JP4090051B2 - Method for producing Al-Zn alloy-plated steel strip - Google Patents

Description

  The present invention relates to a method for producing an Al—Zn alloy-plated steel strip having a reduced yield point and improved workability.

  In interior building materials, exterior building materials, etc., hot-dip galvanized steel sheets having good corrosion resistance have been conventionally used as coating original sheets. However, with the progress of air pollution, acidification of the atmosphere and rainwater due to sulfur oxides, nitrogen oxides, etc. has been remarkable in recent years. Since corrosion of the hot dip plating layer is promoted, there is a concern about durability as an interior building material, an exterior building material, and the like. For example, the corrosion of the flat portion is caused by corrosive ions such as Cl ions permeating the coating film to promote the corrosion of the hot dip galvanized layer, and the coating film is pushed up by the volume expansion zinc-based corrosion product. Observed as a bulge.

  In order to cope with environmental deterioration, the use of a hot-dip Zn—Al alloy-plated steel sheet has been promoted as a coating original sheet that exhibits corrosion resistance superior to that of a hot-dip galvanized steel sheet. In the hot-dip Zn—Al alloy-plated steel sheet, the corrosion resistance of the flat part and the coating-flawed part improves as the Al content of the Zn—Al alloy hot-plated layer increases. However, as the Al content increases, the workability of the hot dipped layer decreases. For example, a large working crack is generated in the hot-plated layer in the bent portion of the coated hot-dip Zn-55% Al alloy-plated steel sheet, and the working crack is also propagated to the coating film on the hot-plated layer. When the coating film cracks become large enough to be detected by visual observation, the design properties of the molded product are deteriorated.

Process cracks generated in the hot dipped layer and coating promote the penetration of corrosive ions. Corrosive ions selectively react with Zn in the hot dipped layer to prevent corrosion of the base steel by the sacrificial anticorrosive action of Zn. However, when Zn is exhausted by the corrosion reaction, corrosion of the base steel is started. The corrosion of the base steel appears as red rust, which significantly deteriorates the appearance of the molded product.
In order to improve the workability of the hot-dip Zn—Al alloy-plated steel sheet, Patent Document 1 introduces a method of softening the hot-dip coating layer by post-heating treatment at about 93 to 427 ° C. The Al—Zn alloy plating layer is softened by the post heat treatment, and cracking and peeling of the Al—Zn alloy plating layer in a bent portion or the like are suppressed.
JP 56-87654 A

Although the Al—Zn alloy plated layer is softened by the post heat treatment, the Al—Zn alloy plated steel strip is easily hardened, and becomes harder when left for a long time. In order to form a hardened Al—Zn alloy-plated steel strip into a predetermined shape, a large processing pressure is required, and a load applied to a press machine, a bending machine, and the like is increased. Moreover, as a result of an increase in the difference in mechanical properties between the base steel and the plating layer, cracks and peeling may still occur in the Al—Zn alloy plating layer and the coating film formed thereon.
The tendency of cracks and delamination becomes more prominent as the post-heating conditions are increased to a higher temperature for a shorter time in order to improve productivity. Therefore, the conventional post heat treatment has a limit in improving the workability of the Al—Zn alloy plated steel strip, and the excellent corrosion resistance of the Al—Zn alloy plated layer cannot be fully utilized.

  The present invention has been devised to solve such problems, and on the premise that the Al-Zn alloy-plated steel strip after post-heat treatment is hardened by age hardening, age hardening is suppressed or An object of the present invention is to provide an Al—Zn alloy-plated steel strip in which hardening is suppressed and workability is ensured by incorporating relaxation skin pass rolling.

  In order to achieve the object, the production method of the present invention introduces a steel strip into a hot dipping bath to form an Al—Zn alloy plating layer containing Al: 50 to 60% by mass on the surface of the steel strip, and then 150 to Heating to 350 ° C. for 0.5 hours or more, cooling rate: 0.8 ° C./min or less and post-heating treatment to room temperature, followed by skin pass rolling at an elongation of 0.2 to 3.0% And The Al—Zn alloy plating layer can further contain Si: 3 mass% or less.

  There are ordinary steel, low alloy steel and the like as the plating base plate, and deep drawing steel such as ultra-low carbon Ti and Nb added steel is preferable in applications where machining of complex shapes is planned. After the surface of the plating original plate is activated by reduction annealing according to a conventional method, it is introduced into an Al—Zn alloy hot dipping bath. The hot dipping bath contains Al and Zn at a ratio corresponding to the composition of the Al—Zn alloy plating layer, and is maintained at about 600 ° C. You may add 3 mass% or less Si further to a hot dipping bath. After pulling up the plating original plate from the hot dipping bath, the amount of plating is adjusted by spraying an inert gas on the surface of the steel sheet, and after cooling and solidification, an Al—Zn alloy plating layer having a predetermined composition is formed on the surface of the steel sheet.

