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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or 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
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • 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
  • 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
  • C23C2/14—Removing excess of molten coatings; Controlling or regulating the coating thickness
  • C23C2/16—Removing excess of molten coatings; Controlling or regulating the coating thickness using fluids under pressure, e.g. air knives
• • 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
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • 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

Definitions

• This invention includes the production method of galvanneal (Fe-Zn alloy) coated high strength steel which is cold rolled and annealed in a continuous galvanizing line, with a tensile strength value higher than 440MPa, and it is a steel grade developed for the automotive industry.
• the automotive industry contributes to weight reduction efforts by using high- strength and thinner gauge steels as much as possible in many parts without compromising safety limits. In this way, the production of lighter vehicles reduces fuel savings and CO2. In addition to high strength properties, it demands advanced formability properties in order to produce safety parts in complex geometries in the automotive industry.
• Coating is one of the most successful methods in protecting metallic materials from corrosion. Steels are coated with metals and/or alloys that will help slow down the corrosion rate by forming a passive oxide layer.
• Zinc coating is one of the most common and successful method. Zinc coating can be coated on the steel surface as a hot dip method or electrolytic coating.
• Hot-dip Zn coating method is divided into two main headings as galvanization and galvannealing.
• Galvannealed coating is carried out in a continuous galvanizing line, and it is formed by the transformation of the pure zinc coating into intermetallics consisting of Fe and Zn, by annealing the sheet in a short time (5-20 seconds) after hot dipping, passing through an induction furnace (490-530°C).
• This process called "galvannealing” in short, is a short-term heat treatment that transforms the zinc coating into a multi-phase alloy coating containing iron-zinc intermetallics. With this process, steel sheet production is realized with high corrosion resistance, weldability and paintability properties especially suitable for the automotive industry.
• the delta phase is located between the zeta phase and the gamma phase, and the amount of Fe it contains varies between 8 and 15%. It is the most important phase that provides the optimum properties of the coating. In the zeta phase observed on the outermost surface of the coating, the amount of Fe varies between 5 and 7%. It is the most ductile phase of the coating and its presence more than 10% on the surface increases the surface friction coefficient. Therefore, during cold forming, it can also cause the coating to be removed/ejected due to plastering in the cold stamping die. In general, it is preferred that the total amount of Fe% in the coating is between 8-12%.
• Patent number CN104975226A is a Chinese patent owned by Wuhan Iron & Steel Group Corp. This patent relates to automotive galvanneal-coated steel with a tensile strength of 440 MPa. Steel 0.08-0.11% C, less than or equal to 0.03% Si, 1.10- 1.40% Mn, 0.015-0.030% P, maximum 0.010% Si and 0.020-0.070% by weight contains soluble Al in the range.
• the production method of the aforementioned steel includes the production steps of steelmaking, continuous casting, slab heating, hot rolling, cooling, winding, pickling, cold rolling, continuous annealing, hot dip galvanizing.
• Yield strength is 330-360 MPa, tensile strength is at least 440 MPa, elongation is at least 32%. Surface roughness is 0.6-1.5 urn, PC value is maximum 90/cm. Steel has been described as cheap, highly productive, and advanced in pollination.
• hot rolling parameters annealing parameters and GAF (galvannealing furnace) holding time.
• GAF galvannealing furnace
• no information is given about the steel micro structure.
• Ti was not used in the patent numbered CN104975226A and Ti was limited to 0.030% in our invention. There are coating problems above this value.
• Manganese (Mn) is targeted between 0.90% and 1.70% by weight. Mn improves the hardenability properties of steel. It contributes to the development of yield strength and tensile strength thanks to solid solution hardening and phase transformation. Thanks to its austenite stabilizer behavior, it reduces the transformation temperature of austenite to martensite, and reduces the formation of grain boundary cementite by slowing the formation of carbide.
