least 0.07% by weight.
• an Al content of at least 0.085% by weight is selected in particular.
• Al contents above 0.2% by weight would entail the risk of the formation of coarse Al 2 O 3 particles during steel production, which would impair the mechanical properties of the flat steel product. This is reliably prevented by limiting the Al content to a maximum of 0.2% by weight.
• the Al content can be limited to at most 0.15% by weight, in particular at most 0.13% by weight.
• the presence of Al in the steel of a steel flat product according to the invention can thus be optimally used if a steel flat product according to the invention has Al contents of 0.07-0.15% by weight, in particular 0.085-0.13% by weight.
• Nb also fulfills other tasks in addition to binding nitrogen.
• niobium forms niobium carbides, nitrides and / or carbonitrides at relatively high temperatures, which hinder grain growth before, after and during the rolling process carried out in the manufacture of a flat steel product according to the invention and thus bring about grain refinement and thus an increase in notched impact strength.
• niobium carbides, niobium nitrides and / or niobium carbonitrides can bring about an increase in strength through precipitation strengthening, which is used according to the invention to avoid excessive softening of the heat-affected zone in the area of a weld made on a flat steel product according to the invention.
• Nb contents of 0.005-0.1% by weight are provided in the flat steel product according to the invention.
• the positive effects of Nb are particularly reliable at contents of at least 0.010% by weight, in particular at least 0.015% by weight.
• Nb contents of more than 0.1% by weight there is no longer any increase in the effect of Nb.
• the Nb content is up limited to a maximum of 0.1% by weight.
• Niobium is particularly effective with Nb contents of the flat steel product according to the invention of up to 0.06% by weight, in particular up to 0.04% by weight.
• the presence of Nb in the steel of a steel flat product according to the invention can thus be optimally used if a steel flat product according to the invention has Nb contents of 0.010-0.06% by weight, in particular 0.015-0.04% by weight.
• Chromium (Cr) or molybdenum (Mo) or a combination of chromium (Cr) and molybdenum (Mo) can optionally be present in the flat steel product according to the invention.
• Both chromium (Cr) and molybdenum (Mo) effectively suppress the formation of ferrite and pearlite during the cooling process after hot rolling a flat steel product according to the invention and enable complete martensite or bainite formation even at lower cooling rates, which increases the hardenability, which is particularly advantageous for large thicknesses of the flat steel product.
• This effect can be achieved both by the fact that either Cr or Mo are each present alone in the flat steel product according to the invention, and also because Cr and Mo are present in combination with one another in the flat steel product according to the invention.
• the hardenability-increasing effect of Mo is significantly higher than that of Cr.
• Cr and / or Mo can optionally be present in the flat steel product according to the invention with the proviso that the contents% Cr of Cr and% Mo of Mo, in each case in% by weight, the condition 0, 1% by weight ⁇ % Cr + (3 x% Mo) ⁇ 3% by weight. This requirement is also considered to be fulfilled if only Cr or only Mo are present in sufficient amounts.
• effects of Mo and / or Cr in the flat steel product according to the invention are particularly reliable if the contents% Cr to Cr and / or% Mo to Mo% Cr + (3 x% Mo) 0.2% by weight applies .
• the positive influences of Mo and Cr in the flat steel product according to the invention can be used particularly effectively and economically if the contents% Cr of Cr and% Mo of Mo% Cr + (3 x% Mo) 2% by weight applies.
• titanium can also be added to the steel of a flat steel product according to the invention.
• the addition of Ti can also bind nitrogen to ensure the solubility of boron.
• the titanium nitrides (TiN) that form at high temperatures or directly from the melt also hinder grain growth during the reheating of the slab before hot rolling and thus promote a finer-grain structure and thus higher toughness values.
• at least 0.005% by weight can be added to the flat steel product according to the invention. Titanium can also contribute to grain refinement during the rolling process and to precipitation strengthening through the precipitation of titanium carbonitrides. However, this effect is less than that of niobium.