  The formed Al—Zn alloy plating layer is a mixed crystal in which an Al-rich hard dendrite phase in which Zn is dissolved in a supersaturated state is dispersed in a Zn-rich soft matrix. The Al—Zn alloy plating layer contains Al: 50 to 60% by mass, and if necessary, Si: 3% by mass or less, and the balance has a substantially Zn composition. The Al content of the plating layer is selected in the range of 50 to 60% by mass in order to ensure a mixed crystal structure effective for corrosion resistance. Si added as necessary is effective in suppressing the formation of a hard and brittle Al-Fe-based intermediate layer at the interface between the base steel and the plating layer, but excessive addition exceeding 3% by mass Is not preferable because crystallized Si that hardens the Al—Zn alloy plating layer starts to be scattered.

  Since there is a large difference in hardness between the hard dendrite phase and the soft matrix, the stress introduced by bending or the like is concentrated at the dendrite phase / matrix interface, and cracks are generated and propagated starting from the dendrite phase / matrix interface. To do. The hardness of the dendrite phase is greatly influenced by the concentration of Zn dissolved in the dendrite phase. The smaller the amount of dissolved Zn, the softer the dendrite phase.

  Therefore, in order to promote elution of solute Zn from the dendrite phase, the Al—Zn alloy-plated steel strip is kept in the temperature range of 150 to 350 ° C. for 0.5 hours or more, and then the cooling rate is 0.8 ° C./min or less. To cool to room temperature. When the heat treatment temperature exceeds 350 ° C., the solid solution Zn increases due to the phase change, and the heat treatment temperature below 150 ° C. is not practical because it requires a long-time holding. Moreover, elution of solid solution Zn is not sufficiently ensured by holding for a short time not reaching 0.5 hours, and elution of solid solution Zn is not sufficiently promoted by rapid cooling exceeding 0.8 ° C./second.

  Even if the Al-Zn alloy plating layer is softened by post heat treatment, if the base steel is hardened, a large hardness difference between the base steel and the plating layer may cause processing defects during processing, bending, etc. become. The base steel has a strong tendency to become harder as post-heating treatment is performed at a high temperature in a short time. The inventors of the present invention have predicted that hardening of the base steel is due to age hardening caused by thermal stress and thermal strain brought about by post heat treatment. As a result of investigating and examining a method for suppressing the influence of age hardening as much as possible under such expectation, it was found that skin pass rolling after post-heating treatment is effective.

The reason why skin pass rolling suppresses age hardening is not clear, but is presumed as follows and is supported in the examples described later.
In the Al—Zn alloy-plated steel strip as it is post-heat treated, thermal stress and thermal strain remain in a locally concentrated distribution, and the energy level of stress and strain is locally high. When some external force is applied to the Al-Zn alloy-plated steel strip in this state, the metastable state collapses, elements, compounds, dislocations, etc. that become the core of age hardening appear in the matrix, and age hardening centering on the nucleus proceeds. .

  On the other hand, when skin pass rolling is performed after the post heat treatment, further mechanical stress and strain are introduced into the Al—Zn alloy-plated steel strip, and the energy distribution of stress and strain is averaged as a whole. The thermal stress and strain introduced by the post heat treatment are also dispersed by the skin pass rolling, and the energy level peak is also reduced. As a result, the generation of nuclei causing age hardening is suppressed, and the workability of the Al—Zn alloy-plated steel strip is ensured.

  As a result of investigating the effect of skin pass rolling on the suppression of hardening, it was found that hardening of the Al-Zn alloy-plated steel strip is suppressed to the extent that workability can be secured by skin pass rolling with an elongation of 0.2% or more. It was. However, in the skin pass rolling exceeding 3.0% elongation, the stress and strain introduced by the skin pass rolling become too large, and the workability of the Al—Zn alloy plated steel strip is lowered.

A normal steel plate having a thickness of 0.35 mm was used as a plating base plate, and was subjected to reduction annealing by heating to 700 ° C. in a reducing atmosphere of H ₂ -50 volume% N ₂ , and then 55 mass% Al—Zn maintained at 600 ° C. It was introduced into the alloy plating bath at a line speed of 120 m / min. After being pulled out of the hot dipping bath with an immersion time of 2 seconds and removing excess plating metal by blowing an inert gas, it is cooled at a cooling rate of 20 ° C./min, and the amount of plating deposited per side: 80 g / m ² Al—Zn alloy A plated steel strip was produced.

The manufactured Al—Zn alloy-plated steel strip was subjected to coil annealing at 200 ° C. for 5 hours, and then cooled to room temperature at a cooling rate of 0.1 ° C./min. When the test piece cut out from the Al—Zn alloy plated steel strip after cooling was mechanically tested, the yield point was 393 N / mm ² . The measured value is considerably higher than the yield point of the galvanized steel sheet not subjected to post-heating treatment: 279 N / mm ^2, and the hardening of the base steel has progressed in the process of hot dipping → post-heating treatment. Indicates.

