JP2014506626A - Hot-dip galvanized steel sheet with excellent deep drawability and extremely low temperature joint brittleness resistance and method for producing the same - Google Patents

Abstract

  The present invention relates to a hot-dip galvanized steel sheet having excellent joint brittleness resistance even at extremely low temperatures and a method for producing the hot-dip galvanized steel sheet, and an average diameter of crystal grains of a hot-dip galvanized layer of the hot-dip galvanized steel sheet. A hot dip galvanized steel sheet excellent in deep drawability and extremely low temperature joint brittleness at 150 to 400 μm and having a preferential orientation concentration of 3,000 to 20,000 cps with respect to the (0001) plane of the hot dip galvanized layer; It relates to the manufacturing method.

Description

  The present invention relates to a hot dip galvanized steel sheet, and more particularly to a hot dip galvanized steel sheet having excellent joint brittleness resistance even at extremely low temperatures and excellent deep drawing properties, and a method for producing the same.

  Usually, a galvanized steel sheet is manufactured by passing a steel sheet through a hot dip galvanizing bath and solidifying the hot dip zinc applied to the steel sheet. When the molten zinc attached to the steel sheet solidifies, coarse crystal grains called dendritic spangles are formed on the surface of the hot dip galvanized layer. The spangle is formed by the solidification reaction characteristics of zinc.

  That is, when zinc solidifies, a branch dendrite grows very rapidly from the solidification nucleus at the initial stage of solidification to form a skeleton of the plated structure, and remains between the dendrite. The unsolidified molten zinc pool solidifies to complete the solidification reaction, thereby forming spangles. It can be said that the size of the spangle depends on the size of the skeleton of the plated structure determined in the initial stage of zinc solidification.

  Due to the spangle, the adhesion of the paint to the surface of the plating layer and the corrosion resistance of the steel sheet are poor, and the surface irregularities due to the spangle are not removed even after painting, and the spangle can be seen through the paint, so the surface appearance of the steel sheet is poor. .

  Therefore, in order to minimize the size of spangles formed on the hot dip galvanized steel sheet, the inorganic salt solution is sprayed after the hot dip zinc is adhered to the steel sheet and before solidification. At this time, the solution is sprayed to the steel plate side through an electrode provided on the front surface of the nozzle. By spraying the solution through the electrode, the droplet of the solution becomes electrostatic and adheres to the steel plate by electrical attraction, so that the adhesion efficiency is improved and the plating structure can be reduced. As the inorganic salt solution, a phosphate solution is generally used.

  A plated structure having a spangle size of 150 μm or less can be obtained by spraying the phosphate solution refined by the electric charging. The steel plate with spangles refined to 150 μm or less has a beautiful surface appearance, improves the sharpness and corrosion resistance after coating, and prevents the plating layer from falling off during continuous pressing.

  On the other hand, when molten zinc solidifies, the shape of spangles differs depending on how the hexagonal crystal structure of zinc is formed on the surface of the steel sheet. This means that the hexagonal crystal structure of zinc is at a different angle for each part of the plating layer.

  A hot-dip galvanized steel sheet having a spangle of 150 μm or less and a general hot-dip galvanized steel sheet having a spangle of 400 μm or more are shown in FIGS. As shown in FIG. 1 (b), a general hot-dip galvanized steel sheet with spangles of 400 μm or more is advantageous in terms of brittleness because of its large zinc crystals and random orientation, but with poor surface appearance. There is a problem that there is.

  However, in the case of a hot-dip galvanized steel sheet having a spangle of 150 μm or less in FIG. 1A, the spangle has the same crystal orientation, and the direction is parallel to the steel sheet surface with the (0001) plane being the bottom surface of zinc. Is the direction. It is known that the crystal orientation in which the bottom surface of zinc is parallel to the steel plate surface is the most excellent in terms of corrosion resistance, blackening resistance and chemical stability, and until recently, many developments have been made to improve these properties. I have been.

