JPS6045255B2 - Continuous heating method for steel strips, etc. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a continuous heating method for steel strips, etc., in particular, forming a densely structured chromium layer or its concentrated layer on the surface of a hearth roll in a continuous heating furnace for steel strips, etc. by a chromizing method. The present invention also relates to a method of continuously heating a steel strip or the like in a state where the surface of the chromium layer or the like is oxidized so that chromium oxide is present therein.

Generally, hearth rolls used in continuous heating furnaces for steel strips, etc. are SCH22 (25% Cr, 20% Ni), N
A22H (Cr25%, Ni50%), MORE5 (C
It is made from a Ni-based alloy, a Co-based alloy, etc., such as R25%, Co50%). The reason for this is that high-temperature creep strength is important for hearth rolls.
However, when heating with such a hearth roll, scale build-up occurs during heating, which poses a problem. For this reason, in order to heat the material to prevent scale build-up, consideration was given to prevent the hearth roll from reacting with the oxides that adhere to and generate on the surface of the material to be heated, such as steel strips, and to prevent seizure. Heating is performed on a sleeve roll made of sintered alumina, or on a hearth roll whose surface is thermally sprayed with alumina, zirconia, etc.

However, in the former case, the sintered alumina sleeve on the surface has a problem of spalling resistance, and is particularly undesirable because it may crack due to thermal shock when the heating temperature is changed. Furthermore, in the latter case, the thermally sprayed alumina on the surface transforms from γ to α during heating, and zirconia similarly transforms from cubic to tetragonal, which deteriorates the adhesion of these thermally sprayed layers and increases the porosity. Therefore, this deterioration in adhesion causes the sprayed layer to peel off, and the increase in porosity causes scale to sinter in the pores, become nuclei, enlarge, and cause pick-up, which is undesirable. Also, as mentioned above, SCH22, N
In hearth rolls made of Ni-Cr and Co-Cr alloys such as A22H and MORE5, NiC is deposited on the surface during heating.
r.

O,,CoCr. o. Spinel type oxides containing Ni, Co, etc. are formed, but as described later, Cr has a small self-diffusion coefficient. O,,fuN. o. etc. are not formed stably, completely and uniformly, and furthermore, Cr2O3 etc. are not stably formed, so cracking and peeling are likely to occur. For this reason, iron oxide is formed on the surface of the hearth roll, and intergranular corrosion of Ni and the like occurs there, which is also a cause of scale build-up 5. The present invention aims to solve the above-mentioned drawbacks, and specifically, when heating a steel strip etc. in a continuous heating furnace,
We propose a method for continuously heating steel strips, etc. on the hearth roll without causing scale build-up by forming a densely structured chromium layer or chromium concentrated layer on the hearth roll surface using the chromizing method. do.

The method of the present invention will be explained in detail below.

First, when heating a steel strip or the like in a continuous heating furnace, a chromium layer or a chromium concentrated layer is formed on the surface of the hearth roll of the continuous heating furnace by the chromizing method. A steel strip or the like is continuously heated at a temperature of at least 750° C. with chromium oxide formed thereon.

When heated in this way, chromium oxide is further formed on the chromium layer or chromium concentrated layer on the surface of the hearth roll, and the steel strip is continuously heated through the chromium layer and the chromium oxide on it. During this heating, no scale build-up occurs. To explain the reason for this,
It is as follows. First, we determined the conditions under which scale build-up would not occur on the hearth roll during continuous heating of steel strips, etc., and found that (1) the surface of the hearth roll contains metal ions, oxygen ions, etc. An oxide film with a small diffusion coefficient during diffusion is formed, and continuous heating is performed in the presence of this oxide film; (2) the oxide film is stable during heating without cracking or peeling;
If cracking or peeling of the oxide film occurred during heating, a new oxide film would be immediately formed at that location.

Therefore, 10 of metal ions such as Fe, Ni, CO, Cr, etc.
The self-diffusion coefficient of the oxide, that is, the oxide film at 00°C, is as follows.

