JPS582245B2 - Method for producing hot rolled steel strip with thin black scale - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a hot-rolled steel strip having a thin black scale, and in particular to a method for producing a hot rolled steel strip having thin black scale, and in particular, a method for producing a hot rolled steel strip with improved pickling properties and other properties of the black scale by controlling hot rolling conditions. This paper proposes a method for manufacturing rolled steel strip.

An oxide film generally called black scale is formed on the surface of the steel strip during hot rolling.

When cold-rolling such steel strips, when subjecting them to surface treatments such as plating, or when using them as hot-rolled products for forming processes such as press working, black scale must be removed prior to these processes. It is common to remove scale.

Generally speaking, methods for removing scale include pickling and mechanical treatment such as shot blasting, but as the latter method is not suitable as a complete descaling method, pickling is the main method. It is used in

However, because descaling by pickling takes time, pickling lines are less efficient and become a bottleneck in the process compared to other lines in today's faster manufacturing processes for steel strip products. The reality is that

Therefore, the pickling property of black scale is an important factor that affects the productivity or quality of this type of steel sheet, and its improvement is strongly desired.

By the way, the black scale in hot-rolled steel strips is mostly magnetite, and it is known that this magnetite is produced by a eutectoid transformation reaction in which wustite replaces magnetite and iron during the cooling process after hot rolling. ing.

Magnetite is poorly soluble in dilute hydrochloric acid, dilute sulfuric acid, etc. used in normal pickling lines, and the magnetite layer containing α-Fe grains produced by the above-mentioned eutectoid transformation is dense. It has been thought that for some reason it is difficult to dissolve and peel off with acids.

For this reason, conventional methods for improving the pickling properties of black scale have been broadly divided into the following two methods.

The first method is to apply external mechanical force to generate cracks in the dense black scale layer, thereby improving the permeability of the pickling solution.

A typical example of this is a method in which skin pass rolling or leveler processing is performed before pickling.

However, the biggest drawback of this method is that it increases the number of steps, so it does not lead to an improvement in productivity throughout the entire manufacturing process, and the cold working causes deterioration in material quality, especially elongation.

Further, the effect of improving pickling properties by this method is about 1.3 times or less than in the usual case, and no dramatic improvement can be expected.

The second method is to suppress the progression of the eutectoid transformation reaction of the scale that occurs during the cooling process after hot rolling described above, and to leave wustite, which is highly soluble in acids, to remain.

As a means for achieving this objective, for example, a method is known in which a chemical is attached or applied to the steel surface during the hot rolling process.

However, this method requires a lot of expense, such as not only preparing the chemicals used, but also equipment for applying them, and depending on the type of chemicals, measures to prevent harmful effects from mixing with the environmental water system. It requires effort.

Also, it is important to note that the operating conditions of the pickling line are actually determined by the part of the steel plate that has the poorest pickling properties, so if there is uneven chemical adhesion, etc. Shortening the pickling time is something that cannot be achieved, and when trying to apply a chemical to a high-temperature steel surface that moves at high speed, it is extremely difficult to apply the chemical evenly. No practically effective means to reliably solve this problem has yet been found.

The present invention, as a means to solve the various problems in the conventional method, can easily and reliably blacken the image without going through any additional steps that were added thereto, and therefore without requiring any special incidental equipment. The present invention provides a method that can dramatically improve the pickling properties of skin scale.

The main objective of this invention is to significantly reduce the amount of black scale by controlling hot rolling conditions.

Of course, it is known that the pickling properties of scale and the amount of scale have a large effect, but the inventors' research has shown that the time required for pickling depends on the rate of decrease in the amount of black scale, as shown in Figure 1. It was confirmed that the length was shortened almost proportionally.

As a result of a detailed study of the relationship between hot rolling conditions and the amount of black scale, the inventors found that the most important factors that affect the amount of black scale are the finish rolling end temperature and the cooling conditions immediately after the end of rolling. Ta.

In other words, Fig. 2 shows the relationship between finish rolling finish temperature and the amount of black scale based on the inventors' investigation, and Fig. 3 shows the relationship between the cooling rate immediately after the end of rolling and the black scale when the finish rolling finish temperature is 780°C. This shows the relationship between scale amounts.

It can be seen from FIG. 2 that the amount of black scale decreases as the finish rolling end temperature decreases, and as FIG. 3 shows that the cooling rate after finish rolling decreases.

