WO2023129088A2 - High strength, iron-zinc alloy (galvanil) coated steel sheet and production method for the automotive industry - Google Patents

Abstract

The invention is about high strength steel with a tensile strength higher than 440MPa, which is cold rolled and galvanneal (Fe-Zn alloy) coated by annealing in the continuous galvanizing line, and its feature is; by weight; 0.03 ≤ C ≤ 0.14%, 0.90 ≤ Mn ≤ 1.70%, 0.05 ≤ Si < 0.3%, 0.005 ≤ Al ≤ 0.08%, P ≤ 0.02%, Ti ≤ 0.03%, N < 0.01%, the remainder contains iron and residual elements.

Description

HIGH STRENGTH, IRON-ZINC ALLOY (GALVANIL) COATED STEEL SHEET AND PRODUCTION METHOD FOR THE AUTOMOTIVE INDUSTRY

Technical Field

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.

State of the Art

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.

For the automotive industry, the welding properties of the formed material with other materials is also a very important criterion. Therefore, carbon content and surface properties (coating type, coating method, etc.) have always been important criteria in steel designs.

In addition to these, the resistance of the material against corrosion is also among the important criteria. Corrosion is very important in terms of both safety and steel material life. 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.

Unlike galvanized coating, in galvanneal coating, Fe atoms are allowed to diffuse into the Zn coating with an appropriate heat treatment step. The intermetallic phases seen in the coating to be formed on the steel sheet that is immersed in a liquid zinc bath at 450-500°C and then passed through the galvannealing furnace (470-550°C): r (gamma), 5 (delta) and (zeta) phases. The nucleation sequence of these phases at the steel and coating interface starts with the phase, continues with the 5 phase, and finally, the columnar I" phase is formed. The distribution of the relevant intermetallic phases is of great importance in terms of coating quality. Because of the gamma phase; It is an iron-rich phase formed at the interface between the steel surface and the coating. Approximately the amount of Fe varies between 18-30% and is the most fragile phase. Therefore, it is not preferred to be thicker than 1 pm. It adversely affects the powdering properties of the thick gamma phase material. The delta phase, on the other hand, 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%.

Many of the known strengthening mechanisms (grain refinement, alloying, cold deformation, phase transformation hardening, precipitation hardening) can be used to improve the strength properties of steels. However, the point to be considered here is the selection of the appropriate strengthening mechanism according to the area where the material will be used and the process it will undergo.

Improved formability and better weldability are expected in steel sheet designed for the automotive industry. The achievement of these properties can be obtained by developing relatively lower carbon steels with lower carbon equivalent. At the same time, the type and method of coating to be applied to the steel sheet also brings limitations in chemical composition. Elements with high selective oxidation properties (Mn, Si, Cr, etc.) can cause problems in hot dip coating methods. Surface oxides of the relevant elements cause the formation of uncoated areas in the hot dip process.

When alloy designs are evaluated in particular for galvannealed coatings, the use of materials with high selective oxidation properties causes the formation of heterogeneous coating properties as well as the problem of non-coating areas. High oxides of related elements cause heterogeneity in coating properties due to the formation of different Fe-Zn alloy compositions resulting from uneven Fe diffusion. At the same time, Ti and P elements are among the critical alloying elements for galvannealed coatings. The use of Ti over 0.030% by weight in the alloy causes traces of uneven Fe diffusion on the coating surface. Since the element P is located at the steel grain boundary, it causes to diffuse Fe at different rates at the boundary and in the grain. So it is an alloying element that should be considered for galvannealed coatings.

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. When the related patent and our invention are compared, there is clear differences from this patent in hot rolling parameters, annealing parameters and GAF (galvannealing furnace) holding time. In addition, in the patent numbered CN104975226A, no information is given about the steel micro structure. On the other hand to these basic differences, 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. Therefore, it is allowed to use Ti up to 0.030% in our invention. The fact that the phosphorus element is generally located at the grain boundaries, therefore, secondary deformation hardening and causing different rates of Fe diffusion in the grain boundary and grain in the zinc alloying process were effective in limiting the P element to 0.02%. 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. The next are similarities in the temperature and composition of the zinc pot. 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. In this context, although 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. Although 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. In this patent, 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. For example, 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. Because 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. In the patent numbered CN104093873A, it is stated that 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. Secondary work embrittlement will cause problems, such as tearing and forming defects in the cold forming processes of the material. The amount of P, which is limited to 0.2% in our invention, has been determined as the maximum amount that will not cause a problem in the zinc alloying process. In the patent numbered CN104093873A, Ti, V, Nb additions were observed and it is known that they are critical elements in providing a yield I tensile strength ratio above 0.70. Because these elements increase the yield strength by providing precipitation hardening. However, due to the problems mentioned before, the use of Ti over 0.30% causes unwanted alloying marks formation on the zinc alloy coating surface due to different iron diffusions and creates visual problems. Therefore, in our invention, it is limited to a maximum of 0.30%. In the patent numbered CN104093873A, 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. When the process conditions are examined, 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. In our invention, the A3 temperature was determined depending on the alloy design and the annealing process was carried out above this temperature. In the heat treatment carried out in the continuous galvanizing line, 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.

