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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • 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
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
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  • 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/663—Bell-type furnaces
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Definitions

• the invention relates to a hot-rolled flat steel product, the optimally matched mechanical properties, such as high
• Tensile strength Rm, high yield strengths Rp and high elongation at break A, in combination with a good formability has, which is characterized by a high hole expansion value, for which is introduced as a letter " ⁇ " ("lambda").
• hot-rolled flat steel products according to the invention are distinguished by good fatigue strength and wear resistance.
• the invention relates to a method for producing such a flat steel product.
• the slabs undergo hot rolling, in which they are rolled in a temperature range which is below the recrystallization temperature but above the A3 temperature.
• the resulting hot strip is quenched at a quench rate of at least 20 ° C./s to a quench stop temperature which is within the temperature range between the temperature Ms at which
• the quench stop temperature here is in the range of greater than 200 ° C and less than 400 ° C.
• the quenched hot strip is called a "partitioning
• Treatment is carried out to transfer carbon from the martensitic to the austenitic structural constituents, after which the hot strip treated in this way is cooled down to room temperature, leaving essential parameters of the quenching and partitioning treatment open.
• the object of the invention was to provide a flat steel product with a larger sheet thickness and an optimized combination of properties.
• the invention has this object by the in
• Claim 1 specified hot rolled flat steel product solved.
• the solution according to the invention of the above-mentioned object is that in the production of a flat steel product according to the invention, the steps specified in claim 7 are completed.
• the invention provides a hot rolled flat steel product and a process suitable for its production.
• a hot-rolled flat steel product obtained according to the invention and produced according to the invention consists of a steel having the following composition (in% by weight):
• Ni 0.05-2.0%
• an inventive hot-rolled flat steel product is characterized by
• the flat steel product has a tensile strength Rm of 800-1500 MPa, a yield strength Rp greater than 700 MPa, an elongation at break A of 7 to 25% and a hole widening ⁇ of more than 20%,
• KAM Kernel Average Misalignment
• Carbon "C” is contained in the molten steel processed according to the invention in contents of 0.1-0.3% by weight. In the first place, C plays a major role in austenite formation. A sufficient C concentration allows full austenitization at temperatures up to 930 ° C, which are below the mill finish temperatures commonly used in hot rolling steels of the type in question. When quenching is already a part of the retained austenite by the invention
• the strength of the martensite which occurs during the first cooling step (9Q) or during the Last cooling step ( ⁇ 2) is also highly dependent on the C content of the inventively processed steel composition. At the same time, however, the martensite start temperature is shifted to lower temperatures with increasing C content. Too high a C content would therefore lead to difficulties in production, since the achievable
• Quenching temperature would be shifted to very low temperatures.
• the C content of a steel processed according to the invention most strongly contributes to a higher CE value compared with other alloying elements, which negatively influences the weldability.
• the CE value indicates which alloying elements adversely affect the weldability of the steel.
• the CE value can be calculated as follows:
• the strength level of the final product can be influenced in a targeted manner.
• Manganese “Mn” is an important element for the hardenability of steel.
• manganese reduces the tendency for undesired formation of perlite during cooling.
• the Mn content is limited to 1, 5 - 3.0 wt .-%.
• Adjustment of the strength properties can be achieved by the Mn content is 1, 9 - 2.7 wt .-%.
• Silicon "Si" plays an important role in suppressing perlite formation and carbide formation control. The formation of cementite would bind carbon and would therefore no longer be available for the further stabilization of the retained austenite. On the other hand, too high an Si content deteriorates the elongation at break and the surface quality due to accelerated formation of Rotzunder. A similar effect can be triggered by the addition of AI.
• a minimum of 0.7 wt .-% Si is required for the adjustment of the inventively provided product properties a minimum of 0.7 wt .-% Si is required.
• the desired microstructure can be set particularly reliably if contents of at least 1.0% by weight of Si in the inventive composition
• the Si content can also be adjusted to 0.5-1.1% by weight, as explained in the following paragraph.
• Aluminum “Al” is used for deoxidation and for setting any nitrogen present. Further, Al may also be used to suppress cementite as mentioned above, but is not as effective as Si. By an increased AI addition, however, the
• Austenitizing significantly increased, which is why the Zementit- suppression is preferably realized only by Si.
