Title of Invention	A METHOD OF PRODUCING DEFECT FREE HIGH STRENGTH REPHOSPHORISED INTERSTITIAL FREE GALVANNEALED STEEL SHEET/STRIP
Abstract	This invention relates to a method of producing defect free high strength rephosphorised interstitial free galvannealed steel sheet / strip for automotive industry comprising the steps of removing iron oxide formation carried over from hot rolling stage by employing improved pickling conditions to improve surface condition for zinc wetting as well as iron and zinc diffusion.

Full Text

-2- FIELD OF THE INVENTION In galvannealed steel for automotive industry the uncoated spots are developed on the high strength rephosphorised interstitial free steel due to the presence of the iron oxides. These oxides are carried over from upstream processing (hot rolling stage) and not totally removed during pickling operation and thus cause uncoated spot formation after galvannealing. The present invention employs improved pickling condition which removes the oxide scale totally from the surface of the high strength rephosphorised interstitial free steel and produce defect free galvannealed coated steels sheet / strip. BACKGROUND OF THE INVENTION High strength rephosphorised interstitial free steels are extensively used in automotive industry due to its excellent formability along with strength. Since steel is susceptible to corrosive attack in various environments, most steel sheets are protected from the environment with thin durable coatings. Among these coatings -3- zinc based coated steel sheets have been very extensively used. In zinc based coatings galvannealed coating is the most important. It is due to its inherent advantages such as excellent spot weldability along with superior corrosion resistance and paintability. The steel sheet is first hot rolled. During this stage several iron oxides such as Fe2O3, FeO and Fe3O4 along with the other oxides like MnO, SiO2 and CaO are formed. In the existing practice these oxides are removed prior to cold rolling by pickling operation. After cold rolling, the steel sheet pases through continuous annealing furnace. Then it enters into the zinc bath known as galvanizing followed by a post coating heat treatment in an induction furnace called galvannealing. Several coating defects are observed on both sides of the galvannealed high strength rephosphorised interstitial free steel sheet. -4- The present invention has proposed to remove the above difficulties of prior state of art on: i) determination of root cause of uncoated spot formation on galvannealed high strength rephosphorised interstitial free steel and ii) controlling of the pickling parameters to enhance the surface quality of the high strength rephosphorised interstitial free steel to produce defect free galvannealed coating. DESCRIPTION OF THE INVENTION The positions of the defect in the galvannealed sheet is mainly concentrated on 100 to 300 mm from both edges of the steel sheet. It is therefore necessary to find out root cause of such defect. -5- The proposed invention has developed an assured method of defect free galvannealed high strength rephosphorised steel sheet. First tracking the coil sheet from upstream processing, starting from the hot roiling stage to the final galvannealing stage. To evaluate the root cause of the defect by assessing through cross-sectional scanning electron microscopy (SEM) micrograph along with energy dispersive spectroscopy (EDS) analysis of oxide scale of hot rolled rephosphorised high strength interstitial free steel. Top surface as well as cross-sectional SEM study of the defect area of the galvannealed coating is also carried out. By using X-ray area map of the coating, finding out oxygen content in the defect area and defect free area of the coating carrying out depth profiling analysis of the defect and defect free region of the coating through glow discharge optical emission spectroscopy (GDOES) to observe oxygen content and also segregation characteristics of the coating, carrying out depth profile analysis of the defective area of the coating through anger electron spectroscopy (AES) and preparing anger map of the defect region of the coating on using argon ion sputtering and collecting spectrum and developing pickling characteristics according to the nature of the defective zone of the coating observed through the said analysis on maintenance of appropriate acid concentration of the pickling bath, residence time of hot rolled sheet at the bath to remove all oxides and pickling conditions such as temperature and line speed. -6- Aceording to the invention first objective to carry out pickling of the hot rolled rephosphorised high strength interstitial free steel in HCI acid concentration in tank1: 1 - 4%, tank2: 5 - 8%, tank3: 9 -11% and tank4: 12 -15%, temperature 75 -90°C and line speed: 75 - 120meter/minute According to the invention another objective is to carry out Continuous annealing of the electrolytically cleaned cold rolled (full hard) steel sheet with furnace soaking zone temperature: 815 to 890 °C, composition of gas mixture in continuous annealing furnace: N2: 85 to 95 % and H2: 5 to 15 % and furnace dew point -20 to - 45 °C and jet cooling