JPH0551641B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention provides a wire coil having an iron oxide scale,
The present invention relates to an atmospheric heat treatment method for effectively descaling steel materials such as bars, plates, and strips by subsequent pickling. [Prior art and its problems] Generally, iron oxide scale (also referred to as rolling scale) is generated on the surface of steel materials due to the influence of heat and environment during processing steps such as rolling. This iron oxide scale is usually removed by immersing the steel in a solution of hydrochloric acid, sulfuric acid, potassium permanganate, or the like and cleaning it. To prevent decarburization of steel surfaces
Heat treatment such as annealing is performed in an inert atmosphere such as N ₂ gas or in a vacuum. In that case, it was difficult to completely remove the iron oxide scale unless the above-mentioned pickling was performed twice, before and after the heat treatment, as shown in FIG. In other words, even if heat treatment is performed in an inert atmosphere such as _N2 gas or vacuum, the iron oxide scale formed during rolling will remain in a small amount (1~
10PPm) also contains oxygen, so the oxidation of the iron oxide scale further progresses during heat treatment.Fe ₂ O ₃
Fe ₂ O ₃ is difficult to be eroded by acid even if it is pickled after heat treatment, so it is difficult to completely remove iron oxide scale only by pickling after heat treatment. It was hot. [Means for solving the problem] Therefore, the present inventors developed iron oxide scale FeO,
Of the three forms of Fe ₃ O ₄ and Fe ₂ O ₃ , Fe ₂ O ₃ or Fe ₃ O ₄ is difficult to erode even when soaked in acid;
We learned that FeO is relatively easily eroded, and improved the descaling properties of iron oxide scale.
That is, the first invention according to this application is to heat a steel material in an inert atmosphere or in a vacuum, after passing through the steps of heating up, holding at high temperature, and slow cooling, the partial pressure ratio (H ₂ O/H ₂ ) is 0.5 or less. The steel material is heated in an atmosphere of hydrocarbon fuel combustion gas.
Hold at a temperature of 640℃~680℃ for 0.5 hours~1.5 hours,
This heat treatment method is characterized in that it is then cooled. Further, the second invention according to this application provides carbonization with a partial pressure ratio (H ₂ O/H ₂ ) of 0.5 or less after the steel material undergoes the steps of heating up, holding at high temperature, and slow cooling in an inert atmosphere or vacuum. The steel material is heated in a hydrogen-based fuel combustion gas atmosphere.
Hold at a temperature of 640℃~680℃ for 0.5 hours~1.5 hours,
Furthermore, after that, the steel material was heated to a partial pressure ratio (H ₂ O/H ₂ ) of 10.
This is an atmospheric heat treatment method characterized by cooling in a hydrocarbon fuel combustion gas atmosphere of ~10 ^-2 . [Effect] Iron oxide scale Fe ₃ O ₄ ,
Since Fe ₂ O ₃ is reduced to FeO, descaling performance in subsequent pickling is significantly improved. In addition, cooling after reduction is performed in an atmosphere that maintains equilibrium without oxidizing or reducing FeO, that is, an inert atmosphere such as N ₂ gas or a partial pressure ratio (H ₂ O / H ₂ ) of 10.
By carrying out the process in a neutral atmosphere adjusted to ~10 ^-2 , descaling performance during pickling is not impaired. [Example] Figure 1 shows the arrangement of each treatment zone of the continuous heat treatment furnace used in this atmospheric heat treatment method, and the steel material after rolling is divided into heating zone 1, soaking zone 2, slow cooling zone 3, and reduction zone 4. Pass through cooling zone 5 in this order. Heating zone 1, soaking zone 2, and slow cooling zone 3 are adjusted according to the type of steel being processed.
Maintained in an inert atmosphere such as _N2 gas or vacuum. Radiant tubes are installed on the inner walls of heating zone 1 and soaking zone 2, and are metallurgically determined in order to spheroidize the steel material in heating zone 1, as shown in the temperature diagram in Figure 2. (720° C. to 760° C.) and maintain that temperature in soaking zone 2 for a predetermined period of time. After that, the steel material is passed through slow cooling zone 3 to 640℃~
Cool slowly to 680℃. The next reduction zone 4 is isolated from the slow cooling zone 3 in terms of atmosphere, and the reduction zone 4 contains the hydrocarbon fuel combustion gas combusted in the radiant tubes of the heating zone 1 and soaking zone 2. Adjust ingredients and supply. That is, in the reduction zone 4, the atmosphere is a hydrocarbon fuel combustion gas with a partial pressure ratio ( _H2O / _H2 ) of 0.5 or less, and the steel material is heated to a temperature of 640°C to 680°C for 0.5 hours to 680°C in this atmosphere. Hold for 1.5 hours. In addition, Fig. 3 shows Fe ₂ O ₃ , Fe ₃ O ₄ and
The oxidation and reduction equilibrium phase diagram of FeO is shown at 640℃~
Under atmospheric conditions at 680°C and a partial pressure ratio (H ₂ O/H ₂ ) of 0.5 or less, it has a reducing effect on Fe ₂ O ₃ and Fe ₃ O ₄ . Therefore, the partial pressure ratio of the atmosphere, the steel material temperature, and the holding time are necessary conditions for rapidly reducing Fe ₃ O ₄ to FeO. Furthermore, the steel material is then transferred to the cooling zone 5.
It is rapidly cooled down to 350℃ or less at a rate of 50℃/hour or more. The cooling zone 5 is a neutral gas that does not have any oxidizing or reducing effects on FeO by adjusting the partial pressure ratio (H ₂ O/H ₂ ) of the combustion gas of the hydrocarbon fuel to 10 to 10 ^-2 . It fills the atmosphere. In addition, the third
As shown in the figure, partial pressure ratio (H ₂ O/H ₂ ) = 10 ~
At 10 ^-2 , FeO almost maintains an equilibrium state. Next, regarding the heat treatment method of the present invention, a comparative test of descaling properties was conducted using a carbon steel (S45C) wire rod and a chromium molybdenum steel (SCM435) wire rod, and the results will be described. Carbon steel wire rods and chromium-molybdenum steel wire rods that had not been pickled after rolling were heated to 740°C in 2 hours in an _N2 atmosphere, and after maintaining that temperature for 4 hours,
It was slowly cooled to 660°C over 4 hours. Thereafter, the wire was maintained at 660° C. for 1 hour in a hydrocarbon fuel combustion gas atmosphere with a partial pressure ratio (H ₂ /H ₂ O)=0.1. After that, the sample material was allowed to cool naturally in the air at a cooling rate of 50℃/hour or less, and the sample material was rapidly cooled with a fan at a cooling rate of 150℃/hour.
For sample material, partial pressure ratio (H ₂ O/H ₂ ) = 10 to 10 ^-2
The sample material was cooled at a cooling rate of 50℃/hour in a neutral atmosphere, and the specimen was cooled at a cooling rate of 150℃/hour in the same neutral atmosphere.
Rapid cooling was performed at a cooling rate of °C/hour. The time required for each is shown in the table below.