• the maximum content of titanium if it is actually present in effective contents in the flat steel product according to the invention, is linked to the nitrogen content present in each case according to the proviso that for the contents% Ti to Ti and% N to N, each in% by weight of the steel flat product
• % Ti /% N 5 in particular% Ti /% N 4, resulting in a particularly precise addition of Ti aimed specifically at binding the nitrogen present in the flat steel product when% Ti /% N 3.42 is.
• the maximum content of optionally added Ti in the flat steel product according to the invention is therefore at maximum 0.05% by weight with a maximum N content of 0.01% by weight, with optimum effects resulting in terms of the binding of the nitrogen present in each case, if, with a maximum N content of 0.01% by weight, the Ti content is at most 0.0342% by weight or, with a particularly favorable N content of at most 0.006% by weight, the Ti content is at most 0.021% by weight. -% is.
• a sub-stoichiometric titanium alloy is preferably selected with the proviso that% Ti /% N ⁇ 3.42 applies; for this reason, a maximum Ti content of 0.020% by weight is preferred. elected.
• one or more elements from the group “Si, Ni, Cu, V, Ca, REM” can be present in the flat steel product according to the invention in accordance with the stipulations explained below.
• the flat steel product according to the invention can optionally have silicon (Si) in contents of 0.01-0.5% by weight. From contents of at least 0.01% by weight, Si forms a substitution mixed crystal with the iron lattice and thus brings about a significant increase in strength. In addition, Si suppresses the formation of cementite so that more carbon remains dissolved in the austenite, which in turn promotes the martensitic transformation. In addition, Si reduces the risk of undesired cementite formation in the martensite and thereby increases the resistance to an undesired reduction in strength in the heat-affected zone during welding as well as when starting. To ensure this effect, Si contents of at least 0.05% by weight can be added. High levels of silicon would lead to the formation of red scale.
• Red scale has an insulating effect on the surface of the material and can therefore significantly reduce the effect of the cooling water used for cooling. This in turn has negative effects on the martensitic transformation.
• the content of Si if present in any effective amounts, is limited to at most 0.5% by weight. Negative effects of the optional presence of Si can be avoided particularly reliably by limiting the optional Si content of the flat steel product according to the invention to at most 0.3% by weight, in particular at most 0.1% by weight.
• Nickel (Ni) and copper (Cu) can also optionally be provided in the flat steel product according to the invention to increase the hardenability. Suitable contents of Ni and / or Cu for this purpose are in each case 0.1-1.5% by weight. The effect of Cu and / or Ni in the flat steel product according to the invention can be used particularly effectively if Ni and / or Cu are optionally present in contents of up to 1.0% by weight, in particular up to 0.5% by weight This applies both to the individual addition of either Cu or Ni and, if Cu and Ni are present at the same time, to the sum of the content of both elements or to the respective content of each of the two elements.
• a flat steel product according to the invention can optionally contain 0.0005-0.005% by weight Ca.
• contents of at least 0.001% by weight are preferably added; for reasons of resource efficiency, the Ca content is preferably limited to a maximum of 0.004% by weight.
• Rare earths such as Cerium and lanthanum can bring about a grain refinement in the flat steel product according to the invention and thus an increase in toughness and strength.
• REM contents of 0.001-0.050% by weight can optionally be present in the flat steel product according to the invention.
• Vanadium (V) can also optionally be present in the flat steel product according to the invention in order to bring about precipitation strengthening.
• V contents suitable for this are 0.005-0.10% by weight.
• the remainder of the flat steel product according to the invention not taken up by the above-mentioned mandatory components and optionally present alloying elements consists of iron and impurities which are unavoidably present in the flat steel product according to the invention due to the manufacturing process, but whose contents are kept so low in each case that they have no influence on the properties of a flat steel product according to the invention to have.
