JP2016108614A - Al PLATED STEEL HAVING EXCELLENT WELDABILITY AND CORROSION RESISTANCE AFTER PAINTING FOR HOT PRESSING - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Al plated steel material having a hot-slidability and corrosion resistance after painting, which are better than that of a conventional Al plated steel material, and a good weldability.SOLUTION: An Al plated steel material having an excellent weldability and an excellent corrosion resistance after painting, the Al plated steel material having an Al plating layer including 3 - 15 mass% of Si and a rest of Al and inevitable impurities, and a layer thereon, the layer including ZnO of which a Zn-converted coating amount is 0.1 - 2.0 g/m, and a stainless steel of which a Zn-converted coating amount is 10 - 40%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an Al-plated steel for hot pressing with improved weldability and post-coating corrosion resistance.

  In recent years, the usage rate of high-strength steel sheets has been increasing for the purpose of reducing the weight of automobiles and improving collision safety. However, high-strength steel sheets generally have a small degree of freedom in forming by press molding, poor shape freezeability after pressing, poor dimensional accuracy of the molded product, and reduced press die life. There is.

  As measures for improving these problems, a hot press method for producing a high-strength part by heating and softening a steel plate to 800 ° C. or higher and quenching it by rapid cooling at the same time as press forming has become widespread. In addition to the bare material without plating, Zn plating and Al plating are used as materials used for the hot press method depending on the application. When using Al plating, there are the following problems.

  One is inferior hot slidability. This leads to the problem that Al plating is deposited on the hot press mold and the deposit damages the plating. This is thought to be because the Fe—Al—Si layer produced when the Al plating is heated is hard and the sliding resistance value is high.

  Another is inferior corrosion resistance after painting. This is due to the fact that the Fe—Al—Si layer formed after heating the Al plating is extremely inactive and no chemical conversion film is formed.

  Patent Document 1 discloses forming a film of a wurtzite type compound, for example, ZnO, on an Al-plated steel sheet as means for improving these. By using this method, it is said that a sufficient amount of chemical conversion film is generated, the corrosion resistance after coating is improved, and the hot slidability is also improved.

Japanese Patent No. 4590025

  The Al-plated steel material of Patent Document 1 has a problem in that it tends to scatter during continuous hitting during welding and has poor weldability. In view of this problem, the present invention has an object to provide an Al-plated steel material for hot press with improved hot slidability, post-coating corrosion resistance, and weldability while utilizing the corrosion resistance of the Al plating itself. To do.

  The present inventors diligently studied to solve the above problems. As a result, it is possible to improve weldability by adding ZnO and stainless steel to the surface of the Al-plated steel material, and it is ideal as a material for hot pressing that combines hot slidability, corrosion resistance after coating, and weldability. The present inventors have found that an Al-plated steel material can be provided, and have reached the present invention. The summary is as follows.

(1) It contains 3 to 15% of Si by mass%, the remainder has Al plating consisting of Al and inevitable impurities, and the Zn conversion amount is 0.1 to 2.0 g / m ^{2 on the} upper layer. A hot-pressed Al-plated steel material having excellent weldability and post-coating corrosion resistance, characterized in that it has a layer containing ZnO and 10 to 40% of the amount of deposit in terms of Zn.

  (2) The Al-plated steel for hot pressing having excellent weldability and post-coating corrosion resistance according to (1), wherein the average particle size of the stainless steel particles is 20 to 120 nm.

  According to the present invention, the performance of the Al-plated steel material used as a hot press material is remarkably improved, which is superior in hot slidability and post-coating corrosion resistance than conventional Al-plated steel materials, and also has good weldability. An Al-plated steel material can be provided.

It is a figure which shows the test apparatus used in order to evaluate hot slidability in an Example.

  Hereinafter, the present invention will be described in detail.

  First, Si in Al plating is 3 to 15% by mass. If Si is less than 3%, the alloying reaction between the steel material and the plating proceeds excessively during the production of the plating, and an alloy layer of Al-Fe-Si develops, so that it is difficult to control the amount of plating adhesion. Further, since the plating solution phase temperature rises, stable operation cannot be performed unless the plating bath temperature is maintained at 700 ° C. or higher, and the load on the equipment becomes extremely large.