  As an example, Patent Document 1 discloses the technical content of controlling the size of spangle to 60 to 1000 μm by mist injection using a nozzle, and Patent Document 2 discloses that the size of spangle is 50 μm or less and the surface. The technical content with a roughness satisfying 0.4 to 1.0 μm is disclosed. Patent Document 3 discloses the technical content of making spangles finer to 1000 μm or less by passing droplets through an electric field.

  Recently, however, in the case of automobile manufacturers, conventional welding methods, especially spot welding, which have been used universally for joints between automobile steel plates to save costs, increase stability, reduce work time, and work friendly to the environment. There is an increasing movement to expand from methods to methods using structural adhesives.

  When using the above structural adhesives, unlike spot welding, which is a mechanical joining method, considering the use of automobiles in cold extreme regions, adhesion between plated steel sheets can be achieved at an extremely low temperature of -40 ° C. It is necessary to confirm. However, when an adhesive is used on a steel sheet without spangles whose zinc (0001) plane is parallel to the steel sheet surface, the plating layer peels off at a deep drawing or at a very low temperature of −40 ° C. There is a problem of triggering.

  This is because brittleness increases due to refinement of spangles, and since the (0001) surface of the galvanized layer is the bottom surface and at the same time, it is a slip surface or a cleavage fracture surface, it peels from the plating layer / base iron interface due to external impact. Is expected to occur.

  Therefore, recently, since the joining method of hot dip galvanized steel sheets using structural adhesives has emerged, the appearance of the plating layer is beautiful, and it has excellent deep drawing properties and resistance to joint brittleness at extremely low temperatures. Development for hot-dip galvanized steel sheets is required.

Japanese Published Patent No. 1999-100635 Japanese Published Patent No. 1996-188863 US Patent No. 4500561

  One aspect of the present invention is to provide a hot dip galvanized steel sheet having excellent deep drawability and cryogenic joint brittleness resistance by controlling the structure of the hot dip galvanized layer and the size of crystal grains, and a method for producing the same. The purpose is to provide.

  One aspect of the present invention is that the average diameter of crystal grains of a hot-dip galvanized layer of a hot-dip galvanized steel sheet is 150 to 400 μm, and the concentration degree of preferential orientation with respect to the (0001) plane of the hot-dip galvanized layer is 3,000 to The present invention provides a hot-dip galvanized steel sheet having excellent deep drawability (20,000 cps) and extremely low temperature joint brittleness resistance.

  Another aspect of the present invention is a method for producing a hot dip galvanized steel sheet including a molten zinc adhesion step, a molten zinc adhesion amount adjustment step, an aqueous solution injection step, a cooling step, and a temper rolling step, The step provides a method for producing a hot-dip galvanized steel sheet having excellent deep drawability and extremely low temperature joint brittleness resistance, which is performed by electrifying and spraying desalted water.

  According to the present invention, the deviation of the size of the crystal grains of the hot dip galvanized layer is reduced by electrification jetting of desalted water and the rolling reduction during temper rolling, and the zinc crystal is directed to the (0001) plane. A hot-dip galvanized steel sheet having a reduced crystal orientation and an increased twin structure fraction of the plated layer is produced. Accordingly, a hot-dip galvanized steel sheet having excellent deep drawing resistance such as bending and resistance to joint brittleness of the plated layer at an extremely low temperature can be obtained.

(A) is a schematic diagram showing a crystallographic structure of a hot-dip galvanized steel sheet having a spangle of 150 μm or less, and (b) is a schematic diagram showing a crystallographic structure of a general hot-dip galvanized steel sheet having a spangle of 400 μm or more. (A) is a measurement result of the crystal orientation of the (0001) plane of a hot-dip galvanized steel sheet with spangles of 150 μm or less using X-ray, and (b) is a general hot-dip galvanized steel sheet with spangles of 400 μm or more. It is the graph which showed the measurement result of the crystal orientation of (0001) plane. (A), (b) is the photograph which showed the evaluation result of the ultra-low temperature joint brittle resistance of the comparative example 1 and the invention example 1, respectively.

  Hereinafter, the present invention will be described in detail.

  The average diameter of the crystal grains of the hot dip galvanized layer of the hot dip galvanized steel sheet of the present invention satisfies 150 to 400 μm.

