JPH0551642B2 - - 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] 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 washing the steel by immersing it in a solution such as hydrochloric acid, sulfuric acid, or potassium permanganate, but as shown in Fig. When processing, heat treatment such as annealing is performed to improve workability. In that case, it was considered difficult to completely remove the iron oxide scale unless the above-mentioned pickling was performed twice, before and after the heat treatment. In other words, this heat treatment is usually carried out in a reducing atmosphere to prevent decarburization of the steel, so if the first pickling before heat treatment was omitted, the iron oxide scale attached to the steel would be removed by the heat treatment. Since the iron oxide scale is reduced, even if it is pickled after heat treatment, scale removal is inhibited and a thin film of reduced iron oxide scale (hereinafter referred to as reduced scale) remains on the surface of the steel material.
That is, when iron oxide scale is placed in a reducing atmosphere, it changes in the order of Fe ₂ O ₃ →Fe ₃ O ₄ →FeO, but there is a problem that the reduced scale layer remains on the surface of the steel material. Therefore, the heat treatment method disclosed in Japanese Patent Publication No. 57-28725 involves raising the temperature of a steel material with rolling scales in a reducing atmosphere where the carbon concentration is adjusted, holding it at a high temperature, and
After heat treatment consisting of slow cooling, the steel is heated and maintained at 650℃ to 500℃ in an oxidizing atmosphere, and then allowed to cool in the atmosphere to actively re-oxidize the reduced scale that has adhered to the steel material after heat treatment. The aim was to improve descaling properties during subsequent pickling. [Problems to be solved by the invention] The invention described in Japanese Patent Publication No. 57-28725 is said to be economical in terms of equipment costs because pickling before heat treatment can be omitted. According to tests conducted by the inventors of the application, even if the reduced scale is reoxidized as described in the invention described in this publication, the descaling performance after heat treatment is not necessarily improved to a satisfactory state, and The reality was that the quality was poorer than that which had gone through the pickling process. Therefore, the present inventors further investigated the cause of this problem and were able to accomplish the present invention by learning the following method for improving the descaling property. In other words, iron oxide scale consists of three layers formed around Fe: FeO, Fe ₃ O ₄ and Fe ₂ O ₃ in that order, but the layer near the surface, Fe ₂ O ₃ or Fe ₃ O ₄ is difficult to erode even when soaked in acid, whereas FeO is easily eroded. However, in the reoxidation in the invention of Japanese Patent Publication No. 57-28725, Fe ₃ O ₄ or
A thick layer of Fe ₂ O ₃ is formed, which actually prevents descaling during pickling. [Means for Solving the Problems] Therefore, the present invention provides a method for processing steel materials with iron oxide scale generated by processing such as rolling, after passing through the steps of heating up, holding at high temperature, and slow cooling in a reducing atmosphere. O ₂ concentration is below 10%, CO concentration is below 3%, H ₂ O dew point is below
The steel material is held at a temperature of 640°C to 680°C for 0.5 to 1.5 hours in a hydrocarbon fuel combustion gas atmosphere of 10°C or higher, and then the steel material is heated in an inert atmosphere such as _N2 gas or hydrocarbon gas. This is an atmospheric heat treatment method characterized in that the system fuel combustion gas is cooled in a neutral atmosphere in which the partial pressure ratio (H ₂ O/H ₂ ) is adjusted to 10 to 10 ^-2 . [Function] By holding the steel material after heat treatment under the above atmospheric conditions at that temperature condition for a certain period of time, Fe stops oxidizing mainly at the FeO stage and does not progress to Fe ₃ O ₄ and Fe ₂ O ₃ . Descaling properties in subsequent pickling are significantly improved. In addition, cooling after reoxidation 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 ).
By carrying out the process in a neutral atmosphere adjusted to 10 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. It passes through zone 4 and cooling zone 5 in this order. Inside heating zone 1 and soaking zone 2,