  On the other hand, when the Si concentration exceeds 15%, the volume ratio of the Si layer increases, the plating becomes brittle, and the corrosion resistance is extremely deteriorated.

  Next, an element applied to the upper layer of the Al plating, that is, the surface of the Al plating will be described. ZnO has the effect of improving hot slidability and post-coating corrosion resistance. Regarding the improvement of hot slidability, it is presumed that ZnO hinders direct contact between the Al plating and the mold and further plays a role like a lubricating oil in the heating process. The improvement in post-coating corrosion resistance is considered to be due to the formation of a chemical conversion film by dissolving a part of ZnO in the chemical conversion treatment solution during the chemical conversion treatment step, which is a pre-coating treatment.

In order to exhibit the effect of improving hot slidability and post-coating corrosion resistance, the ZnO deposition amount needs to be 0.1 to 2.0 g / m ² as the deposition amount in terms of Zn. If the ZnO adhesion amount is less than 0.1 g / m ² , the effect of improving these performances is insufficient, and conversely if it exceeds 2.0 g / m ² , the residual stress increases, and ZnO is removed from the Al plating. It becomes easy to peel off, and the hot slidability decreases. In addition, the average crystal grain size of ZnO is preferably in the range of 20 to 100 nm, and the measurement can be performed by a generally used method without any problem. For example, a light scattering method can be used.

  Stainless steel formed on Al plating together with ZnO has the effect of improving weldability and corrosion resistance after coating. Regarding the improvement of weldability, the presence of stainless steel with a high conductivity in a part of the layer containing ZnO with a low conductivity suppresses local energization during resistance welding and prevents rapid heat generation. it is conceivable that. In addition, in the heating process of the hot press, a part of ZnO particles in contact with the stainless steel may be partly changed to Zn, and the resistivity of the layer may be further reduced.

  Regarding the improvement of coating corrosion resistance, a part of ZnO becomes Zn, and this Zn increases the amount of dissolution of the layer containing ZnO during the chemical conversion treatment, thereby increasing the amount of chemical conversion film or forming it more uniformly. It is estimated that That is, it is considered that the weldability and post-coating corrosion resistance are improved as compared with the conventional Al-plated steel material for hot pressing due to the synergistic effect of ZnO particles and stainless steel particles.

  In order to obtain this effect, the adhesion amount of stainless steel needs to be 10 to 40% of the adhesion amount of ZnO in terms of Zn. When the adhesion amount is less than 10% of the Zn conversion amount of ZnO, local energization occurs, so that the weldability is poor, and the post-coating corrosion resistance improvement effect is poor. If it exceeds 40%, the effect of improving hot slidability and post-coating corrosion resistance by ZnO is inferior.

  The stainless steel here is defined as steel containing 11% or more of Cr. The structure of the stainless steel formed on the Al plating of the present invention may be any of ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, or a mixed structure thereof.

  The average particle size of the stainless steel is preferably 20 to 120 nm. If the average particle size exceeds 120 nm, it is too large for ZnO, and it becomes difficult for the stainless steel to be uniformly dispersed, and local energization occurs. When the average particle size is less than 20 nm, it is considered that there is no significant problem in performance. However, since the production becomes extremely difficult, the thickness is preferably 20 nm or more.

  The method for forming the layer containing ZnO and stainless steel on the Al plating surface is not particularly limited. As an example, there is a method in which a suspension containing Zn and stainless steel and a predetermined organic binder are mixed and applied to the Al plating surface and dried.

  Examples of the predetermined organic binder include polyurethane resins, polyester resins, acrylic resins, silane coupling agents, silica, and the like. These organic binders are water soluble so that they can be mixed with a suspension of ZnO and stainless steel.

  The content of the binder component such as the resin component, the silane coupling agent, and silica in the surface film is preferably 5 to 80% in total with respect to ZnO and stainless steel. When the content of the binder component is less than 5%, the binder effect cannot be sufficiently obtained and the coating film can be easily removed. In order to stably obtain the binder effect, the binder component is more preferably 10% or more by mass ratio. When the content of the binder component exceeds 80%, odor generation during heating becomes remarkable, which is not preferable.

  The components, forms, and the like of the steel material that is the base material of the Al-plated steel material for hot pressing of the present invention are not concluded at all. The component may be a soft material, a steel material containing a strengthening element such as Si or Mn, and the form may be a thin plate, a thick plate, a steel pipe, or a molded product.

