Classifications

• • 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
  • C21D1/26—Methods of annealing
  • C21D1/32—Soft annealing, e.g. spheroidising
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/003—Cementite

Definitions

• the present invention further relates to a method of treating a steel product according to the preamble of claim 1. Furthermore, the invention relates to an apparatus for treating a steel product according to the preamble of claim 12. Finally, the invention also relates to a steel product.
• a steel product for example a billet for a finished product, heats up bar steel, then descaled and then rolled in a rolling train to the required final diameter.
• the austenitic rolling stock is successively converted into a cubic body-centered structure.
• first ferrite and on further cooling ferrite and carbides precipitate in the form of perlite as equilibrium phases (sometimes the carbides (M 3 C) are also called cementite).
• M 3 C carbides
• a subsequent heat treatment causes the cementite lamellae of perlite to form as spherical carbides.
• the microstructure consists of ferrite, spherical cementite (iron carbide) and a proportion of unformed cementite lamellae.
• the material has a high ductility according to the set structure, which is reflected in high elongation at break and Brucheinschnürungs exchange. The tensile strength is correspondingly low due to the high ferrite content.
• the object of the present invention is to improve a method and a device of the type mentioned above such that in the steel product, in particular, an improved yield strength and improved tensile strength compared to the prior art can be realized. Furthermore, a correspondingly improved steel product is to be provided.
• the basic idea of the present invention is, in particular, that hot rolling stock - the temperature is preferably selected so that the rolling stock structure is austenitic - a rapid cooling and a subsequent special heat treatment, the "annealing of spherical carbides" is subjected.
• the yield strength and tensile strength of the resulting steel product are higher than those achievable via the known process route.
• a steel product is treated, and the invention is basically not limited to certain types or grades of steel products.
• the steel product may be one which by means of a hot forming process has been reshaped.
• the steel product may be formed as a rolled product which has been produced by means of a rolling process.
• the steel product is first produced by means of a hot forming process, for example a rolling process. Subsequently, the steel product is treated by the method according to the invention.
• a manufacturing process includes in particular the process sections forming and treatment of the steel product.
• the treatment method according to the invention represents an aspect of the production process of a steel product.
• the steel product is a finished product bar steel.
• the starting or starting material used for this purpose for example "billets”, can be heated, for example in the walking beam furnace, from room temperature in the range from -20 to + 30 ° C. for 2 hours to the temperature of 1120 to 1170 ° C. from the furnace removal or drawing temperature.
• the cast structure of the billet transforms homogeneously from cubic body centered at room temperature into face centered cubic, an austenitic structure, at drawing temperature.
• the scale adhering to the billet surface for example iron oxide, can be removed from the billet by means of high-pressure descaling. This is followed by the transformation of the initial dimension billet to the required final diameter of the bar steel in a rolling mill.
• the steel product is cooled at a high cooling rate, in particular by means of water cooling.
• the invention is not limited to particular types or methods of cooling. It is only important that following the forming an intensive cooling of the steel product, for example, the rolling stock takes place. This can preferably be realized in that the cooling takes place by means of water cooling.
• the cooling takes place at a high cooling rate.
• a high cooling rate means in particular that the cooling rate is higher than the cooling rate of a cooling in air, a so-called air cooling.
• air cooling can be done for example on a Rechenkühlbett.
• the cooling with high cooling rate can be influenced in particular by the process boundary conditions, such as rolling speed and / or water temperature and / or cooling line length and / or number of cooling nozzles used and / or cooling water quantity and / or cooling water pressure.
• the steel product may be cooled below the A1 line, for example, cooled to 350 to 550 ° C or about 350 to 550 ° C.
• the steel product is subjected to a further process.
• This is a heat treatment "annealing on spherical carbides".
• the "annealing on spherical carbides” is basically known and familiar to the expert. It is also referred to as "GKZ annealing”.
• Such a heat treatment can be carried out, for example, in a heat treatment furnace.
• GKZ annealing that is, a heat treatment just below or above the A 1 line, followed by a defined cooling, in particular, a microstructure state is achieved with which a transformation of the beam product at room temperature is easier to carry out.
• a structure is necessary, which consists as much as possible of ductile, ie tough ferrite, in which the hard constituents, the iron carbides, are embedded spherical.
• the aim of the GKZ annealing is thus in particular spherically shaped iron carbides in a ferrite matrix.
• the steel product After completion of the treatment process, the steel product has an improved yield strength and improved tensile strength, in particular compared to a steel product produced according to the known process route described above on. Similarly, comparable values for elongation at break and fracture constriction relative to a steel product made by the known process route are feasible.
• the steel product before cooling to a temperature at which the structure of the steel product is present in the austenitic region.
• the inlet temperature of the material in a cooling device, in which the cooling takes place is therefore still so high that the steel product structure, for example, the rolling stock, is in austenitic state.
• the inlet temperature may be in the range of about 900 to 1000 ° C.
• the cooling of the steel product takes place in such a way that the microstructure distribution and / or temperature distribution of the steel product is inhomogeneous after cooling over the cross section.
• the steel product is cooled at such a cooling rate that diffusion-controlled conversion processes do not occur, at least in the near-surface region of the steel product.
• the steel product for example the rolling stock
• the steel product is cooled so rapidly that the diffusion-controlled conversion processes, in particular the transformation of the austenite into ferrite and perlite, are lacking time at least in the near-surface region of the steel product can not run off.
• the extremely hard imbalance structure martensite is formed predominantly in the marginal zone of the steel product with the carbon positively dissolved in the iron matrix lattice.
• the steel product is cooled at such a cooling rate that bainite forms in the direction of the core in a transition zone between the edge region and the core of the steel product.