Technical Problems That The Invention Aims to Solve

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.

The main purpose of the invention is to increase formability. Thus, 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. Thus, 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. Thus, 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. For this purpose, there are limitations to Ti and P elements in chemical composition.

Explanation of Figures

Figure 1: Flow Chart

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

Description of the Invention

In this explanation, 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. In order to avoid these negativities, 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. While determining the target criteria of these elements, it was emphasized that the C element should be preferred in a way that would not adversely affect its weldability. In order to get away from selective oxides that Mn and Si elements may form on the surface, studies were carried out in the atmosphere of the continuous galvanizing line with the target criteria.

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. In 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;

0.03 < C < 0.14%

0.90 < Mn < 1.70%

0.05 < Si < 0.3%

0.005 < Al < 0.08%

P < 0.02%

Ti < 0.03%

N < 0.01%

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.

In this patent;

• C, Mn and Si elements were preferred for solid solution hardening in order to provide a tensile strength higher than 440 MPa.

• The amount of C was chosen at the optimum ratio for the formation of the pearlite phase and contributed to the improvement of the strength.

• It is aimed to keep the C amount as low as possible in order to provide better weldability.

• P and Ti elements are limited as they adversely affect the galvanneal coating quality.

• 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. As it is known, 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. However, depending on the amount of C in achieving the target mechanical properties, the development of the strength is provided by the formation of the pearlite phase. The critical point here 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. For this purpose, Silicon (Si) and Manganese (Mn) elements in the composition also contribute to carbon enrichment of the austenite phase by delaying carbide formation. On the other hand, higher than 0.14% C can cause strength losses due to an increase in the amount of decarburization on the strip surface during the heat treatment of the continuous galvanizing line. Therefore, C is preferably used in the range of 0.05 < C < 0.08.

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. However, when evaluated in terms of steel production conditions, since the use of high Mn causes banding and segregation problems, it should be included in the designs at the optimum rate for the target tensile strength. Therefore, 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. Preferably, the slab is heated up to the temperature range of 1200-1250 °C.

The rolling process takes place in the austenitic phase region so that the high amount of hot deformation applied in the hot rolling process can occur homogeneously throughout the slab thickness. Therefore, the final rolling exit temperature (Tfinish) is aimed to be at least 850°C. Preferably, the final rolling exit temperature should be at least 920 °C (Tfinish). Then, 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.

In cold rolling, 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. With this application, 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. At the end of this process, 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.

In addition to the heat treatment applied in the heating and annealing steps of the continuous galvanizing line, 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. 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. Due to the fact that the carbon content in the steel with the chemical composition considered within the scope of the invention is not high (<0.14% by weight), strength losses due to the ambient atmosphere are not observed, and it offers sustainable end product properties. However, selective oxides of Mn and Si elements in the composition affect the coating quality. The dew point is targeted between -50°C and -25°C in order to minimize the negative effects that will affect it.

Then, 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.

As 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. Here, 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. 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. Here, 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.

Since the affinity of iron to aluminum is higher than that of zinc, 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. In addition, at the level of 0.125-0.140% Al, the dross formation in the crucible remains at a minimum level minimum. In addition, 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.

Therefore, 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. There may be changes in the morphology and distribution of the pearlite phase in the steel micro structure due to the low carbon design of the chemical analysis and the GAF operations. Because of these factors, 60% or more of the pearlite phase is composed of degenerated perlite and 10% or less of spherical cementite. SEM (scanning electron microscopy) image containing ferrite, degenerate pearlite, pearlite and spherical cementite is given in Figure 3. It is aimed that the tensile strength of the steel with the specified micro structure properties is higher than 440MPa, the yield strength is higher than 275 MPa and the total elongation value is higher than 25%.

Table 1.

• 1st cooling exit temperature 625 - 675 °C

• 2nd cooling exit temperature 460 - 500 °C

• GAF temperature 470 - 530 °C

• GAF duration 5 - 20 sec

In Table 1, the mechanical values of the steel, which were tested on 12 coils in 3 different compositions and 2 different main process parameters (annealing temperature and annealing time) in different combinations, are presented in the table. Industrial Application of the Invention

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.