• austenite stabilizer behavior it reduces the transformation temperature of austenite to martensite, and reduces the formation of grain boundary cementite by slowing the formation of carbide.
• the amount of Al in the pot, the pot temperature and the travelling time of the strip in the ladle should be considered as a whole.
• the amount of Al and temperatures in our invention are partially similar, comparison cannot be made since only the amount of Al and the temperature of the pot are given in the patent numbered CN104975226A.
• the tensile strength values of 440 MPa are the same in our invention and patent CN104975226A, there are significant differences in hot rolling parameters, annealing parameters, GAF annealing time and alloy design.
• Patent number CN104093873A is a Chinese patent owned by JFE Steel Corp.
• a galvanneal-coated automotive steel with a yield I tensile ratio of over 0.70, a tensile strength of at least 590 MPa, and a ferrite + martensite microstructure is described.
• Our invention differs from this patent in terms of microstructure, chemical composition and mechanical properties.
• our invention which consists of ferrite and pearlite phases in micro structure, is different from JFE Steel Corp., which has a dual-phase steel structure (ferrite + martensite).
• One of the biggest factor in this differentiation is the high Mn content.
• Mn enables the production of multi-phase steels by facilitating hard phase transformations as well as solid solution hardening Therefore, up to 3% Mn has been used in the relevant patent, and it is known that high Mn use causes banding problem in the steel internal structure and again provides inhomogeneous properties. Therefore, in our invention, which provides a minimum tensile strength of 440 MPa, the Mn content is limited to an amount that will provide solid solution hardening and banding will not cause a problem.
• the element P can be used up to a maximum of 0.1%, and it is known that it will cause significant problems especially in the secondary work embrittlement and zinc alloying process.
• the weight of Si is limited between 0.01% and 0.7%, and the use of high amounts of Si causes oxide formation on the steel surface during annealing, resulting with coating problems. Therefore, in our invention, silicon has been added in an amount (in the range of 0.05% - 0.3%) that will benefit from solid solution hardening and carbide delayer of silicon and will not cause coating problems. In addition, in order to minimize the surface oxide problems caused by silicon, selective oxidation is prevented with the amount of hydrogen and dew point in a controlled atmosphere environment. The reason why the amount of Al is similar in the patent numbered CN104093873A and in our invention is that the deoxidation process is carried out with Al during steel production.
• the annealing temperature was determined as 800 - 900 °C in the patent numbered CN104093873A, and the process was carried out in the ferrite + austenite region in this invention.
• the A3 temperature was determined depending on the alloy design and the annealing process was carried out above this temperature.
• the cooling stage was carried out in 2 stages, slow cooling and fast cooling in our invention, and in the patent numbered CN104093873A the average cooling rate is given.
• the slow cooling step is important in terms of improving the forming properties for the steel, which is the subject of our invention. Therefore, slow and fast cooling steps are also defined in our invention. Since the ferrite + martensitic phase distribution in the final microstructure is targeted in the patent numbered CN104093873A, the average cooling rate has gained critical importance.
• the present invention includes the production method of galvanneal (Fe-Zn alloy) coated high strength steel which is cold rolled and annealed in a continuous galvanizing line, with a tensile strength value higher than 440MPa, and it is a steel grade developed for the automotive industry.
• galvanneal Fe-Zn alloy
• the main purpose of the invention is to increase formability.
• the use of galvanneal-coated high-strength steels in the automotive industry has led to the development of passenger safety standards.
• Another aim of the invention is to provide weight reduction in automotive.
• fuel savings and reduction of CO2 emissions will be achieved by the use of high-strength galvanneal steel grades.
• Another object of the invention is to increase corrosion resistance.
• Galvanneal coating provides enhanced atmospheric corrosion resistance and high coating adhesion.
• Another object of the invention is to improve weldability.
• spot welding performance developed for the automotive industry.
• Another aim of the invention is to provide homogeneous Fe diffusion and surface properties throughout the material in galvanneal coating.