FeO is 9×10-8 cIt/Secl NlO is 1×10-11 CIt/SeClCOO is 3×
10-9 cr1/Secl cr2O3 is 3 x 10-14 cI1/Secl,
It can be seen that among these, Cr2O3 is the most stable and satisfies the condition (1).

From this point, in the method of the present invention, a chromium oxide film is interposed on the surface of the hearth roll when a steel strip or the like is continuously heated on the surface of the high speed roll of a continuous heating furnace.

Further, in order to provide a chromium oxide film that is stable during heating, a chromium layer or the like is formed on the hearth roll surface, and a chromium oxide film is formed on the chromium layer or the like. In other words, without directly forming a chromium oxide film on the surface of the hearth roll, if a chromium 7 oxide film is formed on the chromium layer, etc., and then continuously heated in this state, the chromium oxide film and the chromium layer are separated. The concentration of Cr at the alloy interface between is greater than the amount of Cr consumed by oxidation during continuous heating through the chromium oxide film.

Therefore, even if a part of the chromium O oxide film peels off or falls off during continuous heating, or Cr is consumed, a new chromium oxide film is generated and transferred to sufficiently compensate for the problem. There is no. In short, in the method of the present invention, the chromium oxide film on the surface does not crack or peel off during continuous heating, and even if cracks occur, the oxide film is immediately regenerated without any problem.

In order to stably form a chromium oxide film on the chromium layer during continuous heating, it is necessary to heat the chromium layer at a temperature of at least 750'C or higher. As described above, the method of the present invention increases the Cr content on the surface of the hearth roll of a continuous heating furnace, and this increase makes it stable during heating to form a chromium oxide film, and continuous heating under these conditions makes it possible to form a chromium oxide film. This prevents scale build-up from occurring. In addition, in order to increase the Cr content on the hearth roll surface in advance, it is appropriate to form a chromium layer or a chromium concentrated layer on the hearth roll surface by a chromizing method, and this chromium layer A dense layer thereof has a dense structure that satisfies the condition (1) above, has no pores, and completely suppresses the occurrence of scale build-up. In addition, in connection with increasing the Cr content on the surface of the hearth roll as described above, the hearth roll is also constructed of a high chromium alloy containing, for example, 35 to 50% Cr. . However, when it is constructed from such a high chromium alloy, problems arise in weldability and workability, which is not preferable. Next, an example will be described. Example 1 1 First, chromium powder 75 containing 99.5% Cr
25 parts by weight of activated alumina is mixed with 25 parts by weight of activated alumina, 0.5 parts by weight of ammonium chloride is added to this mixture and stirred, and a hearth roll body made of a Ni-Cr alloy is embedded in this penetrant. The treatment was carried out under conditions of 1100'C x 2@ in a hydrogen stream.

Next, a chromium layer was formed on the surface of the hearth roll body through this treatment, and the steel strip was heated on the hearth roll in this state to a temperature of 900°C in a continuous heating furnace. A film of chromium oxide was formed on the surface of the steel, and almost no pickup occurred.

In addition, for comparison, when the above-mentioned eight-throat roll body was used as is and continuously heated as usual, the hearth roll had to be replaced about once every four months, but with the heat treatment as described above, It was sufficient to replace the hearth roll once every two years.

As explained in detail above, the method of the present invention involves continuously heating a steel strip or the like at at least 750°C on a hearth roll having a chromium layer or a chromium-concentrated layer on the surface, and
It generates stable chromium oxide when heated.

Claims (1)

[Claims] 1. A chromium layer or a chromium-concentrated layer is formed on the surface of a hearth roll by a chromizing method, and a chromium oxide is formed on the surface of the chromium layer or chromium-concentrated layer on the hearth roll. A method for continuously heating a steel strip, etc., characterized by continuously heating the steel plate, etc. to at least 750°C or higher.