JPH0569883B2 - - Google Patents

Description

[Detailed description of the invention] (Industrial application field) Steel materials, especially materials containing about 0.05% or more of Si, such as Si-killed steel (simply referred to as Si-containing steel in this specification), are subjected to hot rolling processing. The technical content described in this specification regarding the improvement of the heating method during heating is related to the development results for suppressing the growth of scale based on new findings from the investigation of the scale growth behavior during heating, especially in the high temperature range. In particular, it is positioned in the technical field to which plate or strip rolling belongs.

(Prior Art) In general, when heating steel materials such as slabs, blooms, or billets for hot working, the so-called baking temperature at the end of heating is determined by the heating gradient (heatable temperature) that should reach that temperature. Regarding the furnace atmosphere temperature, it means the rate of temperature increase.), but at least the temperature of the steel material during heating is usually calculated from the average temperature in the thickness direction of the steel material from the atmospheric temperature transition in the heating furnace. In many cases, a value is predicted.

However, for some steel types that have special problems such as the scale generation behavior mentioned above, the surface temperature through the heating process must be given more importance than the atmospheric temperature in the heating furnace. Nevertheless, it is extremely difficult to measure the surface temperature continuously and with high precision in the furnace, so there are few cases where heating control is performed directly by surface temperature. The influence of the temperature change in the atmosphere and the causal relationship between them remain unclear, and the surface quality of rolled products, especially those made of Si-containing steel, has often been impaired.

On the other hand, in cases where the baking temperature is strictly regulated, or when hot rolling high-strength steel, etc., the heating operation for steel materials is performed in a temperature range that is biased toward the extraction point after charging into the heating furnace. The so-called initial rapid heating method was adopted, in which the rate of heating is slowed down and the heating rate is slowed once the temperature approaches . However, in this case, the unavoidable disadvantage is that the heating unit consumption increases.

Therefore, initial slow heating and final rapid heating methods aimed at energy-saving operation are often used for general materials, and heating methods with an intermediate pattern between both of these methods are also advantageously used.

However, when applying the above energy-saving method to heating Si-containing steel, that is, controlling the furnace temperature with slow initial heating, rapid heating at the end, or an intermediate pattern, it has been experienced that flaking defects often occur.

Here, flaking defects are caused by scale remaining on the plate surface that could not be removed by the scale breaker prior to hot rolling, pushed into the plate surface during rough rolling, and leading to descaling immediately before the subsequent finish rolling. This peeling defect is a serious cause of product defects, as it is caused by peeling off from the surface and creating dents.

(Problem to be solved by the invention) Such defects are a phenomenon that can be said to be unique to Si-containing steel, and in the case of Si < 0.05%, most of the scale remains on the sheet surface due to descaling during hot rolling. Therefore, the above problem does not occur.

This relationship is clear from the comparative illustrations in FIGS. 1a and 1b.

In general, the surface scale of Si-containing steel is as shown in Figure 2.
S-enriched layer 2 and feuyarite (2FeO SiO ₂ )
A layer 3 and a layer containing pores 5 and cracks 6 as a scale layer 4 are formed by heating.

Feyarite layer 3 at this metal-scale interface
is generated on the metal side and seriously impedes descaling properties.Especially at temperatures above 1170℃ during heating, it becomes a liquid layer and promotes the diffusion of Fe and Si in the steel, and this diffusion rate increases at high temperatures. to improve the degree of
On the other hand, when the Si content is high, the amount of scale formed rises sharply at around 1200℃ (see Figure 3), and on the other hand, the pore size of the pores in the scale layer 4 increases as the temperature increases (Figure 4). (see figure) also acts to inhibit descaling.

Therefore, when heating Si-containing steel materials for hot rolling, we have developed a method that effectively suppresses the growth of the ferrite layer and advantageously prevents the occurrence of flaking defects on the surface of the rolled product. It is an object of this invention to provide a method.

(Means for resolving the problem) The above purpose can be effectively achieved by the procedure that consists of the following items.

In other words, when heating the steel material during hot rolling of Si-containing steel, the rate of increase in the heating furnace atmosphere temperature from around 1200°C to the heating completion temperature is set at a maximum of 4°C per minute at around 1200°C. This is a heating method for Si-containing steel materials during hot rolling, which is characterized by suppressing the formation of feyarite at the metal interface on the surface scale of the steel material by gradually decreasing the heating rate at a maximum of 2°C per minute near 1300°C.

By controlling the temperature increase of the heating furnace atmosphere temperature in this manner, it is possible to effectively control feuyarite during baking of the Si-containing steel material for hot rolling.

From the viewpoint of energy conservation, it is more desirable to interpose a rapid heating operation to quickly increase the temperature increase rate immediately before starting the gradual decrease in the temperature increase rate.

As already mentioned, when heating steel for hot rolling, the baking temperature at the end of heating and the heating gradient during heating are important, but in general, the temperature of the steel material during heating is usually The atmospheric temperature in the heating furnace is monitored, and the average value in the thickness direction predicted based on this is used, and in most cases, the surface temperature is not controlled, and the same applies to the present invention.

This is because, even though surface temperature is considered important, it is extremely difficult to measure surface temperature continuously and accurately within a heating furnace.

