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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
• • 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
  • C22C38/06—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/008—Martensite

Definitions

• the invention relates to a steel, a flat steel product, a steel component produced therefrom and a method for producing a steel component.
• Hot-formed, press-hardened components made of manganese-boron steels are particularly suitable for crash-relevant automotive components.
• a typical example of this steel quality is the MnB steel known under the name "22MnB5" (material number 1.5528).
• MnB steels Possible uses of MnB steels, Press-hardened components are eg B-pillar, B-pillar reinforcement and bumper of car bodies.
• Combined hot forming and press hardening can be used to produce components with complex geometries and highest strengths (R m : approx. 1500 MPa, R P 0 , 2 : approx. 1100 MPa).
• the components thus obtained are characterized by a predominantly martensitic structure.
• Their high strength basically allows a significant reduction in wall thickness and thus a significantly reduced weight of the component.
• hot-press hardened components typically have only a low ductility from MnB steels (A 80 : approx. 5 - 6%).
• the sheet thickness of hot-press-hardened components is therefore designed in practice much more pronounced for safety reasons, as would normally be necessary taking into account their strength in practice.
• body components are made of so-called "tailored blanks".
• These are sheet metal blanks, which are composed of blanks of different grades of steel.
• a "tailored blank” is made available for the production of a B pillar of a car body, the area of which associated with the upper part of the B pillar consists of a 22MnB5 steel.
• a steel grade is then provided which also after hot-press hardening indicates a higher ductility.
• a suitable steel is known under the name H340LAD (material number 1.0933).
• the areas made of the ductile material in the critical area of the respective component generally have to have a higher sheet thickness in order to achieve the same in normal operation To be able to absorb component loadings. This in turn has a correspondingly higher weight for the overall component.
• a first development direction to meet this requirement is aimed at optimizing the manufacturing process.
• a steel grade with a martensitic structure and improved elongation at break should be able to be produced.
• An example of this procedure is in the EP 1 642 991 B1 described and provides for reaching the martensite stop temperature before a high and then a slower cooling rate. In this way, a self-tempered martensite is produced, which has an improved elongation at break.
• An alternative development direction is the optimization of the process for producing a quality with a multi-phase structure by means of the so-called "semi-hot forming".
• the flat steel product to be formed to the respective component is heated to a temperature between the A C1 and the A C3 temperature at which the steel has a two-phase structure.
• the finished component has a lower martensite content and higher proportions of more ductile phases, such as ferrite or austenite, after cooling compared to conventionally austenitized and hardened components.
• the components still have a comparably high strength.
• tensile strengths R m of 800 - 1000 MPa are achieved with only slightly reduced elongation at break values (A 80 approx. 10-20%) compared to the initial condition.
• Such a procedure is for example in the WO 2007/034063 A1 described.
• the heating temperature is above the A C1 temperature and should be selected taking into account a possible grain growth and the evaporation of the Zn-based coating of the flat steel product from which the component is formed.
• the processed flat steel product is composed according to different alloying concepts.
• the steel in question in wt .-%) 0.15 - 0.25% C, 1.0 - 1.5% Mn, 0.1 - 0.35% Si, max. 0.8% Cr, in particular 0.1-0.4% Cr, max. 0.1% Al, up to 0.05% Nb, in particular max.
• Nb up to 0.01% N, 0.01-0.07 Ti, ⁇ 0.05% P, especially ⁇ 0.03% P, ⁇ 0.03% S,> 0.0005 bis ⁇ 0.008% B, especially at least 0.0015% B, and the balance of unavoidable impurities and iron, wherein the Ti content is 3.4 times greater than the N content.
• the object of the invention was to provide a steel in which it is ensured with high reliability that a component manufactured from it has in each case high strength values and an increased elongation at break.
• a steel flat product made using this steel, a steel component made therefrom, and a method suitable for producing such a steel component should be given.
• the solution according to the invention of the abovementioned object is that such a steel component is designed according to claim 9.
• the invention is based on the recognition that, by selecting a suitable alloy and setting a suitable microstructure composition, it is possible to provide a steel having a high strength of at least 1000 MPa and an elongation at break A 80 after austenitization, thermoforming and hardening each safely above 6%.
