JPH0466615A - Method for heat-treating steel material - Google Patents

Abstract

PURPOSE:To make oxide film thickness thin in a steel material and to make acid pickling for removing the oxide film unnecessary by specifying air ratio in a combustion burner, oxygen concn. in waste gas and waste gas temp. to execute the combustion control at the time of executing heat treatment to the steel material with combustion of the directly firing type combustion burner. CONSTITUTION:In the method of heat-treating the steel material by combustion of the directly firing type combustion burner, where the fuel is directly burnt in the furnace, the combustion with the combustion burner is executed at a ratio of <=0.5 to air, and also, the oxygen concn. (A) in the waste gas and the waste gas temp. (T) are detected. While executing the combustion control so that the oxygen concn. (A) becomes <=100 ppm and the waste gas temp. (T) becomes the temp. shown by the next inequality (1). [T >= 2.5A + 550... (1).] By executing the heat treatment of the steel material under such a condition, the oxide film thickness on the steel material surface can be made to < 20 Angstrom and treatment of the acid pickling, etc., for removing the oxide film after heat treatment, becomes unnecessary.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for heat treating steel materials in a direct-fired heat treatment furnace for brightly annealing steel materials, for example, by directly burning fuel in the furnace.

[Conventional technology]

Conventionally, when bright annealing steel materials such as low chromium alloy steels such as 5Cr-H and SCM materials, a method of indirect heating in an atmospheric gas using an annealing furnace as shown in Fig. 4 has been adopted. ing. This annealing furnace 21 has a large number of radiant tubes 23 inserted into the upper part of an annealing furnace main body 22 formed in a box shape, and a mounting table (not disclosed) for a metal material to be processed in a hearth 24. ), and atmospheric gas introduction holes 26 are formed in the furnace wall 25. An atmospheric gas generator 27 such as a DX gas generator is connected to the atmospheric gas introduction hole 26 . This device 27 includes a conversion furnace 28 that generates DX gas by mixing propane or butane gas and air and heating the mixture, and a cooler 29 that cools the DX gas.

When bright annealing the metal material W in the annealing furnace 21, after charging the metal material into the furnace, the radiant tube 23 is heated to red with a burner while the inside of the furnace is maintained in a DX gas atmosphere, The metal material is heated and soaked so as to form a predetermined annealing pattern, and then cooled. The metal material U that has been brightly annealed in this way is subjected to pickling treatment to remove the oxide film, and then processed into a desired product in a post-process.

[Problems to be Solved by the Invention] However, since the conventional bright annealing method described above employs indirect heating using the radiant tube 23, there are problems in that the thermal efficiency is low and the fuel consumption is large. Moreover, 100 to 200 ppm of oxygen is present in the furnace atmosphere gas during the above-mentioned annealing, and for this reason, when the metal material is steel, an oxide film is formed with a thickness of about 700 mm, especially on chromium molybdenum steel. In this case, there is a problem in that even after pickling to remove the oxide film, the surface quality deteriorates, which adversely affects product quality in subsequent steps.

Therefore, the present applicant has conducted extensive research in order to solve the problems with the conventional bright annealing treatment method as described above, and has first improved the thermal efficiency, reduced the fuel consumption rate, and reduced the thickness of the oxide film. As a heat treatment method for metal materials whose thickness has been reduced to a level that does not cause deterioration of surface properties, a furnace is used to heat treat metal materials by combustion in a direct combustion burner that directly burns fuel in a furnace. We have developed a method for heat treating metal materials by detecting the oxygen concentration in the internal gas and controlling the air ratio of the direct combustion burner so that the oxygen concentration becomes a predetermined concentration depending on the quality of the metal material. did.

(Refer to Japanese Patent Application No. 62-7208) However, in the case of steel materials, even with the above heat treatment method for metal materials, although oxidation can be suppressed to a minimum, it is completely prevented (oxidation film thickness 22
Therefore, treatment such as pickling is still required in the post-process.

The present invention has been made in view of the above circumstances, and includes:
The purpose is to provide a heat treatment method for steel materials that heat-treats steel materials to an oxide film thickness of less than 20% and that eliminates the need for post-process treatments such as pickling to remove the oxide film.

[Means to solve the problem]

In order to achieve the above object, the method for heat treating steel materials according to the present invention is a method for heat treating steel materials by combustion in a direct combustion burner that directly burns fuel in a furnace, in which the combustion by the combustion burner is reduced to an air ratio. 0.5 or less, detect the oxygen concentration (A) in the exhaust gas and the exhaust gas temperature (T), and set the oxygen concentration (A) to 1100 pp or less and the exhaust gas temperature (T) to the temperature shown by the following formula (1). This method heat-treats steel materials while controlling combustion to achieve the desired results.

T≧2.5A + 550 (1) The present invention will be described in detail below.