• the transition zone adjacent to the core does not reach the high cooling rate of the edge area, so that in this area not martensite but bainite forms.
• the steel product is cooled at such a cooling rate that in the core of the steel product in addition to bainite the microstructure troostite, sorbitol and ferrite are formed.
• the nucleation and diffusion-controlled structure troostite, sorbitol and ferrite can be formed in addition to bainite due to the lowest cooling rate over the cross section of the steel product.
• the structure of the jet product is in particular completely or almost completely present in the body centered cubic state.
• the distribution of microstructures - such as marginal martensite, bainite in the transition zone and bainite, troostite, sorbitol and ferrite in the core - and / or the temperature distribution between the surface and the core is / are highly inhomogeneous.
• the steel product is cooled such that the heat content remaining in the core is so high that at least the transition zone between the core and the edge region of the steel product is reheated from the core.
• the cooling can also be chosen so that the heat remaining in the core next to the transition zone and the edge area heated again.
• the surface of the steel product such as the rolling stock surface that was in direct contact with the cooling water, and the adjacent edge zone show the lowest temperature on the steel product. Both are preferably heated or tempered again from the core region, which has a higher temperature.
• the steel product is cooled in a water cooling section.
• a water cooling section it can be provided that the steel product is passed through a water bath or cooling tubes, or that the steel product is sprayed with water.
• the present invention is not limited to particular embodiments of the water cooling system.
• the cooling can take place in a water cooling section that is up to 22 meters long.
• the steel product to be cooled passes through a box filled with water, or that the steel stock passes through a device in which water with defined process parameters is applied to the steel product to be cooled.
• the water with a pre-pressure of 0 to 10 bar and / or with a volume flow of 0 to 1000 m 3 / h is applied to the steel product to be cooled.
• the steel product is cooled, for example to about 350 to 550 ° C. In this case, an equilibrium conversion does not take place, at least in the edge zone of the steel product, and the carbon remains in the metal grid with forced release. After cooling, therefore, it is deliberately not a homogeneous microstructure, but an inhomogeneous microstructure is set, since then followed by a heat treatment.
• the subsequent heat treatment "annealing on spherical carbides” causes in the "near-surface region" of the material, in particular, that the previously forcibly dissolved carbon in martensite in the iron carbide form passes and finely dispersed precipitates as spherical molded cementite.
• the matrix in which this cementite is present after the heat treatment is ferrite.
• the bainite present in the "transitional and core region” precipitates iron carbides in their finest form.
• the cementite lamellae of the sorbitol and trostite present in the core area also form globular cementite in the course of the annealing treatment.
• carbide formation takes place extremely finely and homogeneously due to the very low cementite lamella spacing of the sorbitol and trostite.
• the finely dispersed globular cementite embedded in a ferrite matrix after the annealing treatment was precipitated or formed from the structural constituents present before the heat treatment: tempered martensite, bainite and fine-lamellar perlite - sorbitol and troostite.
• the ductility of the steel product according to the invention is due to the high ferrite content in the structure and the homogeneously molded cementite comparable to that which can be adjusted via the known process route. Both the yield strength and the tensile strength of the steel product produced according to the invention Due to the finely dispersed and globular molded cementite, they are significantly above those of the conventional route.
• HSA process Hardened and Soft Annealed
• an apparatus for treating a steel product, in particular a rolled product, after a hot working process, in particular for improving yield strength and tensile strength characterized by means for cooling the steel product after high speed hot working, in particular by means of water cooling, which may be formed as a water cooling section, for example, and a device for heat treatment "annealing on spherical carbides" of the cooled steel product, which may be formed as a heat treatment furnace.
• the device has, in particular, means for carrying out the method according to the invention as described above, so that with respect to the design and mode of operation of the device, reference is made in full to the above statements on the method according to the invention and reference is made.
• the steel product is obtainable, in particular, by a method according to the invention as described above, so that reference is made in full and to the above explanations concerning the method according to the invention with respect to the nature and properties of the steel product.
• the steel product can be produced in a device according to the invention as described above be treated, so that reference is also made to the relevant statements to the device and referenced.
• a steel product in the form of a rolled product is first produced by means of a forming process, a rolling process and then treated.
• the values for yield strength and tensile strength can be improved by the present invention compared to the known process route. Toughness properties such as elongation at break and fracture neck can be approximated.
• the starting material is first heated in a process unit designed as a lifting beam furnace.
• a process unit designed as a lifting beam furnace for example, the following process boundary conditions to be set: heating from ambient temperature that is -20 to + 30 ° C to drawing temperature 1120 to 1170 ° C in about 2 hours.
• the starting material is descaled in a process unit designed as a scale scrubber.
• the following process boundary conditions can be set: Descaling pressure: about 250 bar.
• the descaling pressure is that pressure that is present before the descaling nozzles.
• the process boundary conditions for rapid water cooling of approximately 900 to 1000 ° C. to, for example, 350 to 550 ° C. are influenced in particular by the rolling speed and / or the water temperature and / or the cooling line length and / or the number of cooling nozzles used and / or the amount of cooling water and / or cooling water pressure.
• This rapid cooling can be followed by a slow or rapid further cooling, for example at the computing cooling bed.
• a heat treatment of the cooled rolling stock in a processing unit which is designed, for example, as a heat treatment furnace.
• the process boundary conditions are adjusted so that an annealing on spherical carbides, the so-called GKZ annealing occurs.

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

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• 2011
  • 2011-07-08 DE DE201110051682 patent/DE102011051682B4/de not_active Expired - Fee Related
• 2012
  • 2012-07-02 EP EP12401136A patent/EP2543744A1/fr not_active Withdrawn

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