JPH0448062A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To increase the alloying speed of a steel sheet as basis material and a hot-dip galvanizing layer and to produce a galvannealed steel sheet with high productivity by subjecting a P-containing steel sheet as a basis material to continuous annealing in a specific atmosphere in a furnace and then to galvannealing. CONSTITUTION:At the time of producing a galvannealed steel sheet containing >=0.025wt.% P as a chemical component of a steel sheet as a basis material, the atmosphere in a furnace is regulated so that it has <=-30 deg.C dew point, >=3% H2 volume concentration, and <=1,000ppm O2 volume concentration in the region of >=300 deg.C steel sheet temp. in a continuous annealing furnace prior to hot dipping. By this continuous annealing, the surface concentration of P accelerated by the presence of oxidation phenomenon can be inhibited. By this method, the alloying reaction velocity of the P-added steel can be improved without increasing equipment costs and treatment costs in particular.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention is based on the chemical composition of a base steel sheet of 0.025% by weight.
The present invention relates to a method for increasing the rate of alloying between a base steel sheet and a hot-dip galvanized layer when producing the above alloyed hot-dip galvanized steel sheet containing P.

(Prior Art) Corrosion resistance has become increasingly important in recent years as a factor that determines the commercial value of various durable consumer products such as automobiles and home appliances. Among these, alloyed hot-dip galvanized steel sheets have excellent corrosion resistance after painting, so there is a remarkable growth in demand in such industrial fields where painting is the premise.

Alloyed hot-dip galvanized steel sheets are generally made of Ag after recrystallization heat treatment in a continuous heat treatment line.

It is manufactured by applying hot-dip galvanizing by immersing it in a hot-dip zinc bath containing trace components such as Fc, and then subjecting the base steel sheet and the galvanized layer to a thermal diffusion treatment.

The amount of heat required for the thermal diffusion treatment is determined by the mutual diffusion rate between the base steel sheet and the galvanized layer (hereinafter referred to as alloying reaction rate), and in particular, P contained in the base steel sheet reduces the alloying reaction rate. It is known as a stimulating element.

In recent years, as the need for lighter vehicle bodies has increased with zero focus on improving fuel efficiency, attempts have been made to increase the strength of steel sheets and reduce their thickness, and since P is the most efficient reinforcing element, active efforts are being made to It tends to be added as a steel component.

As a result, the low alloying reaction rate when producing alloyed hot-dip galvanized steel sheets from P-containing steel sheets has become a problem that reduces productivity.

As a conventional technique for increasing the alloying speed of P-containing steel sheets and hot-dip galvanized layers, an electroplating layer of Fc, Ni, etc. is applied to the base steel sheet before passing it through a continuous hot-dip galvanizing line to enrich P in the surface layer. A method of coating a layer (hereinafter referred to as a pre-plating method) and the like are disclosed.

Since the pre-plating method requires special electroplating equipment, it leads to a significant increase in equipment cost and processing cost, and cannot be called a fundamental improvement technology.

(Problems to be Solved by the Invention) An object of the present invention is to suppress the concentration of P in the surface layer when manufacturing an alloyed hot-dip galvanized steel sheet containing P in the steel sheet, and to The object of the present invention is to provide a manufacturing method that increases the alloying speed of the alloy and improves productivity.

(Means for Solving the Problems) The present invention provides 0.025% by weight of the chemical composition of the base steel sheet.
When manufacturing an alloyed hot-dip galvanized steel sheet containing the above P, the atmosphere in the furnace must be maintained at a dew point ≦-30°C in a range where the steel plate temperature is ≧300°C in a continuous annealing furnace prior to hot-dipping.
°C, H2 volume concentration ≧3%, O2 volume concentration ≦11000p
This is a method for manufacturing an alloyed hot-dip galvanized steel sheet, characterized in that p.

The present inventors first studied in detail the basic mechanism of the surface concentration phenomenon of P in a continuous hot-dip galvanizing line, and obtained the following new findings.

Firstly, the reason why the alloying reaction rate of P-added steel is inferior is that the concentrated P on the surface of the base steel plate forms a strong barrier with the Ag in the zinc bath, and the mutual diffusion of the base steel and molten zinc is inhibited. This is to extremely suppress the Second, P is well known as an element that segregates at interfaces, but the degree of concentration differs between grain boundaries and the surface, with significant concentration occurring on the surface. Third, concentration on the surface is simply due to the concentration on the surface. Besides being a freer interface, it is also significantly promoted by the intervention of oxidation phenomena.

Furthermore, the surface concentration of P promoted by the intervention of such an oxidation phenomenon can be prevented by controlling the atmosphere in the continuous annealing furnace prior to hot-dip galvanizing. This can be suppressed by setting the atmosphere to have a dew point≦-30°C, a H2 volume concentration≧3%, and an O2 volume concentration≦11000pp.

The present invention has been completed based on the above new findings.

The P content is 0.025ffi as a chemical component of the base steel sheet.
The reason why the amount is set at % or more is because the reduction in the alloying reaction rate becomes practically noticeable in the P content range.