In the heat treatment method shown in the previous order, the applicant proposed that it is desirable to control oxygen at to 10 pp- or less in the case of steel materials, but in the subsequent research process, the oxygen concentration in the furnace was reduced to 10 pp- or less. It is difficult to control the temperature due to the intrusion of oxygen from outside the furnace, and the limit is to reduce the temperature to about 10 to ppm.Furthermore, the thickness of the oxide film on steel materials heat-treated under these conditions is 20 to ppm.
It has been found that the thickness of the oxide film may vary within a range of 70 Å or less. Therefore, in order to understand the cause of this, we investigated the thickness of the oxide film and
The relationship between the air ratio, the furnace gas temperature, and the oxygen concentration of the furnace gas, which are considered to be factors that affect the thickness of this oxide film, is as follows:
The investigation was conducted using a heat treatment furnace shown in FIG. 1, the details of which will be described later. The results of this investigation are shown in Figures 2 and 3.

Figure 2 is a diagram showing the relationship between the thickness of the oxide film and the air ratio.
B) and 150 people (C) were used, and the air ratio was set to 0.
5. 0.7. 0.9. The thickness of the oxide film on the steel material after annealing was measured when the value was changed to 1.0. As is clear from this figure, when the air ratio is increased, the surface of the steel material is oxidized and the thickness of the oxide film increases, but when the air ratio is 0.5, both steel materials (B) and (C) are oxidized. The thickness of the coating was reduced to about 10 mm, and the steel surface was being annealed while being reduced. However, even if the air ratio is controlled to 0.5, the oxygen concentration of the furnace gas is below 1100 pp, as shown in Figure 3, which shows the relationship between the furnace gas temperature, furnace gas oxygen concentration, and oxidation and reduction. However, it was found that the steel material was oxidized when the gas temperature in the furnace was low. As is clear from this figure, when the furnace gas temperature is the same, as the oxygen concentration of the furnace gas increases, it becomes more likely to oxidize, but by increasing the furnace gas temperature, a reduced state can be obtained. The area is (1) 2 (T≧2.5A→
550).

The present invention is based on the above knowledge, and its gist is that in a method for heat treating steel materials by combustion in a direct combustion burner that directly combusts fuel in a furnace, the combustion by the combustion burner is reduced to an air ratio. 0.5 or less, detect the oxygen concentration (A) in the exhaust gas and the exhaust gas temperature (T), and calculate the oxygen concentration (A) to 100 p4 or less and (Jl - gas temperature (T) using the following formula (1). This method heat-treats the steel material while controlling combustion so that it reaches the temperature shown in .

T≧2.5A + 550 (1) By heat-treating the steel material under these conditions, the thickness of the oxide film on the surface of the steel material can be reduced to less than 20, and after heat treatment Processing such as pickling to remove the oxide film becomes unnecessary.

[Example of implementation]

Embodiments of the present invention will be described below.

FIG. 1 shows a direct-fired annealing furnace for carrying out the method of heat treating steel materials according to the present invention. In the figure, reference numeral 1 denotes a direct-fired annealing furnace body, which is constructed by lining the inner surface of a furnace shell made of shaped steel with a refractory insulation material, and the side wall of the furnace body 1 is not shown. The opening/closing door is arranged so that it can be raised and lowered.

Further, an exhaust gas passage 2 is formed in the lower part of the right side wall 1a of the furnace body 1. Furthermore, the left side wall 1b of the furnace body l
A plurality of combustion burner insertion holes 3 are formed in the upper part of the furnace in the furnace width direction, and a low air ratio direct type combustion burner 4 is inserted and fixed into the insertion holes 3.

An extension tile 5 is installed on the front peripheral wall of the combustion burner 4 to stabilize the combustion state. Furthermore, the nozzle portion of the burner 4 is structured to eject fuel at right angles to the flow direction of the air in order to promote mixing of the fuel and combustion air and improve combustibility.

Further, a fuel supply pipe 6 and a combustion air supply pipe 7 are connected to the combustion burner 4, and each supply pipe 6.7 has a flow rate detector 6a, 7a and a flow rate control valve 6b, 7, respectively.
b is provided.

8 is an oxygen coal making detector that detects the oxygen concentration in exhaust gas. The rear end of an exhaust gas extraction pipe 9 whose tip is inserted into the exhaust gas passage 2 is connected to the detector 8, and a stream of gas flows sequentially from the tip side to the middle of the extraction pipe 9. control valve 10,
A filter 11 is provided. Note that 12 is a suction pump for introducing exhaust gas into the detector 8.

13 is a temperature detector that detects the temperature of exhaust gas. A temperature measuring element 14 inserted into the exhaust gas passage 2 is connected to the detector 13 .

15 is a furnace temperature setting device according to the material of the steel material S; 16 is a fuel flow rate to realize the furnace temperature set in the furnace temperature setting device 15, and the detected flow rate from the fuel flow rate detector 6a is ,
The above fuel flow should be as the above calculated flow rate! While controlling the opening and closing of the i control valve 6b, the detected oxygen concentration (A) from the oxygen production detector rI8 and the exhaust gas temperature (T) from the temperature detector 13 are inputted, and the above (1), which has been manually input in advance, is input. It is calculated whether the formula (T≧2.5A±550) is satisfied or not, and if it is not satisfied, the fuel flow rate is adjusted so that the formula (T≧2.5A±550) is satisfied within the furnace temperature range set in the furnace temperature setting device 15. This is a furnace temperature control device that controls opening and closing of the control valve 6b.