• an Al content of 0-0.03 wt% which is favorable in terms of austenizing temperature, is provided when Si is contained in contents of at least 1.0% by weight. If, however, the Si content, for example, to
• Adjustment of an optimized surface quality limited, ie values between 0.5 to 1, 1 wt .-%, preferably 0.7 to 1, 0 wt .-%, set, so AI must have a minimum content of 0.5 wt. % are added to the cementite suppression.
• the Al content may be particularly production of deoxidized melts to values of at least 0.01% by weight.
• the restriction of the AI content to max. 1, 5 wt .-%, preferably max. 1, 3 wt .-%, is made to avoid problems when casting the steel.
• Phosphorus "P” has an unfavorable effect on weldability. Its content in the hot strip according to the invention or in the melt processed according to the invention is therefore not more than 0.1% by weight, P contents of up to 0.02% by weight, in particular less than 0.02% by weight, may be advantageous.
• Sulfur "S” at higher concentrations leads to the formation of MnS or (Mn, Fe) S, which has a negative effect on the elongation.
• the S-content is limited to max. 0.03 wt .-% limited, with a limitation of the S contents to max. 0.003 wt .-%, in particular less than 0.003 wt .-%, may be advantageous.
• Nitrogen "N” leads to the formation of nitrides, which adversely affect the
• the N content should therefore be less than 0.008 wt .-%. By applying a high degree of technical effort, very low N contents of, for example, less than 0.0010% by weight can be realized. To reduce the technical complexity, the N content may preferably be adjusted to at least 0.0010% by weight and more preferably to at least 0.0015% by weight.
• Chromium is an effective inhibitor of perlite and can thus reduce the required minimum cooling rate.
• Cr is the Steel processed according to the invention or the steel of the hot-rolled flat steel product according to the invention is added.
• a minimum proportion of 0.10 wt.% Cr, preferably 0.15 wt.% Cr is required.
• the strength is greatly increased by the addition of Cr and there is also a risk of pronounced
• the Cr content to a maximum of 0.30 wt .-%, preferably max. 0.25% by weight, limited.
• Molybdenum "Mo" is also a very effective element for suppressing perlite formation.
• at least 0.05% by weight in particular at least 0.1% by weight, may optionally be added to the steel. Additions of more than 0.25% by weight are below
• Nickel “Ni”, like Cr, is an inhibitor of perlite and is effective even in small amounts. With optional alloying with Ni of at least 0.05% by weight, in particular at least 0.1% by weight, at least 0.2% by weight or at least 0.3% by weight, this supporting effect can be achieved. With regard to the desired adjustment of the mechanical properties, it is at the same time expedient to limit the Ni content to a maximum of 2.0% by weight, with Ni contents of not more than 1.0% by weight, in particular 0.5% by weight. -% have been found to be particularly practical.
• the steel of a flat steel product according to the invention may optionally also include micro-alloying elements, such as vanadium "V”, titanium “Ti” or niobium “Nb”, which, by forming very finely divided carbides (or carbonitrides in the simultaneous presence of nitrogen "N") to a higher
• Effect of B is saturated at a level of about 0.0020 wt .-%, which is also listed as the upper limit.
• a hot-rolled flat steel product according to the invention has a tensile strength Rm of 800-1500 MPa, a yield strength Rp of more than 700 MPa and an elongation at break A of 7-25%, the tensile strength Rm, the yield strength Rp and the elongation at break A according to DIN EN ISO 6892-1 -2009-12.
• hot strip according to the invention is characterized by a very good Formability, which is reflected in a determined according to DIN ISO 16630 hole expansion ⁇ of more than 20%.
• hot strip produced according to the invention has a structure of tempered and unannealed martensite with retained austenite content, whereby bainite, polygonal ferrite, non-polygonal ferrite and cementite may also be present in minor proportions in the structure.
• the martensite portion of the structure is at least 85 area%, preferably at least 90 area%, of which at least half is tempered martensite. Accordingly, the proportion of retained austenite in a hot-rolled flat steel product according to the invention is at most 15% by volume. In each case, up to 15 area% bainite, up to 15% each, at the expense of the rest of it
• Area% polygonal ferrite, up to 5 area% cementite and / or up to 5 area% non-polygonal ferrite in the structure is 0 area%, since in this case the values for the hole expansion due to the delayed crack formation in a predominantly martensitic structure with a uniform hardness are particularly high.