  Next, an Example demonstrates this invention in detail.

[Example 1]
From the ZnO and stainless steel, a polyurethane resin is used as the organic binder in the laboratory roll coater on the surface of the hot-pressed hot-dip Al-plated steel sheet having a plate thickness of 1.6 mm and a size of 300 mm × 200 mm shown in Table 1. The layer to be formed was formed by changing the adhesion amount and the content rate, and used as a test material. Then, according to the following methods, hot slidability, post-coating corrosion resistance, and weldability were evaluated. As the stainless steel particles, commercially available SUS304 nanopowder, SUS316 nanopowder, and SUS430 nanopowder manufactured by Nippon Ion Co., Ltd. were used.

[Hot slidability]
With the apparatus shown in FIG. 1, a test material cut to a size of 150 mm × 200 mm is heated to 900 ° C., and a steel ball is pressed from above at 700 ° C., and the pressing load and the pulling load are measured. The hot slidability was evaluated by determining the pressing load as a dynamic friction coefficient. If the dynamic friction coefficient was 0.8% or less, it was judged as acceptable.

[Corrosion resistance after painting]
A 300 mm × 200 mm test material was placed in an atmospheric furnace maintained at 900 ° C. for 5 minutes, then taken out and immediately cooled by being sandwiched between stainless steel molds. The cooling rate was about 150 ° C./second. Next, the test material was cut into 150 × 60 mm, and after chemical conversion treatment with a chemical conversion treatment liquid (PB-SX35) manufactured by Nippon Parkerizing Co., Ltd., an electrodeposition paint (Powernics 110) manufactured by Nippon Paint Co., Ltd. Was coated to a thickness of 20 μm, baked at 170 ° C., and used as a corrosion resistance test material after coating.

  The post-painting corrosion resistance evaluation was performed by the method prescribed in JASO M609 established by the Automotive Engineering Association. A crosscut was put into the coating film with a cutter in advance, and the width (maximum value on one side) of the swelling from the crosscut after 180 cycles (60 days) of the corrosion test was measured.

[Weldability]
A 300 mm × 200 mm test material was placed in an atmospheric furnace maintained at 900 ° C. for 5 minutes, then taken out and immediately cooled by being sandwiched between stainless steel molds. The cooling rate was about 150 ° C./second. Next, it cut | disconnected to 125 mm x 40 mm, and was set as the weldability test material. The welding test was evaluated by determining an appropriate current range (upper limit current-lower limit current) during AC spot welding and a scattering occurrence rate during continuous hitting. If the appropriate current range is 1.5 kA or more, it is acceptable, and if it is less than 1.5 kA, it is unacceptable. The scattering rate at the time of continuous hitting was determined to be acceptable if it was 15% or less, and rejected if exceeding 15%.

  The welding conditions are as shown below. The lower limit current was defined as the current value when the nugget diameter was 5.6 mm or more, and the upper limit current was defined as the scatter generation current.

[Welding conditions]
Electrode: Chrome copper, DR (tip 6mmφ is 40R)
Plate assembly: Welding test material combination welding Pressurization: 400kgf
Energizing time: 20 cycles (60 Hz)
Welding current: When measuring the appropriate current range; increased from 5 kA to scattering
At the time of continuous hitting: A value 0.5 kA smaller than the upper limit value of the appropriate current range.

  The results are shown in Table 2. When the constituent component of the Al plating upper layer is within the range of the present invention, it was confirmed that the hot slidability, post-coating corrosion resistance, and weldability are all good.

DESCRIPTION OF SYMBOLS 1 Test piece 11 Elema heater 12 Furnace body drive device 13 Ballway 14 Load cell

Claims (2)

It contains 3 to 15% Si by mass%, and the remainder has Al plating consisting of Al and inevitable impurities,
Welding characterized in that it has a layer containing ZnO with a Zn conversion amount of 0.1 to 2.0 g / m ² and stainless steel of 10 to 40% of the Zn conversion amount on the upper layer. Al-plated steel material for hot pressing that excels in heat resistance and corrosion resistance after painting.   The average particle size of the stainless steel particles is 20 to 120 nm, and the Al-plated steel material for hot press having excellent weldability and post-coating corrosion resistance according to claim 1.

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