The inside of the slow cooling zone 3 is maintained in a reducing atmosphere such as an exothermic gas or an endothermic gas to prevent decarburization depending on the type of steel material being processed. Radiant tubes are installed on the inner walls of heating zone 1 and soaking zone 2, and metallurgical tubes are used to spheroidize the steel material in heating zone 1 without decarburizing it, as shown in the temperature diagram in Figure 2. It is heated to a scientifically determined temperature (720° C. to 760° C.) and maintained at that temperature in the soaking zone 2 for a predetermined period of time. After that, the steel material is passed through slow cooling zone 3 to a temperature of 640℃ to 680℃.
Cool slowly to ℃. The next reoxidation zone 4 is isolated from the slow cooling zone 3 in terms of atmosphere, and the reoxidation zone 4 contains hydrocarbon fuel combustion gas combusted in the radiant tubes of the heating zone 1 and soaking zone 2. The ingredients are adjusted and supplied. That is, in the reoxidation zone 4, the atmosphere is a hydrocarbon fuel combustion gas with an O ₂ concentration of 10% or less, a CO concentration of 3% or less, or a H ₂ O dew point of 10°C or more, and in this atmosphere, Hold the steel at a temperature of 640°C to 680°C for 0.5 to 1.5 hours. Note that when the O ₂ concentration is 10% or more, the oxidizing power is too strong and Fe ₃ O ₄ or Fe ₂ O ₃ is generated, and on the other hand, when the CO concentration is 3% or more, the oxidizing power is insufficient and Fe is converted to FeO.
cannot be reoxidized to Of course, these concentrations are also related to the temperature of the steel material and its holding time, so the temperature conditions and holding time of 0.5 to 1.5 hours at 640°C to 680°C must also be strictly observed.
Under these conditions, reoxidation of Fe can be stopped in the FeO state. 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 provided with an inert atmosphere such as N ₂ gas or a partial pressure ratio of combustion gas of hydrocarbon fuel (H ₂
By adjusting O/H ₂ ) from 10 to 10 ^-2
It is filled with a neutral atmosphere that has neither oxidizing nor reducing effects on FeO. In addition, as shown in the oxidation and reduction equilibrium state diagram of FeO in FIG. 3, FeO maintains a substantially equilibrium state when the partial pressure ratio (H ₂ O/H ₂ ) is 10 to 10 ^-2 . Next, we will discuss the results of a comparative test of descaling properties of the heat treatment method of the present invention using carbon steel (S45C) wire and chromium molybdenum steel (SCM435) wire. Carbon steel wire rods and chromium-molybdenum steel wire rods that have not been pickled after rolling were heated to 740℃ in 2 hours in a reducing atmosphere, maintained at that temperature for 4 hours, and then heated to 660℃ over 4 hours. It was slowly cooled. after that,
The wire was heated to an O ₂ concentration of 5%, a CO concentration of 0%, and a H ₂ O dew point of 40.
It was held at 660°C for 1 hour in a hydrocarbon fuel combustion gas atmosphere at 660°C. After that, the sample material was allowed to cool naturally in the atmosphere at a cooling rate of 50℃/hour or less.
The sample material was rapidly cooled at a cooling rate of 150°C/hour using a fan, and the sample material was cooled at 50°C/hour in a neutral atmosphere with a partial pressure ratio (H ₂ O/H ₂ ) of 10 to 10 ^-2 .
The samples were rapidly cooled at a cooling rate of 150°C/hour in the same neutral atmosphere. The results and the time required for pickling are shown in the table below.

【table】

[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, because the reaction time of rapid cooling is short, the rate at which FeO generated by reoxidation changes to Fe ₃ O ₄ or Fe ₂ O ₃ is small, and cracks occur due to shrinkage of scale due to rapid cooling, causing the pickling solution to deteriorate. make it easier 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, and Fig. 3 is a diagram of a heat treatment temperature diagram of steel materials.
The oxidation and reduction equilibrium state diagram of FeO, FIG. 4, is a diagram showing the conventional treatment process for steel materials.

Claims (1)

[Claims] 1. After going through the steps of heating up, holding at high temperature, and slow cooling in a reducing atmosphere a steel material with iron oxide scale generated from processing such as rolling, the O ₂ concentration is 10% or less.
The steel material was heated to 640℃ in a hydrocarbon fuel combustion gas atmosphere with a CO concentration of 3% or less and a H ₂ O dew point of 10℃ or more.
The steel material is maintained at a temperature of ~680°C for 0.5 to 1.5 hours, and then heated in an inert atmosphere such as _N2 gas or at a partial pressure ratio of hydrocarbon fuel combustion gas ( _H2O /
An atmospheric heat treatment method characterized by cooling in a neutral atmosphere in which _H2 ) is adjusted to 10 to ^10-2 .