• Figure 2 Microstructure Showing the Separation of Ferrite and Perlite Phases
• Figure 3 Degenerate Perlite, Perlite, Spherical Cementite SEM Image
• Figure 4 Fe-Zn phase diagram
• high-strength steel with a tensile strength value higher than 440MPa, cold rolled and galvanneal (Fe-Zn alloy) coated, annealed in continuous galvanizing line, and the production method of this steel are explained only for a better understanding of the subject and without any limiting effect.
• the invention relates to cold rolled and galvanneal (Fe-Zn alloy) coated high strength steel with a tensile strength value higher than 440MPa, annealed in a continuous galvanizing line, and its production method.
• the use of large amounts of P for solid solution hardening and the use of large amounts of Ti to provide precipitation hardening indicate known analysis designs and cause negative effects for galvanneal coating quality to produce high-strength steels with a tensile strength value higher than 440MPa.
• limitations were placed on P and Ti elements during the analysis design stage and it was aimed to benefit from solid solution hardening of C, Mn, Si elements as a basis to provide the target mechanical properties.
• This invention includes the production method of galvanneal (Fe-Zn alloy) coated high strength steel with a tensile strength higher than 440MPa, which is cold rolled and annealed in a continuous galvanizing line.
• galvanneal Fe-Zn alloy
• This content analysis design and hot rolling, cold rolling and continuous galvanizing line designs are specified for the targeted microstructure, mechanical properties and optimum coating properties.
• Analysis design for high strength steel sheet, containing ferrite-perlite microstructure, galvanneal-coated by hot-dip method, which is the subject of the invention, includes by weight;
• Steel sheet subject to the invention contains: 0.03 ⁇ C ⁇ 0.14%, 0.90 ⁇ Mn ⁇ 1.70%, 0.05 ⁇ Si ⁇ 0.3%, 0.005 ⁇ Al ⁇ 0.08%, P ⁇ 0.02%, Ti ⁇ 0.035%, N ⁇ 0.02% and Fe (balance) elements, as well as impurity elements that remain from the steel production process and cannot be disposed of.
• N Although there is no special limitation for N, it is kept as low as possible so that the natural aging performance of the material is not adversely affected and there is no loss in formability properties.
• the metallurgical effects of the alloying elements in the material which offers a tensile strength of at least 440 MPa, better weldability and higher corrosion resistance thanks to the galvanneal coating, are as follows;
• Carbon (C) is targeted between 0.03% and 0.14% by weight.
• carbon is one of the critical elements in obtaining improved mechanical properties, but the optimum amount should be preferred in analysis designs, since its high use affects the weldability properties negatively. Especially the weldability feature, which is aimed to be used in the automotive industry, comes to the fore even more.
• the critical point is to provide the required amount of perlite with appropriate heat treatment steps by enriching the austenite with sufficient amount of carbon during the high temperature annealing and slow cooling process steps.
• Silicon (Si) and Manganese (Mn) elements in the composition also contribute to carbon enrichment of the austenite phase by delaying carbide formation.
• C is preferably used in the range of 0.05 ⁇ C ⁇ 0.08.
• Mn Manganese
• Mn is targeted between 0.90% and 1.70% by weight. Mn improves the hardenability properties of steel. It contributes to the development of yield strength and tensile strength thanks to solid solution hardening and phase transformation. Thanks to its austenite stabilizer feature, it reduces the transformation temperature of austenite to martensite, and reduces the formation of grain boundary cementite by slowing the formation of carbide.
• the Mn content is preferably used in the range of 1.30 ⁇ Mn ⁇ 1.40.
• Silicon (Si) is targeted between 0.05% and 0.3% by weight. Silicon not only contributes directly to the development of yield strength but also slows the carbon diffusion of austenite during cooling and prevents the formation of cementite, thanks to its carbide retarding property. In this way, it increases the austenite stability. However, when 0.3% by weight or more is used, the continuous galvanizing line can cause surface oxides to form during the annealing process, and uncoated areas to form in the hot-dip coating. In addition, surface oxides prevent Fe diffusion during galvanneal coating, resulting in heterogeneous coating properties.