• the microstructure of the hot strip according to the invention is very fine, so that its assessment by means of conventional optical microscopy is hardly possible. Therefore, an evaluation by means of scanning electron microscopy (SEM) and a magnification of at least 5000 times is recommended. However, the maximum permissible retained austenite content is difficult to determine even after high magnification. Therefore, quantitative determination of residual austenite by X-ray diffraction (XRD) according to ASTM E975 is recommended.
• SEM scanning electron microscopy
• XRD X-ray diffraction
• the microstructure of the hot-rolled flat steel product according to the invention is characterized by a defined, local misorientation in the crystal lattice. This applies in particular to the intended proportion of primary species sesit, ie the at the first cooling martensite formed.
• the said local misorientation is quantified by the so-called "kernel average misorientation", in short "KAM”, which is greater than or equal to 1.50 °, preferably greater than 1.55 °.
• KAM value should be at least 1, 50 °, because then there is a homogeneous deformation resistance due to uniform lattice distortion in the grain. Thus, a locally limited pre-damage of the multi-phase structure at the beginning of a deformation can be avoided. If the KAM value is below 1.50 °, there is an excessively tempered structure which causes strength properties outside of the desired spectrum according to the invention.
• Crucial for the mechanical properties of a steel product designed and produced according to the invention is therefore, in addition to the pure phase fractions, above all the distortion of the crystal lattice.
• Lattice distortion is a measure of the initial resistance to plastic deformation, which determines the properties of the desired strength ranges.
• a suitable method for the measurement and thus quantification of the lattice distortion is the Electron Backscatter Diffraction (EBSD). With EBSD a lot of local diffraction measurements are generated and put together to small ones
• KAM Kernel Average Misalignment
• a process according to the invention for producing a hot-rolled flat steel product according to the invention comprises at least the following working steps: a) melting a steel alloy whose composition and variants have already been explained in connection with the hot-rolled flat steel product according to the invention and which accordingly has the following composition (in% by weight) : 0.1 - 0.3% C, 1, 5 - 3.0% Mn, 0.5 - 1, 8% Si, up to 1, 5% Al, up to 0.1% P, up to 0 , 03% S, up to 0.008% N, optionally one or more elements of the group
• Hot rolling end temperature TET is terminated, for the applies
• TET> (A3 - 100 ° C), where "A3" indicates the respective A3 temperature of the steel; e) first quenching of the hot strip starting from the
• % Mo Mo content of the steel
• FIG. 1 The process engineering production of hot strip according to the invention is shown schematically in FIG. 1 and will be explained in detail below.
• a precursor is cast, which will typically be a slab or thin slab.
• the precursor is heated to a heating temperature TWE which is in the temperature range in which austenite forms in the steel according to the invention.
• the heating temperature TWE of the steels according to the invention should therefore be at least 1000 ° C. in the process according to the invention, since at lower heating temperatures during the following
• the heating temperature should be at most 1300 ° C, to a partial
• the heating temperature TWE is preferably at least 1 150 ° C, because in this way microstructural inhomogeneities, for example, by
• 1 150 - 1250 ° C set a defined structural state and a targeted Resolution of excretions reached.
• the heating to the temperature TWE can be carried out in a conventional shock or walking beam furnace.
• Thin slab caster in which the composite steel according to the invention is cast into thin slabs with a thickness of typically 40-120 mm (see DE 4104001 A1), the heating can also take place in the oven which has passed through the casting and is directly connected to the casting plant.
• the precursor After heating, the precursor is hot rolled to hot strip with final thicknesses between 1, 0 and 20 mm, preferably between 1, 5 and 10 mm. Depending on the available system technology, this can be
• Hot rolling comprises an optionally reversing pre-rolling in a roughing stand and subsequent finish rolling in a so-called finishing scale, which consists of several, typically five or seven continuous in a continuous sequence rolling stands.
• finishing scale which consists of several, typically five or seven continuous in a continuous sequence rolling stands.
• the final rolling temperature TET of hot rolling is as specified
• the steel is quenched in a first quenching step from the hot rolling end temperature TET with a high cooling rate to a quenching temperature TQ.
• the cooling rate 9Q is more than 30 K / s.
• the quench temperature TQ targeted during the cooling is not below the room temperature. On the other hand, it is at most 100 ° C higher than the martensite start temperature TMS at which the martensitic transformation starts.