strip temperature 460 to 485 °C, snout strip temperature 460 to 465 °C, Yet another objective of the invention is to galvanizing the steel strip / sheet in zinc bath at 460 to 470° C and galvannealing subsequently at 500° to 550° C. -7- DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS Figure 1 represents galvannealed coating defects on high strength rephosphorised interstitial free steel. Figure 2 shows cross sectional SEM along with EDS analysis of hot rolled high strength rephosphorised interstitial free steel. Figure 3 shows a top surface SEM of defect region of the galvannealed coating on high strength rephosphorised interstitial free steel. Figure 4 shows X-ray maps of coating defect on high strength rephosphorised interstitial free steel showing maps of 0, Al, Si, P, Ti, Nb, Mn, Fe and Zn. -8- Figure 5 shows top surface SEM along with EDS analysis from the defect region of the coating on high strength rephosphorised interstitial free steel. Figure 6 shows cross sectional SEM along with EDS analysis from the defect region of the coating on high strength rephosphorised interstitial free steel. Figure 7 shows cross sectional SEM along with EDS analysis from the defect free region of the coating on high strength rephosphorised interstitial free steel. Figure 8(a) and (b) show quantitative depth profiling using glow discharge optical emission spectroscopy (GDOES) of galvannealed coating (a) defect and (b) defect free region on high strength rephosphorised interstitial free steel. -9- Figure 9 shows auger maps of defect region of the galvannealed coating on high strength rephosphorised interstitial free steel after 600 min sputtering. As shown in Figure 1, the coating defects formed on high strength rephosphorised interstitial free galvannealed steel are not purely uncoated, where substrate steel is exposed to atmosphere. These are whitish in nature and easily visible by naked eyes. The defects are mostly elongated towards the rolling direction (RD). The position of such kind of defect is mainly concentrated on 100 to 300 mm from both edges of the steel sheet. To find out the root cause of such kind of defect, coil is tracked from upstream processing, starting from hot rolling stage to the final galvannealing stage. Several characterization techniques have been used to determine the root cause of such type of defect. These are as follows: -10- Figure 2 shows the cross sectional SEM micrograph along with EDS analysis of hot rolled high strength rephosphorised interstitial free steel From the above figure it is clearly observed that the entire oxide scale contains iron oxide with little amount of silicon and manganese oxides. Top surface as well as cross sectional SEM study was also carried out over the defect area of the galvannealed coating. Circled region of the Figure 3 represents the defect region along with defect-free area of the coating. X-ray maps of oxygen (0), aluminium (Al), silicon (Si), phosphorus (P), titanium (Ti), niobium (Nb), manganese (Mn), iron (Fe) and zinc (Zn) of the circled region of the micrograph shown in Figure 3 was taken and shown in Figure 4. From this figure, it is evident that oxygen content in the defect region is high and Fe content is low. No preferential segregation of Ti, Nb, P, Mn, Al and Si were found. Area EDS analyses of the defect region as well as defect-free region were also carried out, shown in Figure 5. It can be seen again that the oxygen content in the defect area of coating is much higher than the defect-free area. -11- Cross-sectional SEM micrograph with EDS analyses of the defect and defect free regions of coating were also carried out, shown in Figures 6 and 7. It was observed that the oxygen content in the defect region was much higher whereas iron content was much lower compared to the defect-free region of the coating. Depth profiling analyses of the defect as well as defect-free region of the coating were carried out using glow discharge optical emission spectroscope (GDOES). The GDOES analysis of the defect and defect-free area of the galvannealed coating are shown in Figure 8 (a, b). Here the oxygen content of the defect region of coating is higher compared to defect free region. No preferential segregation of Si, Mn or B was found. Depth profile analysis of the defect area of the coating was also carried out using auger electron spectroscope (AES). In this study coating was removed layer by layer using argon ion sputtering and then the spectra were collected after 50, 100, 200, 300, 400, 450 and 600 minutes sputtering. Figure 9 shows the auger maps of the defect region of the coating after 600 minutes sputtering. It was apparent from these maps that Zn was completely removed in the sputtering place, whereas considerable amount of iron and oxygen were present. -12- During galvanizing of high strength rephosphorised interstitial free steel, wetting of sheet surface by liquid zinc plays a vital role in the formation of defect free coating. The wettability mainly depends on surface conditions of the steel sheet. During hot rolling, several layers of oxides are formed. These oxides are mainly consisting of wustite (FeO), magnetite (FesO4) and hematite (Fe2O3) along