• Nitrogen (N) forms nitrides with B, Al and Nb and, if they are present, with Ti and V.
• N Nitrogen
• boron nitrides is particularly undesirable in order to be able to use the hardenability-increasing effect of boron.
• nitrogen contents of up to 0.01% by weight can be accepted as an unavoidable contamination, whereby N -Contents of at most 0.008% by weight, in particular at most 0.006% by weight, as particularly favorable for the reliable production of flat steel products have shown with a profile of properties according to the invention.
• Contents of less than 0.002% by weight can only be avoided technically with great effort, which is why, for economic reasons, in particular a content of at least 0.002% by weight is tolerated.
• Arsenic (As) and tin (Sn) can accumulate on grain boundaries at temperatures around 500 ° C and thus cause embrittlement.
• the content of As and Sn in the flat steel product according to the invention should be limited in the usual way to a maximum of 0.05% by weight.
• sulfur (S) forms sulphides with iron or manganese (FeS or MnS). These have a negative influence on deformability and toughness.
• the sulfur content is therefore limited to at most 0.010% by weight, preferably to 0.008% by weight and particularly preferably to 0.006% by weight.
• Phosphorus (P) has a very negative influence on the toughness, so that its content in the flat steel product according to the invention is limited to 0.02% by weight.
• Oxygen (O) combines in particular with aluminum to form oxides (Al 2 O 3 ). These reduce both toughness and fatigue strength. The oxygen content is therefore restricted to ⁇ 0.01% by weight.
• Hydrogen (H) can lead to the formation of cracks in the material if the content is too high.
• its content in the flat steel product according to the invention is limited to a maximum of 0.0004% by weight, in particular less than ⁇ 0.0001% by weight.
• Co Co
• Co has a negative influence on hardenability and toughness. For technical reasons, however, traces of cobalt usually remain in steels. Since the negative influences of cobalt generally only occur above 0.2% by weight, its content is limited to a maximum of 0.2% by weight.
• Tungsten (W) forms a Laves phase with molybdenum above certain levels. This can have a negative effect on the impact strength. For technical reasons, however, the tungsten content cannot usually be reduced to any desired extent, but in accordance with the invention may not exceed 0.2% by weight to avoid negative influences.
• step a) of the method according to the invention a melt is thus melted with a composition corresponding to the above provisions of the invention, which can be varied in the manner also explained above in order to set or develop certain properties of the hot-rolled flat steel product to be produced according to the invention.
• This melt is cast in a conventional manner in step b) to form a preliminary product with a thickness d V.
• This intermediate product is typically a slab. However, casting to form thin slabs, cast strips or blocks is also possible.
• step c) the respective preliminary product is heated to an austenitizing temperature T WE , which heating can consist of bringing the preliminary product to the respective austenitizing temperature T WE or keeping it at the austenitizing temperature T WE until the preliminary product is completely heated.
• the austenitizing temperature T WE is 1100-1350 ° C., an austenitizing temperature T WE of at least 1220 ° C. proving to be favorable with regard to avoiding solidification in the subsequent hot rolling process. Melting of the surface of the preliminary product and excessive coarsening of its grain as a result of the heating of the austenitic structure can be reliably avoided by limiting the austenitizing temperature T WE to a maximum of 1320 ° C. In the temperature range of 1220-1320 ° C., an optimally homogeneous initial structure is also established and previously existing precipitations of the micro-alloy elements provided in the steel alloys according to the invention are reliably dissolved.
• the temperature of the flat steel product rolled from the preliminary product drops to the hot rolling end temperature T E at which the finished hot rolled flat steel product leaves the last pass of hot rolling.
• the hot rolling end temperature T E In order to suppress ferrite formation in the flat steel product during hot rolling, the hot rolling end temperature T E must be at least 770 ° C, with a hot rolling temperature T E that is at least 20 ° C higher than the Ar 3 temperature of the steel from which the inventive Flat steel product is manufactured, ferrite formation can be avoided particularly reliably.