• the steel according to the invention contains (in% by weight) 0.15-0.40% C, 1.0-2.0% Mn, 0.2-1.6 Al, up to 1.4% Si, where the sum of the contents of Si and Al is 0.25 - 1.6%, up to 0.10% P, 0 - 0.03% S, up to 0.5% Cr, up to 1.0% Mo, up to 0.01% N, up to 2.0% Ni, 0.012-0.25% Nb, up to 0.40% Ti, and 0.0015-0.0050 B with the remainder iron and unavoidable impurities.
• a flat steel product according to the invention has at least one region which consists of a steel according to the invention.
• a flat steel product according to the invention can be designed as a tailored blank, in which one region is produced from a steel according to the invention, while another region is produced from a different steel.
• the area produced by the steel according to the invention of the tailored blanks according to the invention forms on the finished, from the steel flat product produced steel component then a high-strength area in which a high strength combined with a good elongation at break.
• a steel component produced from such a flat steel product according to the invention then has at each point the advantageous combination of high strength and good extensibility achieved by the steel alloy according to the invention.
• a steel component according to the invention is correspondingly characterized in that it consists of a steel according to the invention at least in one region and that its structure in the region of the high-strength steel according to the invention is composed of martensite, austenite and up to 20 area% ferrite.
• a flat steel product according to the invention is first of all made available. This flat steel product is then heated through to an austenitizing temperature of 850-950 ° C. The required hold time is typically 2 to 10 minutes.
• the flat steel product is usually transported to a thermoforming mold to be thermoformed there.
• the transport time should be limited to 5 - 12 seconds.
• the thermoforming itself can be carried out in a conventional manner as compression molding.
• the steel component is cooled down so rapidly that the steel component obtained after cooling has a structure consisting of martensite, austenite and up to 20 area% of ferrite.
• the typically required cooling rates are in the range of at least 25 ° C / s.
• the hot forming and cooling can be carried out in one or two stages. In single-stage hot press hardening, hot forming and hardening are performed in one go together in one tool. In contrast, in the two-stage process, cold forming takes place first (up to 100%) and only then is the final hot forming, including the production of the hardened structure.
• the component according to the invention When the respective processed flat steel product has been austenitized within the abovementioned temperatures, the component according to the invention, after hot forming and accelerated cooling in the region consisting of a steel according to the invention, has a structure which is characterized by a combination of a hard phase ( Martensite) and at least one more ductile phase (austenite and ferrite).
• Martensite hard phase
• austenite and ferrite ductile phase
• the ferrite content is limited by the inventively given composition of the processed steel to 20 area%, since an improvement in the elongation values and an increase in the energy absorption by austenite are preferred.
• austenite and a maximum of 20 area% ferrite are the mechanical technological properties of components according to the invention over the entire temperature range of the present invention carried out at 850 - 950 ° C austenitization reliably obtained.
• the stability of the mechanical-technological properties of the component produced according to the invention is ensured by the analysis concept according to the invention.
• the structure consisting of a combination of hard (martensite) and ductile (austenite and ferrite) phases of a component according to the invention ensures optimum behavior in the event of a crash load.
• the austenite to martensite phase transformation that occurs during the deformation of the hot formed component causes the component to subsequently harden when deformed at high kinetic energy in the event of a crash.
• the combination of high strength, good elongation at break and optimum crash behavior in the region of its high-strength region which is the aim of the invention is achieved with particular certainty if the martensite content of the microstructure in the high-strength region in question is at least 75 area% in a component according to the invention.
• the required high elongation at break can be ensured by the fact that the austenite content of the structure of the component according to the invention is at least 2 area%.
• the tensile strength of a component made from steel according to the invention should not be below 1000 MPa in the region of its high-strength range. So that the Martensithärte necessary for this purpose is achieved,
• the steel alloy according to the invention contains a C content of at least 0.15 wt .-%. At the same time to ensure sufficient welding suitability for practice, the C content of the steel according to the invention is limited to 0.4% by weight at the top.
• the alloying elements Mn, Si and Al of a steel used according to the invention are of particular importance, since they stabilize the austenite at room temperature.
• the Mn present in amounts of at least 1.0% by weight in the steel of the invention serves as an austenite former by lowering the Ac 3 temperature of the steel.
• the result is a microstructure consisting essentially of austenite and martensite after hot working.
• the Mn content is limited to a maximum of 2% by weight.