with some other oxides such as alumina, silica, manganese oxides [Fig. 1, 2]. Pickling operation is carried out on the hot rolled sheet just before cold rolling. In this stage sheet steel is passing through a series of four tanks containing hydrochloric acid (HCI). The tank temperature is maintained around 75 to 85° C. The acid concentration is maintained near 1 to 15 % in different tanks. Through this pickling operation above oxides are removed to produce clean surface of the sheet steel. This clean surface enhances the wettability by liquid zinc during galvanizing and diffusion process during galvannealing, thus finally leads to the formation of defect free galvannealed coating. So the pickling stage plays a vital role in the production of defect free coating. -13- From the above characterization results it is clearly observed that some iron oxide is present in the sub-surface area of the defect region of coating. Oxides carried over from upstream processing (hot rolling stage), are not totally removed during pickling operation. As a result, residual un-pickled oxide goes thorough the cold rolling stage and gets elongated. Due to the presence of iron oxide the complete Fe-Zn diffusion is not taking place and causes whitish, partially alloyed coating defects after galvannealing operation. Pickling time should be controlled in such a way that the residence time of hot rolled sheet at the acid bath should be sufficient to remove all oxides. Pickling condition has been identified as shown in Table 1, which removes the hot rolled oxide scale from the surface of high strength rephosphorised interstitial free steel sheet. Table 1: Pickling parameters for high strength rephosphorised interstitial free steel Hydrochloric Acid Concentration (%) Temperature Line Speed Tank 1 Tank 2 Tank 3 Tank 4 (°C) (Meter/Minute) 1-4 5-8 9-11 12-15 75-90 75 -120 -14- The invention as narrated with an exemplary embodiment should not be read and construed in a restrictive manner as various modifications of test evaluations, alterations of parameters and conditions during processing of sheet / strip formation and appropriate adaptations accordingly are possible within the scope and ambit of the invention as defined in the appended claims. -15- WE CLAIM 1. A method of producing defect free high strength rephosphorised interstitial free galvannealed steel sheet / strip for automotive industry having composition in weight %, C - 0.0025 - 0.0050, Mn - 0.35 - 0.75, S - 0.005 - 0.015, P - 0.025 - 0.050, Si - 0.010 - 0.020, Al - 0.025 - 0.050, Nb - 0.010 - 0.03, Ti - 0.030 - 0.070, B - 0.0006 - 0.001 and N - 20 - 50 ppm, comprising the steps of: a) hot rolling the steel with slab drop out temperature 1100 - 1250° C and roughing mill exit temperature of 1030 - 1125° C and further rolling in a finish rolling mill with entry temperature of 965 - 1070° C, exit temperature of 870 - 910° C and coiling temperature 650 - 690° C; b) pickling the hot rolled steel sheet in acid bath as HCI concentration in the range of as 1 - 15 %, temperature 75 - 90° C and line speed 75 - 120 meters / minute. -16- c) electrolytically cleaning the cold rolled coiled strip / sheet; d) continuous annealing the electrolytically cleaned steel strip / sheet at furnace soaking zone temperature of 815 - 890° C maintained with N2 and H2 gas mixture atmosphere; e) Jet cooling strip / sheet to 460 - 485° C with jet with snout strip temperature at 460 - 465° C; 0 galvanizing the strip / sheet at zinc bath temperature 455 - 470° C and subsequently galvannealing at 500 - 550° C. 2. A method of producing steel strip / sheet as claimed in claim 1, wherein electrolytic cleaning steps of cold rolled coils are carried out at electrolyte conductivity of 100 - 155 mili Siemens and electrolyte temperature at 75 - 150° C. -17- 3. A method producing steel strip / sheet as claimed in claim 1, wherein the step of pickling is carried out through four tanks sequentially maintained with HCI concentrations in the ranges of 1 - 4 %, 5 - 8 %, 9-11 % and 12 - 15 % respectively. 4. A method of producing steel strip / sheet as claimed in the preceding claims, wherein reducing atmosphere in continuous annealing of strip / sheet is maintained by keeping a gas mixture in the annealing furnaces of N2 - 85 to 95 % and H2 - 5 to 15 %. 5. A method as claimed in claim 4, wherein the said annealing furnace is maintained with dew point -20 to -45° C. -18- 6. A high strength rephosphorised interstitial free steel of composition in weight percentage C - 0.0025 - 0.050, Mn - 0.035 - 0.75, S - 0.005 - 0.015, P - 0.025 - 0.050, Si - 0.010 - 0.020, Al - 0.025 - 0.050, Nb - 0.010 - 0.030, Ti - 0.030 - 0.070, B - 0.0006 - 0.0010, and N - 20 - 50 ppm as used to form an improved Zinc wettability and defect free galvannealed steel strip / sheet according to claim 1. This invention relates to a method of producing defect free high strength rephosphorised interstitial free galvannealed steel sheet / strip for automotive industry comprising the steps of removing iron oxide formation carried over from hot rolling stage by employing improved pickling conditions to improve surface condition for zinc wetting as well as iron and zinc diffusion.