• the Ar 3 temperature can be determined by that of Choquet in P.
• the invention provides that two or more passes are carried out above a temperature T NR in the course of hot rolling.
• T NR can be calculated according to that of Boratto in F. Borrato et al .: "Effect of Chemical Composition on Critical Temperatures of Microalloyed Steels", THERMEC '88, Proceedings, Iron and Steel Institute of Japan, Tokyo, 1988, p.
• n W of the rolling passes carried out at a temperature of the flat steel product above the temperature T NR corresponds to the result n W 'of the formula rounded off to an integer n W.
• ⁇ 2 * root Pre-product thickness d V / 6 * Thickness d W. of the finished hot-rolled flat steel product .
• This minimum number n W of roller passes above the temperature T NR is necessary in order to achieve an optimally fine-grained austenitic structure through recrystallization.
• a final hot rolling temperature T E below the temperature T NR can optionally be selected if rolling is carried out with more than n W rolling passes.
• the degree of deformation ⁇ which is achieved via the hot rolling passes, at which the temperature of the hot-rolled flat steel product in each case is below the temperature T NR , is advantageously at least 0.25.
• the flat steel product is cooled in an accelerated manner in step e) in the cooling section at a cooling rate ⁇ Q of at least 40 K / s, in particular at least 60 K / s.
• ⁇ Q cooling rate
• the term "immediately" in this document means that a maximum of 8 s may pass between the exit of the material from the roll gap of the last pass, up to accelerated cooling begins.
• a particularly suitable coolant here is water, which can be applied to the flat steel product in a conventional cooling section.
• the cooling stop temperature T KS for the flat steel product according to the invention is at least 250 ° C lower than the hot rolling end temperature, with cooling stop temperatures T KS of at most 550 ° C, in particular 500 ° C, provided that they are not above T E - 250 ° C.
• Hot-rolled flat steel products according to the invention with a yield point R e of less than 890 MPa and a structure consisting predominantly, ie at least 50% by area, of bainite can be produced by choosing a cooling stop temperature T KS that is below the bainite start temperature B S , but not more than 30 ° C below the martensite start temperature M S (T KS ⁇ M S - 30 ° C) of the respective steel.
• the proportion of bainite in the structure can be determined by setting a cooling stop temperature T KS . For example, at a cooling stop temperature T KS of Approx. 50 ° C below the bainite start temperature B S (T KS ⁇ B S - 50 ° C) with a structural proportion of 50% bainite area.
• a bainite proportion of 100% by area ie a completely bainitic structure
• a cooling stop temperature T KS that is around 120 ° C below the bainite start temperature B S (T KS ⁇ B S - 120 ° C ).
• the other structural components are up to 50% by area ferrite and up to 10% by area, in particular up to 5% by area, martensite, whereby the proportions of ferrite and martensite with correspondingly high proportions of the other structural component in each case also " 0 "can be.
• a cooling stop temperature T KS is selected that is at least 100 ° C below martensite -Start temperature M S is (T KS ⁇ M S - 100 ° C).
• a cooling stop temperature T KS is required that is approximately 380 ° C. below the martensite start temperature M S (T KS ⁇ M S 380 ° C.).
• the rapid cooling to the cooling stop temperature T KS carried out in step e) is followed in step f) by slow cooling of the hot-rolled flat steel product according to the invention to room temperature.
• the cooling rate ⁇ Q ' should not exceed 0.1 K / s, in particular 0.05 K / s.
• the slow cooling causes tempering effects in a structure with martensite components. Using the cooling stop temperature T KS in combination with the slow cooling rate ⁇ Q ', the proportion of tempered martensite in the structure can therefore be controlled very precisely during the subsequent cooling to room temperature. This allows the mechanical properties to be set very precisely.
• the invention thus provides a hot-rolled flat steel product which has a high yield point R e , a high tensile strength Rm and a high elongation at break A in combination with a good bending ability.
• the hot-rolled flat steel product according to the invention is characterized by good tempering resistance and excellent notched impact strength, especially in the heat-affected zone of weld seams.