The effects of finish rolling finish temperature and subsequent cooling rate on the amount of black scale scale have a synergistic effect with each other, and it is recommended that finish rolling finish temperature be less than 800°C and cooling rate immediately after finish rolling. When it is 306C/S or more, the effect becomes extremely large, and it becomes possible to significantly reduce the amount of melasma scale.

Note that the above effects are almost unrelated to the temperature at the entrance of the finishing mill.

This seems to be because the surface scale generated in the finishing mill row is repeatedly crushed and peeled off in each pass, so the final amount of black scale has little influence on the surface temperature before the final pass.

According to the present invention, as can be seen from the examples described later, the amount of black scale is much smaller than that obtained by the normal rolling method, so that the pickling properties are extremely good.

This invention is based on the above-mentioned knowledge, and has the finishing rolling finishing temperature, the cooling conditions immediately after finishing finishing rolling, and the winding temperature as constituent elements, and the reasons for these limitations are as follows.

Finish rolling end temperature; The temperature should be less than 800°C and more than 650°C.

The reason for this is that if the temperature is 800°C or higher, the rate of black scale formation is high, making it impossible to achieve the purpose of this invention, and if the temperature is lower than 650°C, the effect will be saturated and the ductility of the material will deteriorate. This is because the disadvantages due to the adverse effects of

Cooling conditions after finish rolling; If the temperature of the rolled material after finish rolling exceeds 650°C, start cooling immediately, and cool so that the average cooling rate up to 650°C is 30°C/S or more. do.

The reason for this is that when the cooling rate is less than 30°C/S, there is little synergistic effect with the finish rolling end temperature, and the amount of black scale can be reduced.
This is because the growth rate of melasma scale is small in the temperature range below 0° C., and the influence of the cooling rate is not a substantial problem.

Winding temperature; The temperature shall be less than 650°C.

The reason for this is that the growth rate of black scale is high in a temperature range of 650° C. or higher, and growth of black scale occurs after winding.

Next, examples of the present invention will be described.

Table 1 shows examples in which the present invention was applied to the production of high carbon steel (S45C) hot-rolled steel strips. It is clear that the performance is significantly improved.

In this way, the marked reduction in the amount of black scale leads to a marked improvement in pickling properties, but it is also particularly advantageous that an increase in the yield of steel is obtained as a result.

In other words, the weight percentage of black scale in a normal hot rolled steel sheet is, for example, about 0.60% in the comparison material shown in Table 1.
However, in the example with a black scale deposit of 20 g/m2, it is 0.16%, so the weight reduction ratio before and after pickling in both cases is 0.44% (approximately 0.34% for pure Fe content).
%), and in mass production factories, the benefits that can be obtained continuously are extremely large.

Under the rolling conditions of the present invention, almost the same reduction ratio in the amount of black scale was achieved not only in the above-mentioned high carbon steel but also in the hot rolling operation of low steel content.

As demonstrated in the examples above, the black skin scale obtained by this invention is extremely thin and can be easily removed by pickling. It is.

This is based on the essential property that the thinner the film is, the harder it is to peel off, as well as the fact that the black scale is structurally dense and consists of a magnetite layer that has excellent adhesion to the base steel.

Therefore, even when the hot-rolled steel sheet according to the present invention is subjected to forming processing without descaling such as pickling and with black scale attached, almost no peeling of black scale occurs during processing. In recent years, there has been an increase in the use of processing black scale as it is in order to eliminate the equipment and economic burden of descaling, but the characteristics of black scale obtained by this invention are suitable for such uses. It is also suitable.

Based on the above, the effects of this invention can be summarized as follows.

1. Pickling properties can be easily and reliably improved without going through a pretreatment process or using special incidental equipment.

2. It is possible to improve the yield of steel plate products.

[Brief explanation of drawings]

Figure 1 is a graph of the relationship between black scale scale thickness and pickling property, Figure 2 is a graph of the relationship between black scale scale thickness and finish rolling end temperature, and Figure 3 is a graph of black scale thickness and cooling rate after finish rolling. It is a relationship graph.

Claims (1)

[Claims] 1 During hot rolling, finish rolling is performed at an exit temperature of 650
If the temperature of the material to be rolled exceeds 650°C after finish rolling, cooling is started immediately and the temperature range from the finishing temperature to 650°C is 30°C/30°C. Cooling at an average cooling rate of S or more,
A method for producing a hot-rolled steel strip having a thin black scale, the method comprising then winding the strip at a temperature of less than 650°C.