• the martensite start temperature TMS can be estimated using the following equation (2) developed by van Bohemen:
• Martensite start temperature TMS - 250 ° C is, that is:
• a quench temperature TQ between the martensite start temperature TMS and the martensite start temperature TMS -150 ° C. ((TMS -150 ° C.) ⁇ TQ ⁇ TMS) has proved to be particularly favorable.
• quench temperature TQ low temperatures such as a temperature lying in the range of room temperature.
• Step f) this can be quenched to the quench temperature TQ
• the temperature of the flat steel product may thereby fall by at most 80 ° C below the quench temperature TQ.
• the hot rolled flat steel product cooled to the quench temperature TQ is held for a period of 0.1-48 hours in a temperature range of TQ-80 ° C to TQ + 80 ° C, for the targeted conversions and when using the micro-alloying elements to ensure the formation of finely divided carbides.
• Holding within the temperature range from TQ -80 ° C to TQ +80 ° C can be done both isothermally, ie at constant temperature, as well as non-isothermal, ie at falling or rising or oscillating temperature.
• the maximum permitted cooling rate is 0.05 K / s.
• Conversion operations may, however, also be exothermic so that heat of conversion is released which results in an increase in the temperature of the flat steel product.
• the heat of conversion then counteracts a possible cooling.
• Microstructure development is a maximum of 0.01 K / s.
• the rate at which temperature changes occur during holding is thus typically in the range from -0.05 K / s to +0.01 K / s, based on the respective quench temperature TQ.
• the holding conditions must be selected so that the specified temperature window of TQ +/- 80 ° C despite the self-adjusting
• the aim of this step is to set a microstructure of martensite, tempered martensite and possibly retained austenite.
• step h the flat steel product is brought to a partitioning temperature TP starting from its temperature set after step g) or, if the partitioning temperature TP is in the order of +/- 80 ° C. around the Quench temperature TQ fluctuating range, held there to enrich the retained austenite with carbon from the supersaturated martensite.
• the partitioning temperature TP should advantageously be at least as high as the quench temperature TQ, but preferably at least 50 ° C. higher, in particular at least 100 ° C. higher.
• partitioning temperature TP is smaller than the temperature present after step g) (quench temperature TQ +/- 80 ° C.), then the partitioning temperature TP is smaller than the temperature present after step g) (quench temperature TQ +/- 80 ° C.), then the partitioning temperature TP is smaller than the temperature present after step g) (quench temperature TQ +/- 80 ° C.), then the partitioning temperature TP is smaller than the temperature present after step g) (quench temperature TQ +/- 80 ° C.), then the
• the partitioning temperature TP is for the steels according to the invention a maximum of 500 ° C, in particular a maximum of 470 ° C, to the optimum
• the partitioning time tPT is between 30 minutes and 30 hours, in order to allow a sufficient redistribution of the carbon without causing the disintegration of the residual austenite present in the microstructure.
• the partitioning time tPT is composed of the time tPR (heating ramp) required for the heating process and the time tPI provided for the isothermal hold, where tPI can also be zero.
• the proportions of the times tPR and tPI at the partitioning time tPT are variable as long as the total partitioning time tPT predetermined according to the invention is adhered to.
• the heating in step h) takes place with flat steel products wound into a coil
• the heating of the hot strip is optimally carried out with a heating rate ⁇ 1 of up to 1 K / s. Heating rates ⁇ 1 below 0.005 K / s do not appear practical. At heating rates ⁇ 1> 1 K / s, impermissible differences in the temperature between the outer, middle and inner windings of the wound hot strip may occur.
• time tPI is equal to zero.
• desired texture alone during the heating process i. in time tPR, set.
• the partitioning temperature can also be equal to the
• step h is preferably carried out batchwise in one
• Heating rate depends on the one hand on the target temperature and on the other hand, according to the respective operating weight in the hood. If heated too quickly, the belt is not completely evenly heated. This leads to a non-uniform structure, in particular to a different one
• Martensite morphology which further partitioning behavior and thus the Final structure influenced. This is the case in particular with heating units which are integrated directly into the hot strip mill (continuous annealing or induction in-line annealing, as for example in US 2014/0299237).
• a non-uniform structure leads to poor deformability, in particular to a poorer hole widening.