• Silicon is present in the steel of the present invention at levels of up to 1.4% by weight. It affects the hardenability and serves in the melting of the steel of the component according to the invention as a deoxidizer. At the same time, Si increases the yield strength, stabilizes the ferrite and austenite at room temperature, and prevents unwanted carbide precipitation in austenite during cooling. However, too high an Si content causes surface defects. Therefore, the Si content of a steel of the present invention is limited to 1.4% by weight.
• Aluminum contributes to the stabilization of the ferrite and the austenite at room temperature in the steel according to the invention, similar to Si, and effects a grain size control. These effects are certainly achieved when the contents of Al in the inventive manner to 0.2 to 1.6 wt .-% are limited, with Al contents of at least 0.4 wt .-% particularly positive on the properties of an inventive Impact component.
• the carbide formation is suppressed during the heat treatment and thus the inventively provided proportion of austenite of preferably at least 2 area% stabilized in the thermoformed structure.
• phase constellation according to the invention Due to the phase constellation according to the invention, a reduction of the scattering of the mechanical properties of a steel according to the invention after its austenitization, hot working and cooling is achieved. Surprisingly, it has been shown here that the mechanical properties of a component produced according to the invention can be achieved with high reliability over a comparatively large temperature range of the austenitizing temperatures. Thus, despite the tolerances inevitably occurring in practice when setting the austenitizing temperature, the desired properties of components according to the invention can be ensured with high reliability and stability of the work result.
• the sum of the Al and Si contents of a steel component according to the invention can be increased to at least 0.5 wt .-%.
• Mo may be present in a steel of the invention at levels of up to 1.0% by weight.
• the presence of Mo promotes martensite formation and improves the toughness of the steel.
• an excessive Mo content may cause cold cracking.
• the hardenability can be increased.
• the Cr content should not be higher to avoid surface defects. Certainly, these effects can be achieved when the Cr content is limited to 0.1 wt%.
• P can be alloyed in amounts of up to 0.10 wt .-% to increase the yield strength and thus to secure the mechanical properties. Too high a P content, however, damages the ductility and toughness of a steel made in accordance with the present invention.
• Ti in amounts of up to 0.40 wt .-% increases the yield strength both dissolved and by precipitation formation (eg of Ti-carbonitrides).
• Ti binds N to TiN, thus promoting the efficiency of B in terms of conversion behavior. This effect can be ensured by the Ti content of the steel according to the invention being the condition % Ti - 3 . 42 x % N > 0 . 005 weight , - % where% Ti is its respective Ti content and% N is its respective N content.
• the hardenability of a steel according to the invention is improved by retarding the ferrite transformation during cooling towards longer transformation times.
• the boron present in the steel according to the invention stabilizes the mechanical properties for a wide temperature range of the hot forming process.
• N stabilizes the austenite and increases the yield strength of a steel according to the invention.
• the nitrogen present in the alloy steel according to the invention is not completely bound by Ti, it reacts in combination with boron to give boron nitrides.
• These boron nitrides cause a grain refining of the initial microstructure and thus a refining of the martensitic thermoformed microstructure. As a result, the susceptibility to cracking of a steel processed according to the invention is thus reduced.
• the boron nitrides contribute significantly to increasing the strength of the steel according to the invention.
• the N content which is not bound to Ti can be adjusted in a targeted manner such that, in the presence of Ti, the N content of the steel according to the invention is the condition 0 . 0015 ⁇ % N - % Ti / 3 . 42 ⁇ 0 . 0060 weight , - % where% Ti is its respective Ti content and% N is its respective N content.
• Nb at levels of 0.012-0.25% by weight in a steel alloyed according to the invention promotes the combination of high tensile strength values with increased elongation at break, resulting overall in an increase in the energy absorption capacity of steel components according to the invention.
• Nb in composite steel according to the invention increases the yield strength by means of carbide precipitation and, due to austenitic grain refinement, causes a fine martensite structure which has a high stability against crack propagation.
• Nb precipitates may act as hydrogen traps, thereby reducing susceptibility to hydrogen-induced cracking.
• the positive effect of niobium is particularly safe if the Nb content of the steel according to the invention is limited to 0.012-0.040% by weight.
• Ni in amounts of up to 2.0% by weight contributes to increasing the yield strength and the elongation at break.
• the S content of the steel of a component according to the invention is limited to max. 0.03 wt .-% limited, because S has a strong negative impact on the weldability and the possibilities of surface finishing. Also, this limitation is intended to prevent the formation of harmful, elongated MnS excretions.