Claims

1. The invention relates to high strength steel, which is cold rolled and galvanneal (Fe-Zn alloy) coated by annealing in the continuous galvanizing line, and comprising of; by weight;

0.03 < C < 0.14%

0.90 < Mn < 1.70%

0.05 < Si < 0.3%

0.005 < Al < 0.08%

P < 0.02%

Ti < 0.03% the remainder contains iron and residual elements.

2. A steel according to claim 1 wherein; C content is 0.05 < C < 0.08.

3. A steel according to claim 1 or 2 wherein; Mn content is 1.30 < Mn < 1.40.

4. A steel according to claim 1 - 3 wherein; Si content is 0.10 < Si < 0.20.

5. A steel according to claim 1 - 4 wherein; Al content is 0.020 < Al < 0.050.

6. A steel according to claim 1 - 5 wherein; P content is preferably absent.

7. A steel according to claim 1 - 6 wherein; The Ti content is preferably absent.

8. A steel according to claim 1 - 7 wherein; The tensile strength is higher than 440MPa, the yield strength is higher than 275 MPa, and the total elongation value is higher than 25%.

9. The invention is a high-strength steel production method with a tensile strength value higher than 440MPa, which is cold rolled and galvanneal (Fe-Zn alloy) coated by annealing in the continuous galvanizing line, wherein;

- Reheating the slabs in the slab heating furnace up to the temperature range of 1150-1275 °C,

- Finishing temperature must be minimum (Tfinish) 850 °C, - Coiling temperature of hot rolled product between 700 °C and 750 °C,

- Application of cold reduction between 45% and 85% in cold rolling,

- The dew point of the continuous galvanizing line is between -50°C and -25°C,

- Heating the cold rolled material/strip in the continuous annealing line at a heating rate between 1 °C and 40 °C and holding it at the annealing temperature,

- After cooling the annealed material to A1 temperature with a cooling rate between 1 °C and 15 °C per second, cooling the strip/material to the Zn pot immersion temperature with a cooling rate between 5 °C and 25 °C,

- In the Zn pot, the strip stays in the molten zinc bath at 450 - 470 °C for 5 - 15 seconds and reaches the galvanneal furnace,

- As the strip comes out of the Zn pot, it passes through the air knife section where it is exposed to N2 gas injection in order to strip the excess zinc on its surface,

- It consists of the process steps of passing the steel sheet through the galvanneal furnace (GAF) at 470 - 530 °C, by keeping it in the furnace for 5 - 20 seconds.

10. Method according to claim 9, wherein; heating the slabs in the slab reheating furnace up to the temperature range of 1200-1250 °C.

11. Method according to claim 9 - 10, wherein; the final rolling exit temperature is minimum (Tfinish) 920 °C.

12. Method according to claim 9 - 11, wherein; the coiling of the hot rolled product at 720 °C.

13. Method according to claim 9 - 12, wherein; the application of cold reduction between 50% and 70% in cold rolling.

14. Method according to claim 9 - 13 wherein; said target annealing temperature is between 820 - 880 °C above the A3 temperature. It is preferably between 840 - 860°C.

15. Method according to claim 9 - 14, wherein; said target annealing temperature is between 10 sec and 300 sec. It is preferably annealed between 60 - 180 seconds.

16. Method according to claim 9 - 15, wherein; the temperature at which said annealed material is cooled down is between 580 °C and 720 °C. Preferably, it is cooled down between 625 °C - 675 °C.

17. Method according to claim 9 - 16, wherein; cooling the matenal/stnp in the rapid cooling section to 440 - 550 °C (Zn pot immersion temperature), preferably cooling the material/strip to 460 - 500 °C.

18. Method according to claim 9 - 17, wherein; material/strip remains in the zinc pot at 460 °C for 8-12 seconds.

19. Method according to claim 9 - 18, wherein; Zinc pot contains 0.11-0.16% Al, less than 0.05% Fe, preferably 0.125-0.140% Al.

20. Method according to claim 9 - 19, wherein; the target coating thickness after the air knife is between 4 and 12 pm.

21. Method according to claim 9 - 20, wherein; the GAF temperature is between 485 - 520 °C.

22. Method according to claim 9 - 21, wherein; the microstructure of the galvanneal- coated steel contains more than 85% ferrite and less than 15% pearlite phases.

23. Method according to claim 9 - 22, wherein; the ferrite phase grain size in the matrix is between 5 pm and 20 pm.

24. Method according to claim 9 - 23, wherein; the pearlite phase consists of 60% or more degenerate perlite and 10% or less of spherical cementite.