17 is an air ratio setter for setting the air ratio to 0.5 or less; 18 is the air flow rate detector 7 which calculates the air flow rate to realize the air ratio set in the air ratio setter 17;
This combustion air regulating device controls the opening and closing of the air flow regulating valve 7b so that the detected flow rate from point a becomes the calculated flow rate.

Further, in this example, a secondary combustion chamber 19 and a heat exchanger 20 are connected to the exhaust gas passage 2, and the exhaust gas from the furnace body 1 is subjected to secondary combustion in the secondary combustion chamber 19 and is completely combusted. The heat exchanger 20 is then discharged to the atmosphere through a flue. In the heat exchanger 20, air for the combustion burner 4 and air for secondary combustion are heated and supplied.

Next, a method of brightly annealing the steel material S using the direct-fired annealing furnace 1 of this embodiment will be described.

In the annealing furnace 1 of this embodiment, two steel materials S are placed in the furnace! place

Then, the combustion burner 4 is ignited, and the temperature is raised, soaked, and cooled so that the steel material S has a predetermined annealing pattern. At this time, an air ratio corresponding to the steel material S selected from an air ratio of 0.5 or less is set in the air ratio setting device 17, the combustion burner 4 is combusted under that condition, and the combustion exhaust gas obtained by the combustion is Gas is used as an atmospheric gas, and the temperature of the atmospheric gas, that is, the temperature inside the furnace, is adjusted to the above-mentioned furnace temperature setting ri.
The furnace temperature is controlled to be within a predetermined range set at 15, and the atmospheric gas is controlled to be in a reducing atmosphere state within this predetermined range of furnace temperature. That is, the detected oxygen concentration K (A) from the oxygen concentration detector 8 and the exhaust gas temperature (T) from the temperature detector 13 are input to the furnace temperature control device 16. The above formula (1) (
It is calculated whether T≧2.5A+550) is satisfied, and if it is not satisfied, the fuel flow rate control valve 6b is adjusted so that the furnace temperature is satisfied within a predetermined range set in the furnace temperature setting device 15. Control opening and closing. In this way, the combustion burner 4 burns at an air ratio of 0.5 or less, the oxygen concentration (A) in the exhaust gas is 1100 pp or less, and the exhaust gas temperature (
T) is maintained at a temperature that satisfies the above formula (1) (T≧2.5A −550).

According to the method for annealing steel material S according to this embodiment, the oxygen concentration of the atmospheric gas in the direct-fired annealing furnace is set to 1100p.
Since bright annealing is performed while adjusting the exhaust gas temperature to below p and controlling the exhaust gas temperature to a reducing atmosphere state, the thickness of the oxide film adhering to the steel material S can be kept to less than 20.

〔Effect of the invention〕

As described above, according to the heat treatment method for steel materials according to the present invention, the thickness of the oxide film on the surface of the steel material can be kept to less than 20, and treatments such as pickling to remove the oxide film after heat treatment can be carried out. It is no longer necessary, improving the productivity of heat-treated steel materials.

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration diagram of a direct-fired annealing furnace for explaining the method of heat treatment of steel materials according to the present invention, Fig. 2 is a diagram showing the relationship between the thickness of the oxide film and the air ratio, and Fig. 3 is a diagram showing the relationship between the thickness of the oxide film and the air ratio. , A diagram showing the relationship between furnace gas temperature, furnace gas oxygen concentration, oxidation, and reduction,
FIG. 4 is a schematic configuration diagram for explaining a conventional bright annealing method. 1i! j Fire type annealing furnace main body 2 Exhaust gas passage 3 Combustion burner insertion hole 4 Direct type combustion burner 6 Fuel supply pipe
7 Combustion air supply pipe 6a, 7a flow rate detector
6b, 7b Flow control valve 8 Oxygen concentration detector
13 Temperature detector 14 Temperature measuring element 15
Furnace temperature setting device 16 Furnace temperature control device 17 Combustion air adjustment official guest 19 Secondary combustion chamber 20 Heat exchanger Fig. 1 Patent applicant Kobe Steel, Ltd.

Claims (1)

[Claims] In a method of heat treating steel materials by combustion in a direct combustion burner that directly burns fuel in a furnace, the combustion in the combustion burner is performed at an air ratio of 0.5 or less, and the oxygen concentration in the exhaust gas is (A) and the exhaust gas temperature (T), and keep the oxygen concentration (A) below 100 ppm and the exhaust gas temperature (T).
A method for heat treating steel materials, the method comprising heat treating the steel materials while controlling combustion so that the temperature reaches the temperature expressed by the following equation (1). T≧2.5A+550---(1)