• Aluminum (Al) is included in the analysis originating from the deoxidation process in the steel subject to the invention. Therefore, the presence of Al in the range of 0.005% - 0.08% by weight did not have a negative effect. Therefore, the Al content is preferably kept in the range of 0.020 ⁇ Al ⁇ 0.050. Since Phosphorus (P) and Titanium (Ti) cause undesirable heterogeneity in galvanneal coating quality, the maximum is limited to 0.02% and 0.030%, respectively. The fact that the P element is generally located at the grain boundaries, therefore, secondary work embrittlement and the grain boundary in the zinc alloying process and the different rate of Fe diffusion within the grain limited the P element to 0.02%. In steel grades containing Ti, the fact that the Fe2Als compound formed between the coating and the steel with the outburst reaction has different stability levels causes inhomogeneous Fe diffusion. Therefore, it has been found appropriate that there is preferably no addition of Ti and P.
• the remainder of the composition consists of impurity elements, which are analyzed from iron and steel production processes and have no metallurgical effect on the final product.
• the slabs produced in line with the target chemical composition are reheated in the slab furnaces in order to achieve the high rate of plastic deformation to be exposed in the hot rolling process equally and homogeneously.
• This heating temperature is carried out at temperatures where the entire austenitic microstructure can be achieved and is selected as at least 1150°C for the homogenization of the process.
• the highest reheat temperature of 1275 °C is preferred in order to reduce the austenite grain coarsening and to improve the surface quality of the hot rolled product.
• the slab is heated up to the temperature range of 1200-1250 °C.
• the final rolling exit temperature is aimed to be at least 850°C.
• the final rolling exit temperature should be at least 920 °C (Tfinish).
• the hot rolled product is coiled in the range of 700°C to 750°C, preferably at 720°C. This temperature range was chosen high (>A1) for the least amount of hard phase formation, and small amounts of pearlite and carbide dispersed in the coarse ferrite matrix were allowed in the hot product. In this way, it allowed high cold high deformation in cold rolling operations.
• cold rolling is applied between 45% and 85%, preferably between 50% and 70%, and the target thickness of the input material is produced for the continuous annealing and coating process.
• the best recrystallization kinetic was achieved in the annealing process and optimum grain size was obtained in the annealed and coated final product.
• the cold rolled, high dislocation density material is heated to the target annealing temperature in the continuous annealing line at a heating rate between 1°C and 40°C per second.
• the target annealing temperature is annealed above the A3 temperature, at a temperature between 820°C and 880°C, preferably between 840 - 860°C for 10 seconds to 300 seconds, preferably between 60 and 180 seconds in order to enable recrystallization and normalization steps in the material.
• the internal structure of the material is completely austenitic.
• the annealed material is cooled to a temperature close to A1 temperature close to A1 with a cooling rate between 1°C and 15°C per second, allowing C diffusion and increasing its austenitic stability. In this way, pearlitic transformation is facilitated. In addition, with slow cooling texture (111) develops, an improvement in the formability property of the material is obtained in the final product. With the mentioned cooling rate, the material is cooled to a temperature between 580°C and 720°C, preferably between 625 °C and 675 °C, and the material passes to the rapid cooling section where the inlet temperature of the Zn pot is adjusted.
• the material/strip is cooled to preferably 460 - 500 °C with a cooling rate between 5°C and 25°C per second and the pot immersion temperature between 440-550°C. With this cooling rate, the alloying elements dissolved in the ferrite phase are preserved and the strength of the final product is improved.
• the ambient atmosphere is also effective in both the coating quality and the mechanical properties of the final product.
• the ambient atmosphere contains HNx (3-7% H2) gas mixture, traces of 02 and water vapor.