  When the average diameter of the crystal particles is less than 150 μm, the surface is beautiful due to the refinement of spangles. However, when the average diameter exceeds 400 μm, the resistance to cryogenic bonding is excellent, but the size of spangles is excellent. It becomes coarse and the surface appearance and sharpness are poor, and the plating layer falls off during continuous pressing, resulting in poor performance.

  In the hot-dip galvanized steel sheet of the present invention, the minimum diameter of the crystal particles is preferably 30 μm, and the diameter deviation of the crystal particles is preferably 40% or less of the average diameter of the crystal particles.

  If the crystal structure of the plating layer is 30 μm or less, it becomes more brittle than the surrounding crystal structure, which may act as a starting point of cracking, causing the plating layer to fall off during bending of the hot dip galvanized steel sheet. It may reduce the sex.

  Moreover, it is preferable that the crystal deviation of the crystal grains of the hot dip galvanized layer of the hot dip galvanized steel sheet of the present invention has a uniform spangle size of 40% or less of the average diameter of the crystal grains. If the deviation exceeds 40% and the difference in size of the zinc crystal structure is not uniform, the plating layer cannot receive a uniform force during plastic deformation, and there is a problem that the plating layer falls off locally, It is preferable that the crystal deviation of the crystal grains satisfy 40% or less so that brittleness does not occur at the joint when joining.

  Moreover, it is preferable that the hot dip galvanized steel sheet of the present invention has a concentration degree of priority orientation with respect to the (0001) plane of the hot dip galvanized layer of 3,000 to 20,000 cps (counter per second). The hot dip galvanized steel sheet according to the present invention has a Max. Concentration of preferential orientation of Max. With respect to the zinc crystal (0001) plane when irradiated with X-ray having a voltage of 20 KV and a current of 10 mA. As a result of measurement by Intensity (average value of rotation angles 0 to 360 ° measured every 5 ° with reference to the inclination angle 5 ° of the base material), 3,000 to 20,000 cps is satisfied.

  On the other hand, as shown in FIG. 2 (a), the conventional hot dip galvanized steel sheet having a spangle of 150 μm or less has a concentration degree of preferential orientation exceeding 20,000 cps, and the conventional galvanized steel sheet shown in FIG. The hot-dip galvanized steel sheet having a spangle of 400 μm or more does not reach 3,000 cps.

  In the present invention, the reason why the concentration of the preferential orientation is limited to 3,000 to 20,000 cps is that if the concentration of preferential orientation with respect to the (0001) plane is less than 3,000 cps, the brittleness resistance of the plating layer is reduced. Although advantageous in terms of side, it is because the spangle of the plating layer is coarse and the surface appearance is poor. On the other hand, if it exceeds 20,000 cps, spangles of the plating layer become finer and the surface appearance is beautiful, but there is a problem of inducing deep drawability and brittleness of the plating layer at an extremely low temperature.

  In the hot dip galvanized steel sheet of the present invention, it is preferable that the twinned structure fraction of the hot dip galvanized layer satisfies 30% or more in terms of the volume fraction of the entire structure. The twin structure of the plating layer generated by temper rolling acts as an important plastic deformation mechanism of zinc crystals that are dense hexagonal crystals (HCP), and helps to improve deep drawability and brittleness. When the twinning structure fraction of the plating layer is less than 30%, the plastic deformation action of the twinning structure is not sufficient, so that the workability is inferior when the zinc crystal grain size of the plating layer is 150 to 400 μm. is there.

  Hereinafter, the manufacturing method of the hot dip galvanized steel sheet of the present invention will be described in detail.

  The production method of the present invention is a method for producing a hot dip galvanized steel sheet including a molten zinc adhesion step, a molten zinc adhesion amount adjustment step, an aqueous solution injection step, a cooling step, and a temper rolling step. It is performed by charging and spraying salt water (Demineralized Water).

