JPH02282425A - How to heat a steel billet - Google Patents

Abstract

PURPOSE:To prevent the development of decarbonized layer and to obtain a steel slab having the extremely thin surface decarbonized layer in as-roll product by regulating heating time at temp. exceeding A1 transformation point temp. in a walking beam type heating furnace and heating the steel slab. CONSTITUTION:At the time of heating the steel slab in the walking beam type heating furnace, the heating time at the temp. exceeding the A1 transformation point temp., where diffusion phenomenon of C becomes active, is made <=30min. The regulation of the above heating time can be achieved by shortening time in the furnace and heating uniformly, or by suppressing the heating temp. in the heating zone to low and making heating temp. in the soaking zone high to execute rapid heating, etc. By this regulation, the diffusion of C is suppressed to prevent the development of the surface decarbonized layer. By this method, removal of the decarbonized layer in the product after heat treatment is unnecessary and can be used in as-roll.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for heating a steel billet in a walking beam furnace, and more particularly, to a method for heating a steel billet in a walking beam furnace, and more specifically, to heat a steel billet in a manner that does not cause a decarburized layer on the surface of the as-rolled product. Regarding heating method.

[Prior art] Generally, when a steel product has a decarburized layer, the C (carbon) content is low, so when quenching the part, the hardness of that part does not increase, and when used, the progress of (Al wear) Faster, f
Problems arise such as easy BL tearing and short C1 fatigue life.

Therefore, for example, for construction machine parts and automobile parts, processes such as polishing and peeling are required for the purpose of removing the surface decarburized layer.

On the other hand, there is also a method of adjusting the heating furnace temperature in order to suppress the formation of a surface decarburized layer. In other words, the surface decarburized layer thickness is greatly affected by the heating temperature and the furnace time, but in a walking beam type heating furnace, the cylinder stroke is usually 250 to 350-pitch, so the furnace time in the furnace depends on the surface decarburization layer thickness. is long, and the average time from A1 metamorphosis point to extraction is about 70
However, the generation of a decarburized layer was suppressed by controlling the heating temperature without changing the in-furnace time.

[Problems to be Solved by the Invention] However, as mentioned in (■) above, performing polishing and peeling treatments for the purpose of removing the surface decarburized layer increases costs due to an increase in the number of steps, and it is also difficult to respond quickly in terms of delivery time. The problem is that it is not possible.

On the other hand, since the above method (1) relies only on regulating the heating furnace temperature, the effect of suppressing the decarburized layer is not sufficient.

Therefore, in the present invention, the generation of a decarburized layer is prevented by improving the heating method without requiring the addition of a polishing or beering process after rolling, and the heating of a steel billet results in an extremely small surface decarburized layer in the as-rolled material. The purpose is to provide a method.

[Means for Solving the Problems] The present invention for solving the above problems is such that when heating a steel billet in a walking beam heating furnace, the heating time at a temperature exceeding the A transformation point temperature is 30 minutes or less. It is characterized by:

"Effect" Generally, when heating a steel billet from room temperature, C=0.77
% or less, it starts from the state of ferrite + pearlite, and when the transformation point (A) exceeds the transformation point (approximately 727°C),
When austenite begins to precipitate and exceeds the A3 transformation point,
Austenite phase.

In this process, the C diffusion phenomenon that determines the progress of decarburization gradually becomes more intense from the transformation point at which austenite precipitates, becomes most active at the transformation point, and weakens once beyond the transformation point. , after which it gradually becomes more active.

In the present invention, when heating a steel billet, the heating time is limited to 30 minutes or less when the temperature exceeds the transformation temperature of A, so as described later, the diffusion phenomenon of C is suppressed as much as possible, and the scale The amount of decarburization of the advanced product can be suppressed, and the amount of decarburization of the as-rolled product can be extremely reduced.

Furthermore, by applying the present invention, it is possible to improve fuel consumption rate and yield by reducing scale loss.

[Specific structure of the invention] The present invention will be explained in more detail below.

Suppression of the surface decarburization layer in a heating furnace can be achieved relatively easily by performing extraction at as low a temperature as possible, but at such low temperatures, due to the problem of mill load when rolling after extraction,
It becomes difficult to roll.