The area where the atmosphere is controlled in the continuous annealing furnace has a steel plate temperature of 30
The reason for setting the temperature in the range of 0°C or higher is that oxidation of the steel sheet and surface concentration of P become noticeable in the temperature range of 300°C or higher. It is possible to perform similar atmosphere control even at lower temperatures.

The atmosphere in the furnace is dew point ≦-30℃, H2 body vtfIi degree ≧3
%, 02 volume concentration≦11,000 pp is because if the above condition ranges are simultaneously satisfied, the surface concentration of P can be reliably suppressed. The dew point is an index corresponding to the amount of H2O in the atmosphere, and the H2 volume concentration affects the degree of oxidation/reduction on the surface of the steel sheet depending on the relative relationship with the temperature and dew point.

The 0° volume concentration is also an index for determining the degree of surface oxidation, and according to the study results of the present inventors, within the above atmospheric conditions, P
surface concentration is reduced to a virtually non-problematic level, but
Desirably, in a region where the steel plate temperature is 300°C or higher, the atmosphere in the furnace is such that the dew point is ≦-40°C, the H volume concentration is ≧4%, and the 02 volume concentration is ≦200 pp-.

Other trace gas components in the furnace may be those commonly used.

Regarding the manufacturing method of the base steel sheet, in order to improve its mechanical properties, after hot rolling it is rolled at a high temperature of 700℃ or higher, or it is rolled into a box shape before being passed through a continuous hot-dip galvanizing line. Annealing may be applied. In such a case, surface concentration of P has already occurred in these steps,
The present invention is also effective for such manufacturing processes.

In addition, in order to mechanically or chemically remove P concentrated on the surface of the base steel sheet, surface grinding, pickling, etc. Chemical treatment may be carried out. The present invention is also effective when used in combination with such treatment, and in fact, even more significant effects can be obtained.

It is known that the alloying reaction rate is influenced by the An) concentration in the alloy, and by limiting the Afi concentration in the alloy to a low range, the alloying rate of P-added steel can be improved. The present invention is also effective when limiting trace components such as Ap in the bath. Further, the plate temperature entering the bath, the bath temperature, etc. can also be applied to all ranges.

From the viewpoint of satisfying the atmospheric conditions, the continuous annealing furnace is preferably an indirect heating method using a radiant tube, a direct heating method using a reducing flame, or a combination of both.

The present invention described above makes it possible to improve the alloying reaction rate of P-added steel without increasing special equipment costs or processing costs.

(Example) Steel sheets having P contents shown in Table 1 were passed through a hot-dip galvanizing line to produce alloyed hot-dip galvanized steel sheets.

Chemical components in steel other than P are C: 0.0015 to 0.0
48, SI: 0. Old~0.65, Mn:0.10~!
,00,S:O,OOl~0.050,l:0
．． 001~0.080, N: 0.0005~o,
ooeo, Nb: O ~ 0.035, TI: 0 ~ 0.09
0, B: 0 to 0.050 (wt%), and other unavoidable impurities, with P showing the most dominant influence on the alloying rate in the following examples.

The base steel having the chemical composition was heated to a thickness of 4.0 at a heating temperature of 1250°C, a finishing temperature of 920°C, and a winding temperature of 730°C.
After hot rolling to a thickness of 0.2 mm, descaling treatment was performed by pickling with hydrochloric acid, and cold rolling was performed to a thickness of 0.2 mm. It was used as a material for hot-dip galvanizing to 1 mm. In the hot dip galvanizing line,
Annealing was performed under the atmospheric conditions in the continuous annealing furnace shown in Table 1.

Continuous furnace types include: NO and I2 are indirect heating furnaces using a radiant tube method, No. 7.14 is a combination of direct heating using a reducing flame and indirect heating using a radiant tube method, and the others are non-oxidizing furnaces (NOF) and indirect heating. A furnace is used in combination (in the case of a combination type, indirect heating is used in the high temperature range). The maximum plate temperature is 750 to 850°C. The temperature of the plate entering the bath is 460
~520°C, bath temperature: 450~470°C, and bath components are shown in Table 1.

Alloying treatment was carried out by controlling the galvanizing distance to 60 to 62 g/d and applying a constant amount of heat. The interstitial plate temperature and bath temperature within the range of this example did not give any significant difference in the alloying rate.

The alloying speed was determined by the maximum line speed (reciprocal of the time required for alloying) at which alloying to the surface was completed.

From the results shown in Table 1, it is clear that according to the present invention, the alloying reaction rate of P-added steel with the hot-dip galvanized layer is significantly improved.

(Effects of the Invention) The present invention enables highly productive production of alloyed hot-dip galvanized steel sheets using P-added steel as a raw material, and has extremely great significance.

Claims (1)

[Claims] 0.025% by weight or more of P as a chemical component of the base steel sheet
When manufacturing an alloyed hot-dip galvanized steel sheet containing
In the range of °C, the atmosphere inside the furnace has a dew point ≦ -30 °C, H_
2 volume concentration≧3% and O_2 volume concentration≦1000 ppm.