• a flat steel product according to the invention is outstandingly suitable for punching and mechanical cutting.
• Thermal cutting processes such as laser or plasma cutting can also be used without problems in the processing of flat steel products according to the invention.
• a flat steel product according to the invention can be used for bending and edging without any special pretreatment and can be used, for example, to produce highly rigid structural components by roll profiling.
• melts A - I and O - Q according to the invention for comparison, melts J - N not according to the invention with the compositions given in Table 1 are melted and formed into slabs, thin slabs or strips with a thickness d V of 2.5 - 260 mm cast.
• the slabs cast from the melts A - P have each been reheated to an austenitizing temperature T WE , at which they entered a conventional reversing stand and then a conventional rolling mill, in order to form a steel strip with a thickness d W between at a final hot rolling temperature T E 4 mm and 8 mm to be hot rolled.
• T WE austenitizing temperature
• a band with a thickness of 3 mm was cast from melt Q, to be subsequently hot rolled to a thickness of 1.5 mm.
• Tests with different thicknesses d V (and d W ) showed similar properties and are therefore not shown in detail here.
• the flat steel products were initially rolled over a minimum number n W of rolling passes at a temperature which was above the temperature T NR .
• the number n W has been determined in the manner explained above from the thickness d V of the slabs and the final thickness d W of the flat steel product hot-rolled in each case in the tests.
• the respective flat steel product After passing through the rolling passes completed at temperatures above the temperature T NR , the respective flat steel product, with the exception of example Q, has been hot-rolled in at least one further rolling pass at a temperature below the temperature T NR .
• the hot-rolled steel strips obtained by hot rolling were accelerated and cooled to a cooling stop temperature T KS at a cooling rate ⁇ Q. After the respective cooling stop temperature T KS was reached, the steel strips were slowly cooled to room temperature at a cooling rate ⁇ Q ′.
• the hot-rolled flat steel products made from steels J and K that are not composed according to the invention have comparable yield strength and tensile strength values, but significantly lower results in the impact strength test than the flat steel products produced from the steels alloyed according to the invention.
• the reason for this is the titanium alloyed in higher contents in steels J and K (Ti / N ratio> 5).
• the flat steel products made from the steels L and M not composed according to the invention have lower values of the yield point, tensile strength and notched impact strength compared to the flat steel products made from the steels according to the invention.
• the boron is bound in nitrides and can no longer develop its hardenability-increasing effect.
• the flat steel products produced from the steel N which is not composed according to the invention have both lower yield strengths and Tensile strength values as well as a lower notched impact strength.
• the low yield strength and tensile strength values are due to the fact that the steel is not alloyed with boron.
• the low impact strength values are due to the increased titanium content.
• Table 1 stolen C. Mn Al Si Nb B. Cr Mon Cr + 3 mo Ti V Ni Cu P S. N Approx Ti / N A. 0.098 1.45 0.084 0.199 0.026 0.0023 0.334 0.207 0.955 - - - - 0.010 0.001 0.0037 0.0013 - B.
• 260 6th 10 0.28 1302 838 92 65 0.001 2 A. 260 6th 10 0.29 1306 827 151 60 0.002 3 A. 260 5 12th 0.35 1297 842 109 50 0.002 4th A. 260 4th 8th 0.45 1306 823 153 45 0.003 5 B. 260 8th 8th 0.35 1297 848 129 70 0.002 6th B. 260 6th 11 0.26 1291 854 106 90 0.002 7th C. 215 4th 8th 0.40 1262 798 82 100 0.001 8th C. 215 5 8th 0.45 1253 809 75 95 0.001 9 C. 215 5 8th 0.42 1248 812 327 50 0.003 10 D.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Heat Treatment Of Steel (AREA)

Priority Applications (2)

Applications Claiming Priority (1)

Publications (1)

Family

ID=66751888

Family Applications (1)

Country Status (2)

Cited By (1)

Citations (5)

• 2019
  • 2019-05-29 EP EP19177255.7A patent/EP3744862A1/fr active Pending
• 2020
  • 2020-05-25 WO PCT/EP2020/064410 patent/WO2020239676A1/fr active Application Filing

Patent Citations (5)

Non-Patent Citations (2)

Also Published As

Similar Documents

Legal Events