• the steel component according to the invention may be coated on its free surface with a protective coating against oxidation.
• a protective coating against oxidation This is preferably already present on the flat steel product from which the component is thermoformed.
• the protective cover can be designed so that it protects against scale formation during heating and thermoforming and / or corrosion during processing or in practical use.
• metallic, organic or inorganic coatings as well as combinations of these coatings can be used.
• the coating of the flat steel product can be carried out by conventional methods. A surface refinement in the hot-dip process is preferred.
• the optional metallic coatings are based on the Zn, Al, Zn-Al, Zn-Mg, Al-Mg, Al-Si and Zn-Al-Mg systems and their unavoidable impurities. Coatings on an Al-Si basis have proven to be particularly useful.
• the pre-oxidation can advantageously be preceded by the hot-dip process.
• an oxide layer which is 10-1000 nm thick is selectively produced on the flat steel product, with particularly good coating qualities resulting when the oxide layer is 70-500 nm thick.
• the adjustment of the oxide layer thickness takes place in an oxidation chamber, as for example from the WO 2007/124781 A1 is known.
• the iron oxide layer is reduced by hydrogen of the annealing atmosphere. It can be on the surface as well as up to a depth of 10 microns oxides of the alloying elements are present.
• the electrolytic coating is particularly suitable for applying the respective coating. Particularly good results are obtained when Zn, ZnFe, ZnMn, ZnNi systems or their combination are used as the coating material.
• PVD Physical Vapor Deposition
• CVD Chemical Vapor Deposition
• an electroless or chemical deposition of metallic (alloy) coatings based on Zn, Zn-Ni, Zn-Fe and their combinations as well as organic / organometallic / inorganic coatings in coil coating systems in the coil coating, spraying or dipping method may be useful.
• Typical thicknesses of the coatings which can be produced by the methods described here are in the range from 1 to 15 ⁇ m.
• a steel sheet is produced in a similar manner from comparison steel V with a composition likewise indicated in Table 1, and a larger number of sheet metal plates have been divided from this steel sheet, which also consisted uniformly of comparative steel V.
• the blanks consisting of the steels E1-E4 and V are each heated to an austenitizing temperature lying in the range of 880-925 ° C, then placed in a thermoforming mold and then hot-formed into a component. After thermoforming, the components thermoformed from the blanks each have been cooled down so rapidly at a cooling rate of at least 25 ° C./s, that hardened structures have formed in them.
• the mechanical properties yield strength R p0.2 , tensile strength R m and elongation A 80 were determined for the components obtained.
• the respective average values R p0,2 , R m and A 80 as well as the associated standard deviations ⁇ R p0,2 , ⁇ R m and ⁇ A 80 are given in Table 2 for the steel components produced from the steels E 1 -E 4 and V.
• the components consisting of the steels E1-E4 according to the invention have a consistently high residual deformation capacity characterized by a high value of the product of tensile strength R m and elongation A 80 and, consequently, high energy absorption capacity.
• the results of the tests show that the mechanical properties R p0,2 , R m and A 80 of the components produced from steels E1-E4 according to the invention can be reproduced with a significantly higher reliability characterized by small values of the respective standard deviation than this in the case of the components produced from the comparison steel V is the case.
• Table 1 (in% by weight) stole C Si Mn P s al Cr Not a word N Ni Nb Ti B E1 0.217 0.39 1.63 0,003 ⁇ 0.001 1.08 0,038 0.0016 0.0011 0,014 0,025 0,036 0.0030 E2 0.217 0.41 1.64 0.005 0,002 0.62 0.027 0.0016 0.0023 0,008 0,029 0,022 0.0024 E3 0,205 0,203 1.64 ⁇ 0,10 ⁇ 0,10 0,690 ⁇ 0.1 0.0041 0,012 0.0010 0.0029 E4 0.211 0,203 1.65 ⁇ 0,10 ⁇ 0,10 0.662 ⁇ 0.1 0.0024 0,013 0.0020 0.0032 v 0.214 0.14 1.62 0.005 0,002 1,386 0.086 ⁇ 0.002 0.0015 0,006 0,006 0.0030 0.0004 stolen R p0.2 [MPa] OR p0.2 [MPa] R m [MPa] ⁇ R m [MPa] A 80 [%]

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• 2010
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