JPH03243749A - Production of alloying hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To produce the plated steel sheet excellent in press formability by previously allowing specific amounts of Ni, Co, or Pd plating to adhere to a steel sheet by means of electroless plating at the time of producing an alloying hot dip galvanized steel sheet. CONSTITUTION:Ni, Co, or Pd plating is previously applied to a steel sheet by means of electroless plating so that coating weight is regulated to 10-1,000mg/m<2>, by which the formation of an Fe-Zn-Al ternary alloy layer in a hot dip galvanizing bath is inhibited. Subsequently, annealing, hot dip galvaniz ing, and alloying are successively applied to the above steel sheet. By this method, the ternary alloy layer control can be facilitated, and the alloying hot dip galvanized steel sheet having satisfactory powdering resistance and flaking resistance can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet.

Alloyed hot-dip galvanized steel sheet is produced by heating the hot-dip galvanized steel sheet after plating to diffuse the iron in the base steel sheet into the coating layer.
It is an iron-zinc alloy.

Because it has superior corrosion resistance compared to galvanized steel sheets, it is widely used as a material for automobiles, building materials, home appliances, etc.

(Conventional Technique w#) In the conventional method for manufacturing alloyed hot-dip galvanized steel sheets, the effective amount of AM (A[%-Fe%)] is set to 0.0, for example, in a hot-dip zinc bath.
The copper strip is plated by passing it through a bath containing an additive of 9 to 0.15%, and after adjusting the basis weight by gas wiping, etc., the strip is passed through an alloying furnace until the metallic luster on the plated surface disappears, that is, until the surface is alloyed. The alloying temperature was controlled by heat treatment until the alloying was completed, and immediately cooled.
223174), the composition of this plating layer is Fe
:8 to 13%, Afl: 0.25 to 0.35%, and the remainder Zn.

However, when a hot-dip galvanized steel sheet with a basis weight of 45 g/rrr or more is alloyed in such a process, the thickness of the r phase generated at the interface between the base metal is, for example, about 1 to 3 μm, and the powdering resistance is not sufficient.

Therefore, we reduced the effective standing time in the bath to about 0.10% or less,
By thinning the Fe-1 alloy layer formed in the bath, Fe-Zn
By making the formation of an alloy phase relatively easy, an alloyed hot-dip galvanized steel sheet can be produced by heat treatment at a lower temperature. The composition of this plating layer is Fe: 6-11%
, AM: 0.05 to 0.25%, the balance being Zn. However, when the basis weight m is 45 g/rrr or more, although there is a condition that the thickness of the r phase is 1 μm or less, the η phase and ζ phase remain on the surface of the plating layer, and the flaking resistance is not sufficient.

In order to solve these drawbacks, for example, the formation of the r phase generated in the lower layer of the plating layer (boundary with the base steel) is suppressed as much as possible, and the upper layer is made of a plating layer consisting of the η phase, δ1 phase, and ζ phase, and It is disclosed that iron plating is applied on the molten alloy layer.
However, at present, satisfactory results have not yet been obtained.

(Problems to be Solved by the Invention) In recent years, due to the demand for improved corrosion resistance, there has been a strong demand for thicker alloyed hot-dip galvanized steel sheets.

However, since alloyed hot-dip galvanized steel sheets are manufactured by thermal diffusion treatment, as the basis weight increases, the iron concentration gradient in the plating layer increases, and a brittle r-phase with a high Fe content is formed at the interface with the base iron. In the vicinity of the surface of the square plating layer, a ζ phase with a low Fe concentration or, in extreme cases, an unalloyed η phase may remain.

If the r-phase is thick, powdering, which causes the plating layer to peel off during press processing, is likely to occur, resulting in scratches caused by plating peeling powder on the product, resulting in adverse effects such as lower yields and lower efficiency due to frequent mold cleaning. Get out.

On the other hand, if the ζ phase is thick or the η phase remains on the surface of the plating layer, the sliding resistance of these phases is large, so mold galling is likely to occur during press processing, resulting in so-called flaking and damage to the mold bead. This also causes a decrease in yield and efficiency in the pressing process.

Such thick coating amount (adhesion amount of 45 g/rrr or more)
The alloyed hot-dip galvanized steel sheet is required to satisfy both powdering resistance and flaking resistance in the pressing process, and it is necessary to control the r and ζ layers. However, with the conventional technology, it is difficult to control the alloy layer as described above.

In addition, various types of pre-plating using electroplating of Ni, Cu, Co, etc. have been proposed for the purpose of controlling alloying, but this has not been a sufficient solution.

In addition, even in the production of alloyed hot-dip galvanized steel sheets with normal basis weight, elements in steel such as P and Si that are present in high-tensile steel materials inhibit alloying, and currently the line speed is reduced at the expense of productivity. Measures such as these have been taken, and easier control of the alloy layer in this high tensile strength material is desired.

(Means for Solving the Problem) As a result of intensive studies, the present inventors have discovered a method for manufacturing an alloyed hot-dip galvanized steel sheet that facilitates alloy layer control and has excellent press workability. That is, in manufacturing an alloyed hot-dip galvanized steel sheet, the present invention applies electroless Ni, Co, or Pd plating to the steel sheet in advance in a coating amount of 10 to 1000 m.
This is a method for producing an alloyed hot-dip galvanized steel sheet, which is characterized by performing annealing, plating, and alloying after applying g/rd.