Therefore, the temperature sufficient for rolling, usually 1000-11
It is necessary to extract at about 00°C.

By the way, as mentioned above, the formation of the decarburized layer starts above the A transformation point and reaches its peak around the A3 transformation point. Therefore, A. The formation of a decarburized layer can be suppressed by shortening the time from the time when the transformation point is exceeded to the time of extraction. In fact, A
As shown in Figure 3, it was found that there is a strong relationship between the time from the point beyond the transformation point to extraction and the degree of decarburization layer thickness. As shown in the figure, in order to obtain a product without decarburization, it is necessary that the time from the A1 transformation point to extraction be 30 minutes or less.

Therefore, as specific means for achieving heating within this condition, for example, the following means (i) and (ii) can be adopted.

(i) As shown in Figure 1, normally in a walking beam type heating furnace, the cylinder stroke is 250~
However, in the present invention, in consideration of shortening the furnace time and uniform heating property in short heating time,
- Charge the steel billets every other time, and make the pitch between the steel billets 500 to 6.
00M and heat it ((b) in the same figure).

(ii) Also, as shown in Figure 2(a), generally
- A king beam furnace basically consists of a pre-heating zone, a heating zone, and a soaking zone.
As shown by the broken line A in the figure (bl), the heating temperature in the heating zone is high.In contrast, in the present invention, the heating temperature in the heating zone is kept low as shown by the solid line B, and on the other hand, the steel billet is rolled. In order to raise the temperature to a sufficient temperature, the heating temperature in the soaking zone is increased and rapid heating is performed.

According to the present invention, it is desirable that the temperature of 028 degrees in the soaking zone is 0.5% or less.

In this way, as shown in FIG. 2 (bl), in accordance with the present invention, in the heating zone, by lowering the temperature of the steel billet than before, the time from the A1 transformation point to the time of extraction can be reduced to 30 minutes or less. .

Thus, as is clear from the schematic diagram of the metal structure of the surface of the product after rolling in Figure 4, in the conventional heating mode, a decarburized layer of, for example, O, L 4 mm was observed as shown in the ta1 diagram. On the other hand, according to the method of the present invention, the decarburized layer can be reduced to zero as shown in the fb1 diagram. In addition, Fig. 4 ta)
, tb+ are diagrams showing the metal structure of round bar products of material 345C and diameter 25.0 at a magnification of 100 times.

There are various types of steel slabs to which the method of the present invention can be applied, but in particular round steel with a diameter of 19 mmφ to 55 mmφ for machine structures (C = 0.1 to 0.55%), carbon steel for machine structures There are alloy steels (Cr series, Cr-Mo series), bearing steels, etc.

[Example] Next, an example will be described.

545C diameter 251n! When heating a steel billet to obtain a round steel of IIφ by rolling, A in the conventional method, the transformation point ~
The furnace time until extraction is 56 minutes, and in the method of the present invention, the A transformation point ~
The furnace time until extraction was 15 minutes, and the decarburization layer thickness (Dmt-
Max), scale loss, and fuel consumption, the results shown in Table 1 were obtained. The manner in which the steel pieces are heated is as shown in FIG. 2.

Table 1 From Table 1, it is clear that by heating with the heating method of the present invention, it is possible to suppress the formation of a decarburized layer, and also to reduce scale loss and fuel usage.

[Effect of the invention 1 As described above, according to the method of the present invention, it is possible not only to suppress the formation of a surface decarburized layer in an as-rolled product, but also to omit the polishing and beering steps for removing the decarburized layer. It is also possible to reduce scale loss and improve fuel consumption.

[Brief explanation of drawings]

Figure 1 is a diagram showing the method of conveying a steel billet in the furnace, and Figure 2 is a comparison of the heating pattern of the present invention method in each temperature range in the walking beam heating furnace with the pattern of the conventional method. FIG. 3 is a diagram showing the effect of suppressing the decarburized layer according to the present invention, and FIG. 4 is a diagram showing the metal structure of a steel piece. Figure 1 Figure 2

Claims (1)

[Claims] (1) When heating a steel billet in a walking beam heating furnace, the heating time at a temperature exceeding the A_1 transformation point temperature is 30
A method of heating a steel billet, characterized in that the heating time is less than 1 minute.