[Table] Although the sample had good descaling properties as described above, the pickling time can be shortened by cooling in a neutral atmosphere. In addition, due to the short reaction time of quenching, it was produced by reduction.
The rate at which FeO changes to Fe ₃ O ₄ or Fe ₂ O ₃ is small, and cracks occur due to scale shrinkage due to rapid cooling, making it easier for the pickling solution to penetrate. Therefore, rapid cooling is considered to further improve descaling properties. [Effects of the Invention] Since the descaling property is significantly improved, pickling treatment after rolling becomes unnecessary, and iron oxide scale can be completely washed away by just one pickling treatment after heat treatment. Therefore, there is an advantage that cleaning costs, equipment costs, etc. can be significantly reduced.

[Brief explanation of the drawing]

Fig. 1 is a layout diagram of a continuous heat treatment furnace in which the present invention is carried out, Fig. 2 is a heat treatment temperature diagram of steel materials, Fig. 3 is a diagram of the oxidation and reduction equilibrium state of iron oxide, and Fig. 4 is a conventional treatment of steel materials. It is a figure showing a process.

Claims (1)

[Claims] 1 Steel materials with iron oxide scale generated by processing such as rolling are heated in an inert atmosphere or in a vacuum, followed by heating, holding at high temperature, and slow cooling until the partial pressure ratio (H ₂ O/H ₂ ) is 0.5. The steel material is heated to a temperature of 640℃ to 680℃ in the following hydrocarbon fuel combustion gas atmosphere.
An atmosphere characterized by holding the steel material for 0.5 to 1.5 hours, and then cooling the steel material in a hydrocarbon fuel combustion gas atmosphere having a partial pressure ratio ( _H2O / _H2 ) of 10 to ^10-2 . Heat treatment method.