Next, the post-heat-treated Al—Zn alloy-plated steel strip was subjected to skin pass rolling, and the influence of skin pass rolling on mechanical properties was investigated. As can be seen from the investigation results in Table 1, when the elongation rate by skin pass rolling was set in the range of 0.2 to 3.0%, the yield point remained at 323 N / mm ² at the maximum. Yield elongation was 5.0 to 12.0% without skin pass rolling, but decreased to 0 to 1.0% with skin pass rolling. There was no difference in tensile strength and total elongation due to the presence or absence of skin pass rolling.

However, the Al-Zn alloy-plated steel strip subjected to skin pass rolling with an elongation of less than 0.2% still shows a high yield point of 350 N / mm ^2, and the effect of skin pass rolling that suppresses hardening is insufficient. there were. Conversely, the Al-Zn alloy-plated steel strip that has been subjected to skin pass rolling at an elongation rate exceeding 3.0% exhibits a yield point at almost the same level as before skin pass rolling, and is hardened by the stress and strain introduced during skin pass rolling. It is redeemed to be.

  Furthermore, the skin-pass rolled Al-Zn alloy-plated steel strip is used for the coating original plate, and after coating pretreatment such as chromate treatment, an epoxy-based undercoating film with a film thickness of 15 μm and a polyester-based overcoating film with a film thickness of 15 μm are applied to the steel plate. Formed on the surface. For the undercoat, an epoxy-based paint blended with strontium chromate (rust-preventing pigment), titanium oxide (colored pigment), barium sulfate, and silica (external pigment) was used.

The test piece cut out from the obtained coated steel sheet was subjected to a bending test and an accelerated corrosion test.
In the bending process test, 100 40 × 50 mm test pieces were prepared, and the test pieces were bent 180 degrees by sandwiching the same coated steel plate as the processed test pieces in a room adjusted to 20 ° C. While measuring the force required for bending deformation, the surface of the coating film outside the bent portion was visually observed, and the workability was evaluated by the number of test pieces in which cracks occurred in the coating film.

  In the accelerated corrosion test, 125 cycles of acid rain combined corrosion test [1 cycle: spraying with 0.1% NaCl corrosion solution (35 ° C. × 1 hour, adjusted to pH 4 with sulfuric acid) → drying (50 ° C. × 4 hours) → wetting ( 50 ° C. × 3 hours, relative humidity 98%)], the surface of the 4t processed part was observed, and the corrosion resistance of the processed part was evaluated by the white rust occurrence rate (%) with respect to the area 100 of the test target part.

As can be seen from the test results in Table 2, by reducing the yield point to 300 N / mm ² or less, the force required for bending deformation is small, and the scratches generated on the plating layer and coating film in the bent portion are greatly reduced. did. As a result, the Zn-Al alloy hot-plated layer is maintained in a healthy state, and the appearance of the coating film is also maintained in a good state that is almost the same as the surface of the coating film before bending. It was confirmed that excellent corrosion resistance was exhibited. On the other hand, in the test piece with a high yield point, rubbing flaws were found on the coating film on the outside of the bent portion, and white rust starting from the coating film defect was detected after the accelerated corrosion test. Further, assuming that the bending stress of the test piece subjected only to post heating is 1.0, the test piece subjected to post heating and skin pass rolling can be processed with a bending stress of 0.7 to 0.78.

  As described above, when skin-pass rolling the Al-Zn alloy-plated steel strip following the post heat treatment for softening the plating layer, the hardening of the base steel is suppressed, the workability is improved, and the processing defects are improved. Occurrence is also reduced. The processing defect suppressing power also contributes effectively to the coating film on the plating layer. Therefore, the original excellent corrosion resistance of the Al-Zn alloy plating layer is utilized, and it is used as an exterior building material, an exterior home appliance covering material, an interior building material, etc. that have excellent durability even in an environment where air pollution is advancing. .

Claims (2)

  After introducing the steel strip into the hot dipping bath and forming an Al—Zn alloy plating layer containing Al: 50 to 60% by mass on the surface of the steel strip, the steel strip is heated to 150 to 350 ° C. for 0.5 hour or longer and the cooling rate is 0. A method for producing an Al—Zn alloy-plated steel strip, which is subjected to a post heat treatment that is cooled to room temperature at a temperature of 0.8 ° C./min or less, and then subjected to skin pass rolling at an elongation of 0.2 to 3.0%.   Furthermore, the manufacturing method of Claim 1 which forms the Al-Zn alloy plating layer containing 3 mass% or less of Si on the steel plate surface.