(Function) Alloying begins when the Fe-Zn-AQ ternary alloy layer produced in the bath is destroyed, and the resulting alloy layer is greatly influenced by the temperature at that time. Further, the alloying reaction usually starts at the grain boundaries of the original plate, and the ternary alloy layer is formed at the grain boundaries. Therefore.

The key to controlling the alloy layer is controlling the reaction that forms the ternary alloy layer at the grain boundaries on the surface of the original plate during the process.

In the case of high-tensile steel materials, during the reduction process during annealing, although the atmosphere is reducing for Fa, it is an oxidizing atmosphere for P and Si, so segregation mainly occurs at the surface grain boundaries of the original plate.
In addition, P stabilizes the ternary alloy at the grain boundaries on the surface of the original plate, which is the starting point of alloying in the bath, and therefore delays alloying. S
i has a similar effect, but in the case of Si, it becomes a stable oxide, which causes plating leakage and inhibition of alloying. The key point is the formation reaction of the ternary alloy layer in Rakuchu.

Currently, the effects of various types of pre-plating are being investigated for the purpose of controlling reactivity with AQ in baths. The characteristics required for pre-plating are reaction control of AQ in the bath at grain boundaries, Fe in the ZnkJM plate, and Si and P elements in the steel. However, since the conventional pre-plating method was electroplating, the plating metal covered the entire surface of the original plate, affecting the reaction within the grains that was originally supposed to be promoted, and the effect of pre-plating was not sufficient. Therefore, alternatives to electroplating are needed. As a method for this purpose, electroless plating, which preferentially deposits on grain boundaries where one reaction is active, is effective because it can also suppress the area weight to a low level. As the chemical plating species, it is effective to use an element that has high reactivity with AQ, suppresses the formation of a ternary alloy layer, and promotes the formation of a single alloy.

Plating species: Ni, Co, and Pd are preferentially deposited on the reactively active surface grain boundaries of the original plate by electroless plating.

Furthermore, these elements have high reactivity with AQ and preferentially form compounds with AQ such as 1QNi, ^Q, and Ni. Therefore, the formation of a ternary alloy layer of Fe-Zn-^a in the bath is suppressed. Therefore, by controlling the amount of electroless plating, the formation reaction of the ternary alloy layer can be controlled for the reasons mentioned above.

Deposition amount: When the amount of electroless plating is less than 10 mg/d, the reaction with 1 is insufficient and there is no effect of suppressing the ternary alloy layer. Also, 1000+mg/rr? Above,
Precipitation reactions occur not only at the grain boundaries of the surface crystals of the original plate but also within the grains. In this case, since the above-mentioned elements have the effect of suppressing alloying reactions, the effect of grain boundary reaction control is inhibited. Therefore, the adhesion amount range is 10 to 100 kg/n.

Plating method: The electroless plating method is not particularly specified, but a method using a reducing agent is preferred because of its high reactivity.For example, a method using sodium hypophosphite or sodium borohydride as a reducing agent is effective.

(Example) Next, examples of the present invention will be listed together with comparative examples.

Materials for plating include CG-i-K steel, P type and high Si.
Using high tensile strength material (0.8t x 800% + It was heat-alloyed.

The effective AQ in the plating bath is 0.10%, and the F in the plating layer is
The e concentration was manufactured by appropriately selecting the heating conditions of the alloying furnace.

Plating was carried out under the conditions that the plate passing speed was 10 to 20 m/min and the immersion time was 2 to 5 seconds.

The layer structure of the plating layer was investigated using the electrolytic stripping method under the following conditions.

NH, Cff1: 150g/Q, electrolytic current: 2■
A/d, the potential shown by each layer was converted into thickness for evaluation.

In addition, workability was evaluated in an actual press test.

Actual press test A part of the fender of a regular passenger car was formed using an actual press. 300 sheets were repeatedly molded, and the degree of adhesion and accumulation of the plated metal on the steel plate or press mold was evaluated relative to each other. The evaluation was based on the degree of blackening of the metal powder transferred to the tape after attaching tape to each area and removing it.

(Long) O-0-Δ-× (Poor) (0,0 means no practical problem) The above evaluation results are shown in the first table along with comparative examples.
Shown in the table.

(Effects of the Invention) As explained above, the method for producing an alloyed hot-dip galvanized steel sheet of the present invention controls the layer structure of the plating layer, improves the alloying properties of high tensile strength materials, and improves r and ζ, which have an adverse effect on workability. It is possible to provide an alloyed hot-dip galvanized steel sheet that can control the phase and has satisfactory powdering properties and flaking properties.

290-

Claims (1)

[Claims] In manufacturing alloyed hot-dip galvanized steel sheets, the steel sheets are electrolessly plated with Ni, Co, and Pd in an amount of 10%.
A method for producing an alloyed hot-dip galvanized steel sheet, characterized in that after applying ~1000 mg/m^2, annealing, plating, and alloying are performed.