JP2895357B2 - Method of controlling phase structure of galvannealed layer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a galvannealed steel sheet.

[0002]

2. Description of the Related Art Generally, in the production process of an alloyed hot-dip galvanized steel sheet, the degree of alloying, that is, the total iron concentration in the plating layer, is used as an index of the alloying treatment. For example, as described in JP-A-1-177351, even if a part of the alloy phase is measured, it is controlled by converting it into an alloying degree (total iron concentration in the plating layer). However, in the conventional manufacturing method, the control target is only the degree of alloying.

[0003]

However, in the conventional method, the conditions of the alloying treatment are set and changed so that the total iron concentration in the plating layer becomes a predetermined value. . In other words, in the alloyed plating layer, the phase (Γ)
Various phases such as ₁ phase), δ ₁ phase and ζ phase are mixed. In some cases, if the heat treatment for alloying is insufficient, an η phase, which is a pure zinc phase, may remain. How these phases are formed (formation ratio) differs depending on the conditions of the alloying treatment. Since these phases have different properties, the quality of the galvannealed steel sheet represented by powdering properties varies depending on the phase structure of the plating phase (the amount of each phase formed). Therefore, even if the total iron concentration in the plating phase does not change, the quality of the plate changes if the phase structure is different. That is, it is impossible to produce a stable quality alloyed hot-dip galvanized steel sheet by the conventional alloying process using the degree of alloying as an index.

[0004] In view of the above circumstances, the present invention provides:
An object of the present invention is to provide a method for controlling a phase structure of a galvannealed layer in which the amounts of alloy phases such as Γ, δ, and ζ can be made to match predetermined amounts.

[0005]

According to the present invention, in order to solve the above-mentioned problems, initial alloying conditions are determined by a model for predicting the phase structure of a plating layer, and alloying is performed by performing alloying. At the time of completion, the phase structure (the amount of each phase formed) is measured, and the ratio of the amount of δ phase and Γ phase: δ / Γ and
The alloying treatment conditions are changed so that both the ratio of the amounts of the vitreous phase and the δ phase: ζ / δ have a predetermined value.

[0006] Thus, even with the same degree of alloying (iron concentration), the heat treatment conditions (sheet passing speed, heating speed, soaking time, etc.)
When the components of the base plate and the base plate are different, the adverse effect of different powdering properties can be avoided.

[0007]

According to the present invention, the amount of each alloyed phase formed is measured online, fed back, and the alloying conditions are set and changed, so that the phase structure of the plating layer has a predetermined ratio and is always the same. We can produce quality galvannealed steel sheets. As the alloying treatment conditions, the basis weight, alloying heat treatment conditions (heating rate, soaking temperature / time, cooling rate), plating bath composition, temperature and composition of the steel sheet, and line speed act in combination.

Generally, in order to improve powdering properties, a phase structure mainly composed of a δ phase is desirable. An example of alloying treatment conditions suitable for the above will be described. 1) When the basis weight, each component of the plating, the temperature, and the composition of the steel sheet are constant, the heating temperature is lowered and the soaking time is prolonged, thereby acting to increase the δ phase. 2) When the Al concentration in the bath decreases under the conditions of the item 1), it works to increase the 増 大 phase and decrease the δ phase at that rate.

Therefore, the ratio of each phase after the formation of the alloy phase is measured online, and the above-mentioned conditions are manipulated to obtain the target alloy phase ratio.

[0010]

FIG. 1 shows an example of the equipment configuration when the present invention is applied. After the strip 1 is galvanized in a hot-dip zinc pot 2, the strip 1 is heat-treated in an alloying furnace after being controlled to a predetermined amount by a gas wiping device 3. The alloying furnace includes a heating zone 4, a holding zone 5, and a cooling zone 6. The structure of the alloy phase after performing this alloying treatment is measured by the alloy phase measuring device 7. This measuring apparatus utilizes diffracted X-rays.

Next, an embodiment of the control method according to the present invention will be described. First, before the strip to be alloyed enters the alloying furnace, alloying conditions are determined so as to have a predetermined alloy phase structure. Here, the heating rate of the heating zone, the furnace temperature in the solitary zone, and the cooling rate of the cooling zone are the values to be determined. In this determination, the calculation is performed by a function using variables such as a line speed (LS), a bath temperature of the pot, a bath component, a plate temperature entering the pot, a target value of zinc deposition amount, and a component of the mother plate. Here, the line speed is a variable as a determined value, but this can be used as an operation amount. The evaluation of the target phase structure is based on the ratio of the δ phase and Γ phase .
And the ratio of the amounts of the ζ and δ phases is used.

Even if the alloying process is determined by the above method, the difference between the actual value when calculating the setting of each variable and when the strip passes through the alloying furnace, and the function and actual process for obtaining the initial conditions. , The phase structure may not reach a predetermined value. Therefore, the amount of each phase is measured by an alloy phase measuring device arranged after the alloying furnace, and the evaluation value of the phase structure is calculated. The alloying treatment conditions are changed so that this value becomes a predetermined value.

FIGS. 2 and 3 show the difference in powdering properties between the conventional method and the present method. When the degree of alloying is controlled as in the prior art, even if the degree of alloying is the same as shown in FIG. Condition 1 was when the Al concentration in the bath was low, the heating time of the alloying treatment was short, and the heating temperature was high. In this case, the condition 3 is a case where the heating temperature is further lowered by the alloying process and the processing time is lengthened.

FIG. 3 shows an example of the result of the present invention. By controlling the ratio of the δ phase and the 一定 phase to be constant, the difference in the powdering property becomes smaller than that in the conventional method, but the difference in the powdering property due to the difference in the conditions still remains. Therefore, if the ratio between the ζ phase and the δ phase is further controlled, the powdering properties have the same value as can be seen in the figure. Here, C ₁ = 0.
05, C ₂ = 0.01 and C ₃ = 0.005.

[0015]

According to the present invention, in performing alloying treatment of an alloyed hot-dip galvanized steel sheet, the structure of the alloy phase is controlled to a predetermined amount instead of controlling the degree of alloying to a predetermined value. This solves the problem that the powdering property differs depending on the difference in the heat treatment conditions for alloying even at the same degree of alloying, which has conventionally occurred, and the same powdering property can always be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of an equipment configuration when the present invention is applied.

FIG. 2 is a graph showing the relationship between the conventional alloying degree and the powdering property when the degree of alloying is controlled.

FIG. 3 is a graph showing the relationship with the powdering property when the phase structure according to the present invention is controlled.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 strip 2 molten zinc pot 3 gas wiping device 4 alloying furnace / heating zone 5 alloying furnace / holding zone 6 alloying furnace / cooling zone 7 alloy phase measuring device

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-44007 (JP, A) JP-A-3-273144 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] In the process of plating molten zinc and then performing a heat treatment to form an alloy phase of iron in the mother plate and zinc in the plating layer, each phase (Γ, δ) of the formed alloy phase is formed. ,
by the model to predict the formation of zeta), each phase to determine the conditions of initial alloying treatment so as to form a predetermined amount, then the amount of the formed phases was measured on-line, [delta] phase and
比 phase amount ratio: δ / Γ and ζ phase and δ phase amount ratio: ζ /
A method for controlling a phase structure of an alloyed hot-dip galvanized layer, characterized by changing alloying treatment conditions so that both δ coincide with predetermined values .