,

1)Cold Rolling Plant,Baoshan Iron & Steel Co.,Ltd.,Shanghai 200941,China;2)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China;3)Zhanjiang Iron & Steel Co.,Ltd.,Zhanjiang 524000,Guangdong,China

Abstract: The inorganic-lubricant-coated galvannealed automobile steel sheet is a new product that improves the press formability of automobile body panels.In this paper,the production process,physical and chemical properties,press formability,and corrosion resistance of this new automobile steel sheet were discussed.Test results show that this inorganic solid-lubricant galvannealed automobile steel sheet has good surface friction characteristics and press formability.This product also meets the automobile industry’s requirements for press formability,phosphate pro-perties,and corrosion resistance after coating.

Key words: galvannealing; inorganic lubricant film; automobile steel sheet

1 Introduction

The galvannealed automobile steel sheet,with its excellent corrosion resistance and surface quality and good weldability and paintability,has been widely applied to many types of high-grade automobiles.However,the surface friction coefficient of the galvannealed automobile steel sheet is large,which indicates high surface friction between the stamping die and the steel strip when stamping complex automobile parts.This high surface friction decreases the stamping formability of the steel plate and means that zinc dust can easily adhere to the surface of the stamping die,which leads to bright spot defects on the surface of the stamping parts,which affect the production and surface quality of the automobile parts.

To improve the surface friction characteristics and stamping formability of the galvannealed automobile steel sheet,foreign steel enterprises in cooperation with research institutions have conducted a large number of investigations and have subsequently developed surface lubrication for the galvannealed steel sheet to improve its stamping formability.Commercially available surface-lubrication-treated galvannealed steel sheets mainly include the lubrication-treated galvannealed steel sheet (GAL) produced by Nippon Steel Corporation (NSC),the JFE advanced zinc (JAZ) steel sheets produced by JFE Steel Corporation,and others[1].In China,Baosteel-NSC Automotive Steel Sheets Co.,Ltd.pro-duces GAL products under authorization from NSC,and Guangzhou JFE Steel Sheet Company Ltd.produces JAZ products patented by JFE Steel Corporation.

Baosteel has developed a new type of inorganic-lubricant-coated galvannealed automobile steel sheet (GAT) with secured independent intellectual pro-perty rights[2],which is currently industrially produced on a new continuous galvanizing line in Zhanjiang.This paper mainly introduces the production process of this new GAT,analyzes its surface characteristics,and introduces approval trials by automakers.

2 Production process of the GAT

A new type of post-treatment process is adopted to produce the GAT.This post-treatment process mainly involves a roll-coating process,roll-coater coating,a combined induction heating/hot-air heating dry process,and an air-flow-jet cooling process[3],as shown in the diagram in Fig.1.

Fig.1 Diagram of the post-treatment process

2.1 Roll-coating process

The GAT is produced on a vertical roll-coating machine,which mainly consists of two coating heads,each with one applicator roll and one pick-up roll,and is designed to apply coatings in both forward and reverse modes.The vertical roll-coating machine has many advantages in producing GAT pro-ducts because of its ease of operation and main-tenance,high quality, and uniform thickness of the post-treatment film.An extremely thin inorganic-lubricant-coated film is coated onto the surface of a galvannealed steel sheet by a vertical roll-coating machine.The main control processes that affect the quality of this inorganic-lubricant-coated film in-clude the following:① The concentration of the inorganic lubricating agent in aqueous solution has a strong impact on the thickness of the inorganic-lubricant-coated film,so the concentration of this inorganic lubricating agent must be strictly con-trolled within the required range.② The thickness of the inorganic-lubricant-coated film must be suitable for the customer’s stamping parts.To pro-duce a film of suitable thickness,the rotation speeds of the applicator roll and the pick-up roll,the strip steel speed,the pressure between the applicator roll and the pick-up roll,and the pressure between the applicator roll and the strip steel must be carefully controlled.③ The surface condition of the appli-cator roll has an important influence on the thickness and quality of the inorganic-lubricant-coated film and must be carefully controlled.Six key parameters affect the surface condition of the applicator roll,including the roll-coating materials,the hardness and roughness of the roll surface,the uniformity of the roll surface,and the dynamic and static balance parameters of the applicator roll.④ The GAT is produced in forward mode,which often causes problems such as liquid being thrown at the end of the pick-up and applicator rolls and a high current and power tripping of the applicator roll motor.Therefore,the configuration of the roller coating machine equipment must be adaptively designed to better match and optimize the production process parameters with respect to the status of the equipment.

2.2 Heating and cooling processes

Drying and cooling treatments must be performed after coating the inorganic lubricating treatment solution onto the surface of the strip steel.Drying treatment methods include induction heating,hot-air heating,and infrared heating.Cooling treatment methods generally involve fan injection cooling.The GAT is baked using a composite drying method that includes induction and hot-air heating.Drying by induction heating quickly heats the strip steel to the required temperature,and hot-air heating removes the remaining water in the inorganic-lubricant-coated film.Thus,the composite drying method ensures the best drying effect of the post-treatment film.The baked treatment strip is then cooled to room temperature by fan injection cooling,which prevents surface quality problems in the inorganic-lubricant-coated film owing to high temperature,adhesions,or scratches.