  In the hot dip galvanizing step in the manufacturing process of the hot dip galvanized steel sheet according to the present invention, the hot dip galvanization is attached to the steel sheet by passing the steel sheet through a galvanizing solution. In the present invention, the hot dip galvanizing step is not particularly limited, but in order to produce a hot dip galvanized steel sheet, the surface of the steel sheet is applied by applying any galvanizing solution and galvanizing conditions generally applied in the technical field. Molten zinc can be adhered to. As the zinc plating solution, generally, a zinc plating solution containing aluminum (Al), antimony (Sb) and / or lead (Pb) may be used, and the present invention is not limited thereby. The steel plate is not particularly limited, and any known steel plate generally used for hot dip galvanizing may be used.

  After the steel sheet is immersed in the zinc plating solution, the amount of plating adhesion is adjusted by removing the plating solution excessively adhered to the steel sheet by air wiping the zinc plating solution in the zinc adhesion amount adjusting step. The plating adhesion amount is not particularly limited, and can be appropriately adjusted by a technician in the technical field within a range generally known in the technical field as required, such as the use of the steel sheet.

  In the present invention, after adjusting the molten zinc adhesion amount of the steel sheet, demineralized water is charged and sprayed at the time of aqueous solution injection for solidification of the hot dip galvanized layer. The reason why the demineralized water is charged and jetted in the present invention is to ensure a uniform plating layer structure with no variation in spangle size. When the solution is charged and sprayed to make spangles fine, the small mist particles collide with the molten plating layer and cause an endothermic reaction to promote solidification. At this time, when an inorganic salt solution such as phosphate is sprayed, the endothermic reaction becomes more active at the part where the core particles such as phosphate collide with the plating layer, and the surface spangle is reduced, and the part where it is not is relatively As a result, the size of the spangle becomes coarse and a deviation of the size of the spangle occurs.

  When the spangle size deviation is large, the stress cannot uniformly act on the plating layer during deep drawing, so the small spangle portion may act as the starting point of cracking. Also causes the plating layer to fall off and deteriorates workability.

In the present invention, when the desalted water is charged and jetted, the desalted water jet pressure of the nozzle is 0.3 to 5.0 kgf / cm ² , the air jet pressure is 0.5 to 7.0 kgf / cm ² , and the pressure of the desalted water. The ratio of / air pressure is preferably 1/10 to 8/10.

If the pressure of the desalted water is less than 0.3 kgf / cm ^2, there is no spangle refinement effect, and if the pressure of the desalted water exceeds 5.0 kgf / cm ² , the solution droplets collide on the steel sheet surface to cause pit marks. Is generated, and the appearance of the plating layer is impaired.

  The size of the charging electrode at the front end of the nozzle is preferably −1 to −25 KV. If the size of the electrode is less than −1 KV, the electric attractive force does not act, and the effect of refining spangles due to the refining of solution particles cannot be obtained. On the other hand, when the size of the electrode exceeds −25 KV, the spangle refinement effect is excellent and the surface of the plating layer is less than 150 μm, and the deep drawability and the cryogenic joint brittleness resistance deteriorate.

  In the present invention, temper rolling is performed after electrifying and spraying the desalted water. Twinning is generated in the hot-dip galvanized layer by the temper rolling. At this time, temper rolling is preferably performed at an elongation of 5% or less.

  In the present invention, temper rolling is preferably performed at an elongation of 5% or less. The twin structure generated during temper rolling acts as an important processing mechanism in a zinc crystal which is a dense hexagonal crystal (HCP) with few deformation tools. In addition, the effect of dispersing the preferential orientation of zinc to the (0001) plane is brought about by physical deformation of temper rolling. Therefore, when temper rolling is not performed, the adhesion of the base iron of the plating layer is deteriorated and the workability and plating adhesion are poor, and when the elongation rate exceeds 5%, the workability and plating adhesion of the plating layer are exceeded. However, it is not preferable because it induces inferiority of the base iron material.

  Examples of the present invention will be described in detail below. However, the present invention is not limited to the following examples.

(Example)
In order to observe the joint brittleness when desalted water treatment is performed after hot dip galvanizing, the size of spangles is adjusted by performing phosphate treatment and desalted water treatment under the conditions shown in Table 1 below. The temper rolling was performed under the conditions of 0.0% and roll rolling force of 200 to 240 tons, and the joint brittleness, surface appearance and sharpness were measured, and the results are shown in Table 1.