Documents:

01003-kol-2007-abstract.pdf

01003-kol-2007-claims.pdf

01003-kol-2007-correspondence others 1.1.pdf

01003-kol-2007-correspondence others 1.2.pdf

01003-kol-2007-correspondence others.pdf

01003-kol-2007-description complete.pdf

01003-kol-2007-drawings.pdf

01003-kol-2007-form 1 1.1.pdf

01003-kol-2007-form 1.pdf

01003-kol-2007-form 18.pdf

01003-kol-2007-form 2.pdf

01003-kol-2007-form 3.pdf

01003-kol-2007-gpa.pdf

1003-KOL-2007-(10-02-2012)-ABSTRACT.pdf

1003-KOL-2007-(10-02-2012)-AMANDED CLAIMS.pdf

1003-KOL-2007-(10-02-2012)-CORRESPONDENCE.pdf

1003-KOL-2007-(10-02-2012)-DESCRIPTION (COMPLETE).pdf

1003-KOL-2007-(10-02-2012)-FORM-1.pdf

1003-KOL-2007-(10-02-2012)-FORM-2.pdf

1003-KOL-2007-(10-02-2012)-OTHERS.pdf

1003-KOL-2007-(20-11-2012)-CORRESPONDENCE.pdf

1003-KOL-2007-(20-11-2012)-FORM-13.pdf

1003-KOL-2007-(20-11-2012)-OTHERS.pdf

1003-KOL-2007-(25-07-2012)-CORRESPONDENCE.pdf

1003-KOL-2007-CANCELLED PAGES.pdf

1003-KOL-2007-CORRESPONDENCE.pdf

1003-KOL-2007-EXAMINATION REPORT.pdf

1003-KOL-2007-FORM 13.pdf

1003-KOL-2007-FORM 18.pdf

1003-KOL-2007-FORM 3.pdf

1003-KOL-2007-GPA.pdf

1003-KOL-2007-GRANTED-ABSTRACT.pdf

1003-KOL-2007-GRANTED-CLAIMS.pdf

1003-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

1003-KOL-2007-GRANTED-DRAWINGS.pdf

1003-KOL-2007-GRANTED-FORM 1.pdf

1003-KOL-2007-GRANTED-FORM 2.pdf

1003-KOL-2007-GRANTED-FORM 3.pdf

1003-KOL-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

1003-KOL-2007-OTHERS.pdf

1003-KOL-2007-REPLY TO EXAMINATION REPORT.pdf