(Function) This invention focuses on the scale formation on the surface of the steel material during heating, and controls the atmospheric temperature in the heating furnace as described above to achieve heating without damaging the surface quality of the product. .

Now, as shown in Figure 5, the rate of increase in the heating furnace atmosphere temperature (°C/sec) has traditionally been determined depending on the purpose.
It is known to change the heating temperature, and A shows a heating temperature increase pattern used for heating temperature-strict materials and high tensile strength steel, whereas C shows a heating method aimed at energy-saving operation with general materials. B is intermediate between A and C.

This invention can be applied to either a heating method oriented toward energy-saving operation, such as (c), or an intermediate heating method, such as (b), but even when heating to the baking temperature rapidly or relatively rapidly, the heating temperature
By regulating the temperature increase rate around 1200°C and its transition around 1300°C, it is possible to eliminate damage to surface quality.

Here, 1200°C is a value learned empirically in a real furnace, and roughly corresponds to the melting point of 1170°C of feuyarite.

In other words, 2FeO at the scale-metal interface,
Feyarite, which has a SiO ₂ composition, becomes a liquid phase at temperatures above 1170°C, promotes the diffusion of Fe and Si in the steel, and grows inside the metal.

The growth of such feuyarite depends on the rise in surface temperature of the Si-containing steel material, and this surface temperature rise becomes more significant as the rate of temperature rise increases under heating furnace atmosphere temperature control. Suppressing the leading rise in steel surface temperature by gradually reducing the temperature increase rate to a maximum of 4°C per minute near 1200°C and a maximum of 2°C per minute near 1300°C can suppress the growth of feuyarite. Useful.

If the temperature increase rate exceeds 4℃ per minute near 1200℃,
It is a good thing that the growth of feuyarite occurs quickly, and when the temperature exceeds 2°C per minute near 1300°C, the rate of temperature increase decreases and it is no longer useful for suppressing the growth of feuyalite.

In this way, the smaller the growth of feuyarite, the thinner the scale film thickness, and the fewer pores, the better the descaling properties during hot rolling, and this relationship has already been discussed in comparison with Figure 1. It is clear from the point of view.

In this way, flaking defects in rolled products can be advantageously reduced.

According to this invention, for example, the baking heating temperature is
When Si-containing steel is heated around 1300℃, especially above 1260℃, pheyarite begins to increase.
By setting the heating rate at a maximum of 4°C per minute near 1200°C to a surface temperature higher than that, and at a heating furnace atmosphere temperature of 1300°C, the heating rate is gradual at a maximum of 2°C per minute. Rapid increases in the surface temperature that would lead to the growth of pheayarite in the steel material are suppressed, thereby reducing descaling inhibition during hot rolling and thus preventing flaking defects in the rolled product.

(Example) Examples showing that the heating method according to the present invention is sufficiently effective will be shown in comparison with non-Si steel.

Figure 6 shows various compositions containing 0.05% or more of Si.
Si-killed steel slab (component C: 0.15-0.25%,
Mn: 0.60~0.70%, Si: 0.1~0.30%, P: 0.15~
0.25%, S: 0.05~0.15%) was soaked to the baking heating temperature (1220~1320℃) and then processed under the following rolling conditions. The effects on the occurrence of exfoliation defects are shown separately when the heating gradient (°C/min), that is, the temperature increase rate, is set at around 1300°C directly above. A similar relationship is shown for slabs with otherwise similar compositions.

Rolling conditions Rough rolling completion temperature: 1000°C - 1100°C Finish rolling completion temperature: 880°C - 880°C Coiling temperature: 500°C - 600°C Descaling: Each rolling machine for rough rolling and finishing rolling with high pressure spray of 150Kg/ ^cm2 (Effects of the Invention) According to the present invention, depending on the heating procedure during soaking of Si-containing steel materials, flaking surface defects that tended to occur frequently only in conventional Si-containing steel materials can be reduced. Appropriate avoidance is achieved.

[Brief explanation of the drawing]

Figures 1a and b are explanatory diagrams comparing the descaling property in hot rolling of Si-containing steel and non-Si-containing steel, Figure 2 is an explanatory diagram of the surface scale properties of Si-killed steel,
Figure 3 is a comparison graph of oxidation loss of Fe-Si alloy at various heating temperatures for 10 minutes, Figure 4 is a graph of correspondence between oxidation temperature and average pore diameter, and Figure 5 is a comparison of heating patterns. Figures 6 and 7 show the heating temperature of Si-containing steel and non-Si-containing steel, and at just above 1200℃,
This is a graph comparing the influence of the temperature increase rate (heating gradient) around 1300°C on the occurrence of peelable surface defects.

Claims (1)

[Claims] 1. When heating the steel material during hot rolling of Si-containing steel, the rate of increase in the heating furnace atmosphere temperature from around 1200°C to the heating completion temperature is set to around 1200°C. A method for heating a Si-containing steel material during hot rolling, characterized by suppressing the formation of feuyarite at the metal interface on the surface scale of the steel material by decreasing the heating rate from a maximum of 4°C per minute to a maximum of 2°C per minute near 1300°C. . 2. The method according to claim 1, wherein immediately before starting the gradual decrease in the temperature increase rate, a rapid heating operation is performed to quickly increase the temperature increase rate.