The material of the hot dip galvanized steel sheet was a soft material IF steel having a thickness of 0.67 mm, and the plating amount during hot dip galvanization was 70 g / m ² .

  The size and deviation of spangles in Table 1 below were measured using an optical microscope and an image analyzer for a plating layer that was not temper-rolled. Bonding brittleness was achieved by joining two hot-dip galvanized steel sheets using Henkel Korea Co., Ltd.'s Sealer Terokal 5089 adhesive for automobile structure, holding at −40 ° C., and using wedges. An impact was applied in the meantime, and the measurement was made based on whether or not the galvanized layer was removed. In Table 1 below, ◯ means no peeling, Δ means 20% or less peeling, and × means 50% or more peeling. On the other hand, the surface appearance and sharpness were measured by visual observation, where ◯ indicates excellent, Δ indicates normal, and X indicates poor.

  From the results of Table 1 above, the inventive examples of the present invention satisfy the size of the spangle of the present invention using demineralized water, the concentration degree of preferential orientation also satisfies 3,000 to 20,000 cps, The deviation is also within the scope of the present invention, and it can be seen that it has excellent joint brittleness resistance and surface appearance.

  Since Comparative Example 1 uses demineralized water, the size of the spangle of the present invention is satisfied, but the deviation of the size of the spangle is very large, and the concentration of priority orientation is out of the scope of the present invention. The brittleness was bad. When using a phosphate like Comparative Examples 2-5, there existed a problem that joint brittleness was bad or the surface appearance worsened. In Comparative Example 6, which is a general hot dip galvanizing, the surface appearance was very bad.

  3 (a) and 3 (b) show photographs in which the joint brittleness of Comparative Example 1 and Invention Example 1 was evaluated. The evaluation of the joint brittleness was judged by the presence or absence of the blue adhesive. In the specimen of Comparative Example 1 indicated by a circle in FIG. 3A, no adhesive remained when the plated layer was broken. It can be seen that Invention Example 1 in FIG. 3B has excellent adhesive brittleness resistance with the adhesive remaining.

Claims (7)

  Including a hot dip galvanized layer, wherein the average diameter of crystal grains of the hot dip galvanized layer is 150 to 400 μm, and the concentration degree of preferential orientation with respect to the (0001) plane of the hot dip galvanized layer is 3,000 to 20,000 cps. A hot-dip galvanized steel sheet with excellent deep drawability and extremely low temperature joint brittleness resistance.   2. The molten zinc excellent in deep drawability and cryogenic joint brittleness resistance according to claim 1, wherein the diameter of the crystal particles is 30 μm or more and the diameter deviation of the crystal particles is 40% or less of the average diameter of the crystal particles. Plated steel sheet.   The hot-dip galvanized steel sheet excellent in deep drawability and cryogenic joint brittleness resistance according to claim 1, wherein the twinned structure of the hot-dip galvanized layer is 30% or more in volume fraction. A method for producing a hot dip galvanized steel sheet, comprising a hot dip galvanizing step, a hot dip zinc adhering amount adjusting step, an aqueous solution spraying step, a cooling step and a temper rolling step
The aqueous solution spraying step is a method for producing a hot-dip galvanized steel sheet excellent in deep drawability and extremely low temperature joint brittleness resistance, which is performed by electrifying and spraying demineralized water. 5. The step of charging and spraying the desalted water is performed at a nozzle desalted water injection pressure of 0.3 to 5.0 kgf / cm ² and an air injection pressure of 0.5 to 7.0 kgf / cm ^2. Of hot-dip galvanized steel sheet with excellent deep drawability and extremely low temperature joint brittleness resistance.   6. The step of electrifying and spraying the desalted water is performed under the condition that the ratio of the pressure of the desalted water / the air pressure is 1/10 to 10/10, and is excellent in deep drawability and cryogenic joint brittleness resistance according to claim 5. Manufacturing method of hot dip galvanized steel sheet.   The method for producing a hot-dip galvanized steel sheet excellent in deep drawability and cryogenic bonding brittleness according to claim 4, wherein the temper rolling step is performed at an elongation of 5% or less.

Images