JPS5864367A - Production of zinc hot dipped steel plate - Google Patents

Abstract

PURPOSE:To produce a titled steel plate which is formed with less blisteres without decreasing productivity by pickling a rimmed steel plate which is subjected to high temp. coiling after hot rolling and reducing the same by heating in an atmosphere contg. gaseous H2 then dipping the steel plate in molten zinc. CONSTITUTION:In the case of rimmed steel, a steel plate is coiled at >=650 deg.C after hot rolling to increase the solid solubilized contents of carbon and nitrogen in the steel. After the rimmed steel is descaled by pickling, the steel is reduced by heating in an atmosphere contg. gaseous H2, whereby the quantity of hydrogen to be occluded therein is decreased. Thereafter such steel plate is dipped in a molten zinc bath whereby the zinc hot dipped steel plate is produced. According to this method, the formation of blisters is decreased without decreasing productivity.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-rolled hot-dip galvanized steel sheet that prevents blisters from occurring in the plating layer after plating.

Conventionally, hot-rolled steel sheets have been used as base sheets for thick hot-dip galvanized steel sheets. Hot-dip galvanized steel sheets using this hot-rolled steel sheet as the base sheet are subjected to pickling as necessary to remove scale, and then processed using an atmospheric gas containing hydrogen gas (in most cases, H2
It is generally carried out by heating reduction with 10 to 75% Nwg and 6 to 25% Nwg, and then immersing it in molten zinc.

However, when a hot-rolled steel sheet is used as the original sheet, hydrogen is absorbed during the heating reduction process and released after plating, which pushes up the η-galvanized layer and causes numerous blisters on the surface of the plated steel sheet. There is a problem in that it causes This problem occurs to varying degrees in plating methods such as the Cirrus method, Sendzimer method, and US Steel method, in which the surface of a steel sheet is heated and reduced in an atmospheric gas containing hydrogen. A solution is proposed.

For example, one method that has been proposed in the past as a solution to this problem is to improve the composition of the zinc bath. In this method, S, Se, P,
A method of adding one or more of As, Te, Sb, and Bi (Japanese Unexamined Patent Publication No. 14325/1983)
, a method in which a steel plate is reduced at a temperature in the range of 750 to Acs point, and then plated in a zinc bath containing 22 to 0.35% AIo (by weight) (@Kaisei No. 54-26928). However, in the former method, in order to uniformly disperse the blister-preventing agent, it must be added in the form of a zinc alloy, which increases the cost of manufacturing the alloy. There are drawbacks. Furthermore, even if the latter method is used, the concentration of AI in the zinc bath increases, resulting in problems such as deterioration of workability and quality, and increase in manufacturing costs.

Further, as another solution, there is a method of lowering the heating reduction temperature. This method conventionally involves thermal reduction of 800 to 850
This is an attempt to implement the method that was used for 600 to 720 t:'' instead of 600 to 720 t:''.
However, if the heating reduction temperature is lowered in this way, the plating rate will be lowered.

Another solution is to dehydrogenate the hydrogen occluded in the thermal reduction process in a subsequent process, and then hot-dip galvanize. This method uses 1-(28-20%,
A method in which a steel plate at a temperature of 450 to 550C is introduced into an atmosphere of 80% N292 to dehydrogenate it (Japanese Patent Laid-Open No. 1983-1999)
However, providing such a dehydration process in an actual plating line requires a large amount of cost and is extremely difficult in terms of equipment and manufacturing technology. Furthermore, the reality is that conventional plating lines do not have space for such a process.

Invention X is a method for producing a hot-dip galvanized steel sheet that prevents blistering by improving the hot-rolled steel sheet itself, without changing the zinc bath composition or heating reduction temperature or adding a dehydrogenation process as described above. This is what we are trying to provide.

In hot-dip galvanizing using a heating reduction method using hydrogen gas, in order to prevent blisters from forming on hot-rolled galvanized steel sheets, it is ideal to reduce the amount of hydrogen absorbed by the steel sheet during heating reduction. be. For this purpose, the steel plate temperature, atmospheric pressure, and hydrogen concentration during thermal reduction should be made as low as possible. However, keeping these factors low reduces productivity and, in extreme cases, leads to unplated defects, ultimately increasing product costs.

Therefore, the inventors of the present invention solved the problem without causing a decrease in productivity.
One way to reduce the occurrence of blisters is to suppress the fine precipitation of elements such as carbon and nitrogen in steel that precipitate after coiling during hot rolling. The research focused on a method of increasing the amount of dissolved hydrogen and reducing the amount of hydrogen absorbed. And as a concrete means,
In the case of rimmed steel, it has been found that it is sufficient to use high-temperature coiling for hot rolling.

For example, in the case of carbon, if the finishing temperature during hot rolling is the same, carbide precipitation that occurs during cooling after coiling will be coarser and larger. In the first section, β) shows the carbide precipitation state in both windings of the rimmed steel, (8) is the normal winding (5ssc), and (B) is the high temperature winding (730C).
In the case of high-temperature winding, the carbide precipitation is coarse and large.