3 Properties of the GAT

The main properties of the GAT,galvannealed steel sheet (GA),and GAL produced by NSC were tested under the same conditions.The test methods used and the property analysis results are presented below.

3.1 Friction characteristics

The surface friction characteristics of the GAT were measured using the draw bead method,under the following measurement conditions:die tempera-ture of 25 ℃,pressure of 2 000 N,drawing speed of 200 mm/min,and drawing length of 110 mm,with 550HN rustproof lubricating oil on the surface of the sample.Fig.2 shows a diagram for measuring the friction characteristics.

Fig.2 Diagram for measuring friction characteristics

The measurement results in Table 1 show that the surface friction coefficient of the GAT is approximately 8%-10% lower than that of the GA product but is roughly equivalent to that of the GAL.A smaller surface friction coefficient is beneficial to the press formability of the steel sheet.

Table 1 Comparison of surface friction coefficients of GAT/GAL/GA products

3.2 Formability characteristics

The formability characteristics of the GAT were measured using a deep drawing test,which con-forms to GB/T 15825.3-2008,with a punch diameter of 50 mm and a punch speed of 30 mm/min,with 550HN rustproof lubricating oil on the surface of the sample.Fig.3 shows a diagram of the measurements obtained in the deep drawing test.

Fig.3 Diagram of measurements obtained in the deep drawing test

Fig.4 shows the results of the deep drawing test.As compared with the results obtained for the GA and GAL products for materials of the same grade and specifications,the GAT results for the stress on the punch of the drawing process and the drawing force were the lowest.This indicates that the fluidity of the GAT is better than that of the GA and GAL.In other words,the stamping formability of the GAT is better than that in the GA and GAL products.

Fig.4 Drawing test results of the GAT,GAL,and GA

3.3 Phosphate characteristics

The phosphate and paintability properties of the GAT,GA,and GAL products were compared and analyzed using the online test method in two different automobile factories.The weight of the phosphate coating and the surface roughness after electrode positioning are shown in Table 2,which show that the GAT,GA,and GAL products have similar phosphate characteristics.The typical microstructure and corrosion resistance of the phosphate coating,as shown in Fig.5,reveal that the GAT products meet the requirements of the conventional phosphate and paintability processes of the automotive industry.

Table 2 Phosphate properties of the GAT,GAL,and GA products

Fig.5 Paintability properties of the GAT,GA,and GAL products

3.4 Welding characteristics

Under the same test conditions,the welding characteristics of the GAT,GAL,and GA products with the same physical and chemical properties were analyzed and compared.The test conditions are shown in Table 3.

Table 3 Welding test conditions

The welding current range and joint strength of the GAT,GA,and GAL products were evaluated under laboratory conditions,the results of which are shown in Figs.6 and 7.Under the same test conditions,the welding current of the GAT products was slightly lower than that of the GA products but higher than that of the GAL products.The welding current range and joint strength of the GAT,GAL,and GA products did not differ significantly.

Fig.6 Welding tests (range of weldable current)

3.5 Adhesive characteristics

The criteria used to evaluate the adhesive performance vary between different automobile factories.In this paper,the Henkel Terostat-3216 anti-flutter adhesive and Dow Betamate-1496F structural adhesive were selected for evaluation.Fig.8 shows a diagram of the adhesive test.The size of the steel sheet specimen was 25 mm×150 mm,and the adhesives were cured for various periods of time and at various temperatures.After complete curing for 24 h in the laboratory,a tensile shear test was conducted to determine the breakage type.In this test,if a break occurs in the interior of the adhesive,the performance is rated as good;if a break occurs at the interface between the adhesive and the steel sheet,the performance is rated as not good.

Fig.7 Welding tests (TSS strength of welding points)

Fig.8 Diagram of adhesive performance test

The results of the adhesive performance tests,as shown in Fig.9,reveal that the GAT steel sheets exhibited good adhesive performance for both the anti-flutter and structural adhesives.

Fig.9 Results of adhesive performance tests

4 Application of GAT products

The industrial trial production of GAT products started in 2019,and some approval trials were com-pleted by two automakers.The mass application of GAT products has been initiated in one automotive fac-tory in China.A high deep-drawing-grade galvannealed steel sheet was used as the original side-panel material on a certain vehicle,but the stamping cracks could not be effectively controlled owing to the complex deformation of the parts.These original materials were replaced with the GAT products,which eliminated these stamping cracks and other defects.The stamped parts are shown in Fig.10.

Fig.10 Automobile body parts pressed in an automobile plant

The laboratory test and the automobile-factory application results proved that GAT products have superior press formability and stability,which means they can be widely used to produce complex parts such as side panels,hood interiors,and middle floor layers.

5 Conclusions

(1) The GAT is a new post-treatment product that improves the stamping formability of the hot-dipped galvannealed sheet for automobiles while also having good surface friction characteristics.

(2) The properties of GAT products are basically the same as those of GAL products,but the surface friction characteristics and stamping formability of the GAT products are better than those of the GA products.

(3) The mass application of GAT products has been initiated in some automotive factories,and they meet the requirements of conventional phosphate and paintability processes of the automotive industry.