JPS6130622A - Atmosphere heat treatment - Google Patents

Abstract

PURPOSE:To obtain steel material descalable effectively in pickling, by heating, high temp. holding and slowly cooling steel material in reducing atmosphere, then holding at a prescribed temp. in HC fuel combustion gas in which respective concn.s of O2 and CO, H2O dew point are prescribed, and slowly cooling said material. CONSTITUTION:Rolled steel material is passed in order through a heating zone 1 and a soaking zone 2 held to reducing atmosphere respectively, heated to 720- 780 deg.C, and held for a prescribed time. Thereafter, said material is passed through a slowly cooling zone 3 to cool it slowly to 640-680 deg.C. Next, said material is sent to a reoxidizing zone 4 whose atmosphere is isolated from the zone 3, and held at 640-680 deg.C for 0.5-1.5hr in HC fuel combustion gas atmosphere having <=10% O2 concn., <=3% CO concn., and >=10 deg.C H2O dew point. In this way, steel material having remarkably improved descaling property is obtained.

Description

[Detailed description of the invention] [Industrial application field] The present invention is a wire coil having iron oxide scale.

The present invention relates to an atmospheric heat treatment method for effectively descaling steel materials such as rods, plates, strips, etc. 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 immersing the steel in a solution of hydrochloric acid, sulfuric acid, potassium permanganate, etc. and cleaning it, but as shown in Figure 4, tertiary processing such as wire drawing is performed after rolling. When applied, 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. Therefore, even if pickling is performed after heat treatment, scale removal is inhibited and a thin film of reduced scale remains on the surface of the steel material.

Therefore, in the heat treatment method disclosed in Japanese Patent Publication No. 57-28725, steel materials with rolling scales are subjected to heat treatment consisting of heating, holding at high temperature, and slow cooling in a reducing atmosphere in which the carbon concentration is adjusted, and then oxidizing. 650℃~500℃ in a sexual atmosphere
C and then allowed to cool in the atmosphere to actively re-oxidize the reduced scale adhering to the steel material after heat treatment, thereby improving 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 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 those that 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, the iron oxide scale consists of Fe around Fe.
2. Three layers of Fe3041 Fe2O3 are formed in that order, and among them, the layer near the surface, F
e2O3 or Fe3O4 is difficult to erode even when immersed in acid, whereas FeO is easily eroded. However, in the re-oxidation in the invention of Japanese Patent Publication No. 57-28725, a thick layer of Fe3O4 or Fe2O3 is formed on the surface of the steel material, which actually inhibits descaling during pickling.

[Means for solving the problem] Therefore, the first invention according to this application is to heat a steel material in a reducing atmosphere, heat it, hold it at a high temperature, and slowly cool it, and then reduce the O2 concentration to 10% or less and CO2. Concentration is 3% or less, H2O dew point is 10
A heat treatment method characterized in that the steel material is held at a temperature of 640°C to 680°C for 0.5 to 1.5 hours in an atmosphere of hydrocarbon fuel combustion gas at a temperature of 0°C or higher, and then cooled. It is.

Further, the second invention according to this application provides that after the steel material undergoes the steps of heating up, holding at high temperature, and slow cooling in a reducing atmosphere, the 02 concentration is 10% or less, the CO concentration is 3% or less, and the H2O dew point is 10
The steel material is maintained at a temperature of 640°C to 680°C for 0.5 to 1.5 hours in an atmosphere of hydrocarbon fuel combustion gas of % or more, and then the steel material is held in an inert atmosphere such as N2 gas or Partial pressure ratio of hydrocarbon fuel combustion gas (H2O/H
2) is an atmospheric heat treatment method characterized by cooling in a neutral atmosphere adjusted to 10 to 10<-2>.

[Function] By keeping the wire material after heat treatment under the above-mentioned atmospheric conditions for a certain period of time, the reduction scale Fe stops being oxidized mainly at the FeO stage, and becomes Fe3O4, Fe2O3.
Since the oxidation does not proceed to the point where the oxidation progresses, the descaling performance in subsequent pickling is significantly improved.

In addition, cooling after reoxidation is performed in an atmosphere that maintains equilibrium with FeO without oxidizing or reducing it, that is, an inert atmosphere such as N2 gas or a partial pressure ratio (H2O/'H2) of 10 to 10-
By carrying out the process in a neutral atmosphere adjusted to 2, descaling performance during pickling is not impaired.