As a result of various investigations into the relationship between the amount of solid solution and the amount of precipitation of carbon and nitrogen in steel in both these conditions, the inventors found that the amount of solid solution is greater in high temperature coiling, and that It was found that the amount of winding was smaller.

The reason why the amount of solid solution of carbon and nitrogen differs depending on the coiling temperature is considered to be due to the density of precipitation nuclei generated during cooling. In other words, precipitated elements such as carbon and nitrogen are gradually cooled in a supersaturated state during cooling, but in cases where there is a considerable amount of precipitation time, such as in hot-rolled coils, the precipitation rate is determined by the amount of precipitation nuclei. It becomes faster when the cores are densely distributed and the distance between the cores is short. Therefore, in the case of normal winding where the density of precipitation nuclei is high, precipitation occurs easily, the amount of solid solution around the precipitates decreases, and the average amount of solid solution for the steel as a whole becomes low. On the other hand, in the case of high-temperature winding, since the precipitation nuclei are rough, it is thought that the elements in the portions away from the precipitation nuclei cannot be easily precipitated and remain in solid solution.

According to the research conducted by the present inventors, in the case of rimmed steel, setting the bright temperature winding temperature to 650 t:' or higher is effective in preventing blisters, but in the case of killed steel, it is extremely effective. It has been proven to be ineffective. Therefore, the present invention is directed to a method of manufacturing a hot-dip galvanized steel sheet using a rimmed hot-rolled steel sheet as an original sheet.

The present invention will be explained below with reference to Examples.

The rimmed steel shown in Table 1 was hot rolled at a constant finishing temperature of 900C, then rolled at 585C (normal winding) and 65C.
150X from the center of a hot rolled steel strip (thickness 2.3cm, width 920cm) wound at 0C and 730C (high temperature winding)°
A sample of 250III+ was taken, and it was pickled with a mixed acid of sulfuric acid and hydrochloric acid to remove scale, and further reduced to 8275%.
N225%, temperature 650C atmosphere (dew 6 points - 400)
The surface of the steel plate was reduced and activated by holding 15 members inside.

When Table 1 is reached, Aso, xsm zinc bath (450c)
Zinc plating was performed by immersing the steel sheet in water for 2 seconds, and the amount of absorbed hydrogen in the steel sheet and the number of blisters generated on the plating surface were investigated.

Table 2 shows the results, and shows that those with a small number of blisters have a small amount of hydrogen storage, and the winding temperature is 650.
It can be seen that when the temperature is C or higher, the number of blisters generated is small.

Table 2 (Note 1) The number of blisters is determined by leaving the plated steel plate at room temperature for one week, and then observing the number per 1 cd of the (0001) crystal plane of the η-Zn plating layer using a 30x magnifying glass or counting. Ta.

(Note 2) Amount of hydrogen (amount is 500 c when a certain amount of sample is charged into a vacuum heating gas mass spectrometer.

(2-5) Based on the integral value of the H2 peak detected under X10 Torr.

In addition, Fig. 2 shows winding temperatures of 585c, 650r and 73
This study investigated the occurrence of blisters in the width direction of the board at 0C, and as the winding temperature is increased, the number of blisters generated across the board width decreases. Furthermore, Figure 3 shows the occurrence of blisters when a hot-rolled steel strip having the chemical composition shown in Table 3 was hot-rolled at a constant finishing temperature of 910c and then coiled at 585c and 730r and plated in the same manner as above. The numbers show that when aluminum killed steel is used as the original plate, unlike the case of rimmed steel (so 1.AI; tr,), even if the coiling temperature is raised, ? ° The effect of reducing the occurrence of listers is not so recognized - As shown in Table 3 (according to the present invention, the number of blisters that occur after plating can be greatly reduced only when a rimmed hot rolled steel sheet is used as the base plate) Furthermore, in the case of the present invention,
By changing the winding temperature during hot rolling, the number of blisters generated is reduced, so no special equipment is required and productivity can be maintained at the same level as before.

[Brief explanation of the drawing]

Figure 1 is a micrograph showing the state of carbide precipitation in a rimmed steel sheet due to differences in coiling temperature after hot rolling. (A) shows the case where the winding temperature is 585iCS (B) is 730C. This figure shows the influence of the hot rolling winding temperature on the relationship between 5olAI and the number of splinters and tars generated in a hot-dip galvanized steel sheet made of MIKILD steel. Fig. 2 Off width (920 tangle O-on Fig. 3 5etf1.Nl (X/-%)

Claims (1)

[Claims] In a method for producing a hot-dip galvanized steel sheet in which a hot-rolled steel sheet is pickled, then heated and reduced in an atmosphere containing hydrogen gas, and then immersed in hot-dip zinc for plating, the hot-rolled steel sheet includes:
A rimmed steel plate that is hot-rolled and then rolled up at a temperature of 650C or higher is used. A method for producing a hot-dip galvanized steel sheet.