• the proportion of carbon-enriched austenite in the final structure can be adjusted. Too rapid heating causes carbon build-up to crystallographic defects, such as, e.g. Phase boundaries and dislocations, and thus promotes the excretion of
• Transitional carbides and / or cementite This leads to a reduction in the amount of carbon available during the partitioning step for stabilizing the austenite and thus to a non-uniform structure.
• the adjustment of the heating conditions adapted to the kinetics of carbon redistribution during the partitioning step thus enables the setting of a uniform texture with improved
• the maximum heating rate ⁇ 1 during the partitioning step is 1 K / s, preferably 0.075 K / s, for the setting of uniform properties both over the length and width of the flat steel product, since otherwise local irregularities occur together with reduced forming properties, in particular a worsened hole widening. It is particularly advantageous if the heating takes place at a heating rate ⁇ 1 of not more than 0.03 K / s in order to ensure optimum homogeneity of the final structure and thus best hole widening and fatigue properties.
• the minimum heating rate ⁇ 1 is for reasons of economy at 0.005 K / s, preferably 0.01 K / s.
• a bell annealing Another advantage of the use of a bell annealing is that the respective annealing temperatures can be set more accurately than in continuous annealing furnaces.
• the annealing is also carried out in a protective gas mixture, whereby a harmful effect on the hot strip surface, z. B.
• the protective gas used is hydrogen, nitrogen and mixtures of hydrogen and nitrogen.
• Cycle time decoupling to the hot rolling mill This allows better utilization of the hot rolling capacities.
• Step g) taking into account the above explained in terms of compliance with the temperature TQ stipulations.
• step h the hot rolled flat steel product is cooled to room temperature. Cooling in step i) should be carried out with a cooling rate ⁇ 2 of no more than 1 K / s in order to reduce the stress in the
• the flat steel product may also be appropriate to provide the flat steel product in a conventional manner with a metallic coating to protect against corrosion. This can be done for example by electrolytic galvanizing.
• the processing of a steel flat product according to the invention or produced according to the invention takes place in the hot-rolled state. This allows thicknesses of the flat steel product of 1 mm and more, with typical thicknesses in the range of 1, 5 - 10 mm.
• the hot-rolled flat steel product according to the invention is particularly suitable for structural lightweight construction, since the higher strength of a
• the flat steel product according to the invention makes it possible to integrate a component, since due to the good formability, despite high strength, it is possible to replace a plurality of components of an assembly with a component made of hot-rolled flat steel product according to the invention.
• Chassis typical cyclic load requires that the material ideally also has a good fatigue strength.
• the improved formability allows for lightweight construction reduced material thickness new component geometry.
• a use of the flat steel products according to the invention in the construction industry is also advantageous because they have improved formability at high strength. Furthermore, they have an increased yield ratio compared to other flat steel products of comparable strength level. These properties ensure improved structural stability under unforeseen load cases such as earthquakes, impact loads or exceeding the design maximum load.
• test melts A - O having the compositions shown in Table 1 were melted.
• Table 2 shows for the steels A - O the A3 - determined according to equation (1) and the martensite starting temperatures TMS determined according to equation (2).
• melts A - O were cast into slabs, which were then heated to a reheating temperature TWE.
• the thus heated slabs were then rolled in a conventional manner to hot strip with a thickness of 2 - 3 mm, wherein the
• Finishing has included and has been completed at each hot rolling end temperature TET.
• the resulting hot-rolled steel strips were each quenched at a cooling rate 9Q to a quenching temperature TQ at which they were subsequently held for a duration tQ.
• Tempering annealing have been wound into a coil between quenching and holding.
• the hot strips were heated at a heating rate ⁇ 1 for a duration tPR to a respective partitioning temperature TP and held there for a duration tPI.
• Heating rate " ⁇ 1”, holding time "tPI”, partitioning temperature “TP” and heating time "tPR" are given for each of experiments 1-47 in Table 3.
• Called partitioning temperature TP When using a bell annealer, it is also indicated in each case whether it was used to increase (“heat”) the temperature or to keep the temperature ("hold”) constant.
• Experiments 1 1 - 13 show the need to roll above the A3 temperature and to maintain a sufficiently long hold time t Q. With melts D and E, it was possible to produce a material with a strength of 1028 - 1500 MPa and a hole expansion of 22 - 87%.
• the melt M has as an example of a variant with optimized
• melt N was produced as a laboratory melt in the vacuum furnace. With the high-purity melt N, it has been possible to produce a material with very good hole widening (see experiment 46).

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