[Example] Fig. 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 placed in heating zone 1. Soaking zone 2. Slow cooling zone 3. Re-oxidation zone4. It passes through 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. and heating zone 1. A radiant tube is installed on the inner wall of the soaking zone 2, and the tube is metallurgically determined to spheroidize the steel material in the heating zone 1 without decarburization, as shown in the temperature diagram in Figure 2. temperature (720° C. to 760° C.) and maintained at that temperature in soaking zone 2 for a predetermined period of time. After that, the steel material is passed through an annealing zone 3 to reach a temperature of 640°C to 68°C.
Cool slowly to 0°C. The next reoxidation zone 4 is the slow cooling zone 3.
The atmosphere is isolated from the reoxidation zone 4, and the reoxidation zone 4 has
The composition of the hydrocarbon fuel combustion gas combusted in the radiant tubes of the heating zone 1 and the soaking zone 2 is adjusted and supplied. That is, in the reoxidation zone 4, the 0□ concentration is 10% or less, the CO concentration is 3% or less, and the atmosphere is N2O dew fuel combustion gas.
Hold at a temperature of 40°C to 680°C for 0.5 to 1.5 hours. Note that when the 02 concentration is 10% or more, the oxidizing power is too strong and F e 504 or Fe2OB is generated, and on the other hand, when the CO concentration is 3% or more, the oxidizing power is insufficient and the Fe is converted into Fe.
It cannot be reoxidized to O. Of course, these concentrations are also related to the temperature of the steel material and its holding time, so 640℃
The temperature conditions and holding time of 0.5 to 1.5 hours at ~680°C must also be strictly observed, and under these conditions, reoxidation of Fe can be stopped in the FeO state. .

After that, the steel material is transferred to cooling zone 5 at 50℃/
It is rapidly cooled down to 350°C or less at a rate of more than an hour. The cooling zone 5 is provided with an inert atmosphere such as N2 gas or a partial pressure ratio (N2O/H2) of combustion gas of hydrocarbon fuel.
By adjusting the temperature to 0 to 10-2, a neutral atmosphere that neither oxidizes nor reduces FeO is filled. In addition, as shown in the oxidation and reduction equilibrium state diagram of FeO in Fig. 3, when the partial pressure ratio (N20/H2) is 10 to 10-
2, FeO maintains approximately 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 (845C) wire rod and a chromium molybdenum steel (C8CM435) wire rod, and the results will be described.

Carbon steel wire rods and chromium molybdenum steel wire rods that have not been pickled after rolling were heated to 740°C in a reducing atmosphere for 2 hours.
The temperature was raised to 660° C., maintained at that temperature for 4 hours, and slowly cooled to 660° C. over 4 hours. After that, the wire was 02 with a concentration of 5%.
, CO concentration θ%, and H2O dew point of 40°C, the temperature was maintained at 660°C for 1 hour. After that, sample material ■ was allowed to cool naturally in the air at a cooling rate of 50°C/hour or less, and sample material ■ was cooled for 15 minutes with a fan.
Rapid cooling was performed at a cooling rate of 0°C/hour, and the partial pressure ratio (H 2 0 / H 2 ) of specimen ① was 10 to 1O-2 (
7) Cool at a cooling rate of 50°C/hour in a neutral atmosphere,
Sample material (1) was 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.

As described above, sample (1) also had good descaling properties, but the pickling time could be made shorter by cooling in a neutral atmosphere. In addition, since the reaction time of rapid cooling is short, the rate at which FeO generated by reoxidation changes to Fe3O4 or Fe2O3 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]

Figure 1 is a layout diagram of a continuous heat treatment furnace in which the present invention is carried out;
The figure shows the heat treatment temperature diagram for steel materials, and Figure 3 shows the oxidation of Feo.
The reduction equilibrium state diagram, FIG. 4, is a diagram showing a conventional treatment process for steel materials.

Claims (1)

[Claims] 1. After the steel material undergoes the steps of heating up, holding at high temperature, and slow cooling in a reducing atmosphere, the O_2 concentration is 10% or less and the CO concentration is 3.
% or less, the steel material was held at a temperature of 640°C to 680°C for 0.5 to 1.5 hours in a hydrocarbon fuel combustion gas atmosphere with an H_2O dew point of 10°C or higher, and then cooled. An atmospheric heat treatment method characterized by: 2. After passing through the steps of heating up the steel material in a reducing atmosphere, holding it at high temperature, and slowly cooling it, the O_2 concentration is 10% or less and the CO concentration is 3.
% or less, 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 with an H_2O dew point of 10°C or more, and then the steel material is heated to N_2 gas. in an inert atmosphere such as or with a partial pressure ratio (H_2O/H_2) of hydrocarbon fuel combustion gas of 10 to 10^
2. An atmospheric heat treatment method characterized by cooling in a neutral atmosphere adjusted to 2. 3. The atmospheric heat treatment method according to claim 1 or 2, wherein the cooling rate after heat treatment is 50° C./hour or more.