• HNx 3-7% H2 gas mixture
• the dew point which is a result of the balance of water vapor and oxygen with H2 io gas, determines the selective oxidation character that will occur on the material surface or inside.
• the dew point of the continuous galvanizing line should be set as low as possible ( ⁇ -45°C) so that no carbon loss due to decarburization will occur in steels with high carbon content.
• the strip is coated by hot dipping method in a zinc pot (0.11-0.16% Al preferably 0.125-0.140%, less than 0.05% Fe) at a temperature of 450-470 °C, preferably 460 °C.
• the strip stays in the molten zinc bath for 5-15 seconds, preferably 8-12 seconds, and reaches the galvanil furnace. During the time the strip spends in the coating bath, excessive aging occurs and the development of the perlite phase continues.
• the strip exits the pot it passes through the air knife region where it is exposed to N2 gas injection in order to scrape off the excess molten zinc on its surface.
• the target coating thickness (4-12 pm) is achieved, along with the cooling effect of the N2 gas injection, so that the coating solidifies to a large extent.
• the strip coming out of the Zn pot is passed through the galvanneal furnace (GAF) zone, where annealing is done with induction heating and electrical heaters, to obtain the targeted coating alloy and coating phase distribution.
• GAF galvanneal furnace
• Fe-Zn intermetallic phases are formed in the coating on the steel sheet, which is immersed in a liquid zinc bath at the temperature range of 450-470 °C and then passed through the galvanneal furnace at 470-530 °C, preferably at 485 - 520 °C.
• nucleation starts with the phase, followed by the 5 phase, and finally, the columnar T phase is formed.
• the formation and amount of these phases are determined by the amount of Al by weight in the coating bath, the residence time of the strip in the pot and the GAF, and the GAF temperature.
• the GAF temperature was selected in the range with the highest delta phase stability and is given in the Fe-Zn phase diagram in Figure 4.
• an intermetallic layer (Fe2Als) is formed by the first reaction occurring preferentially between Fe-AI on the surface during the contact of the strip with the liquid zinc bath.
• the Gl coating bath contains 0.20-0.30% Al.
• the thick Fe2Als layer formed in this Al content acts as a barrier layer preventing Fe diffusion and hence the formation of Fe-Zn phases. If the Al content of the pot is in the range of 0.125-0.140%, optimum alloying of the coating (thin F phase and appropriate ⁇ ⁇ 5 ratio) is achieved by breaking the Fe2Als layer, which is thinner than the Gl coating.
• the lower amount of Al in the pot may cause a thinner Fe2Als layer, easier Fe diffusion, and excessive alloying of the coating.
• the dross formation in the crucible remains at a minimum level minimum.
• the travelling time in the pot, the travelling time in the GAF (5-20 sec) and the temperature of the GAF are one of the main mechanisms that control the thickness of the Fe2Als intermetallic compound, thus the decomposition of the compound (Fe2Als) and the initiation of Fe diffusion.
• the amount of Al in the ladle is one of the important criteria that determines the coating quality. Therefore, in order to balance the amount of Al that changes depending on the production rate, the amount is controlled at certain periods and is balanced with the addition of ingots of appropriate composition.
• the internal structure of the galvanneal-coated steel with a tensile strength higher than 440MPa contains more than 85% ferrite and less than 15% pearlite phases, and an optical microscope microstructure image indicating the separation of ferrite and pearlite phase is given in Figure 2.
• the grain size of the ferrite phase in the matrix varies between 5 pm and 20 pm.
• This invention includes the production method of galvanneal (Fe-Zn alloy) coated high strength steel with a tensile strength value higher than 440MPa, cold rolled and annealed in a continuous galvanizing line, and it is a steel grade developed for the automotive industry.

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• 2022
  • 2022-12-29 EP EP22917087.3A patent/EP4437146A4/de active Pending

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