JP2017222895A - Method for annealing steel product - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for annealing a steel product enabling oxidation of a surface of a steel product to be easily and relatively inexpensively suppressed.SOLUTION: The method for annealing a steel product according to the present invention uses in a heat treatment step, a heating furnace where a heat insulator is arranged inside an outer wall and has a step of heat treating the steel product while blowing non-oxidizable gas to the heating furnace. The method for annealing a steel product satisfies a relationship of F/V×100≥(1/30×X×X)+(58/3), where F [Nm/h] denotes non-oxidizable gas supply amount per 1 hr to the heating furnace, V [m] denotes inner volume of the outer wall, X[%] denotes volume percent of the heat insulator in the inner volume of the outer wall, X[%] denotes porosity of the heat insulator. The heating furnace is a batch type heating furnace and the method for annealing a steel product preferably has a step of replacing atmosphere in the heating furnace with the non-oxidizable gas before temperature rising.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for annealing a steel material.

  When steel is annealed, if moisture is present in the atmosphere in the heating furnace, oxidation of the steel is promoted, so that an oxide film is formed on the surface of the steel and may cause defects such as poor appearance due to discoloration, for example. is there. For this reason, for example, the inside of the furnace is replaced with an inert gas such as nitrogen gas (non-oxidizing gas), but it is difficult to sufficiently suppress the oxidation of the steel material.

  The reason why the oxidation of the steel material cannot be sufficiently suppressed by the replacement in the furnace is considered to be the presence of a heat insulating material used for the furnace wall of the heating furnace. Specifically, since the heat insulating material is formed of a porous material, moisture is adsorbed in the pores, and the moisture content in the furnace is reduced by releasing moisture as the temperature rises during heat treatment. It is thought to increase.

  On the other hand, a method has been proposed in which a non-oxidizing gas is continuously supplied to a heating furnace during a heat treatment for annealing a steel material to reduce the dew point in the heating furnace (see JP-A-9-256074). . In the method described in this publication, the dew point of the atmosphere in the heating furnace is measured, and the supply amount of the non-oxidizing gas is adjusted so that the dew point is within the target range.

  As described in the above publication, in order to adjust the supply amount of the non-oxidizing gas, it is necessary to provide a dew point meter, a control valve for adjusting the gas flow rate, and a controller for adjusting the control valve based on the detected value of the dew point meter. There is an equipment cost. In addition, it is not impossible for the operator to periodically measure the dew point in the furnace and adjust the gas flow rate by manual operation, but since the heat treatment for annealing is performed for a long time, the dew point is manually operated. Managing this is a heavy burden on the operator.

Japanese Patent Laid-Open No. 9-256074

  In view of the above inconveniences, an object of the present invention is to provide a method for annealing a steel material that can easily suppress oxidation of the surface of the steel material at a relatively low cost.

The invention made in order to solve the above-mentioned problems is a method of annealing a steel material comprising a step of heat-treating a steel material while blowing a non-oxidizing gas into the heating furnace using a heating furnace in which a heat insulating material is disposed inside the outer wall. In the heat treatment step, the amount of the non-oxidizing gas supplied to the heating furnace per hour is F [Nm ³ / h], the inner volume of the outer wall is V [m ³ ], and the inner volume of the outer wall is It is a steel material annealing method characterized by satisfying the relationship of the following formula (1), where X _m [%] is the volume ratio of the heat insulating material and X _c [%] is the porosity of the heat insulating material.
F / V × 100 ≧ (1/30 × X _m × X _c ) + (58/3) (1)

  The method for annealing the steel material is to keep the dew point of the atmospheric gas in the furnace low by driving out the water vapor released from the heat insulating material by the non-oxidizing gas by satisfying the above formula (1), Oxidation of the surface of the steel material can be suppressed. Further, the steel material annealing method can be performed relatively inexpensively because it does not require a special control device, and can be performed relatively easily because the burden on the operator is small.

  The heating furnace is a batch annealing furnace, and it is preferable to include a step of replacing the atmosphere in the heating furnace with an inert gas before raising the temperature. Thus, the said heating furnace is a batch type annealing furnace, and it suppresses the oxidation of the surface of steel more reliably by providing the process of substituting the atmosphere in a heating furnace with non-oxidizing gas before temperature rising. Can do.

  Here, the “non-oxidizing gas” means a gas that does not have an oxygen atom or a gas that has a stable chemical bond of oxygen atoms and does not have the ability to supply oxygen at a heat treatment temperature.

  As described above, the method for annealing a steel material according to the present invention can suppress oxidation of the surface of the steel material relatively easily and inexpensively.

It is a schematic diagram which shows the structure of the apparatus used for the annealing method of the steel material of one Embodiment of this invention. It is a flowchart which shows the procedure of the annealing method of the steel materials using the apparatus of FIG. It is a graph which shows the relationship between the volume rate and porosity of a heat insulating material, the supply amount of non-oxidizing gas, and discoloration by the oxidation of the steel material surface.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

[Steel annealing method]
A steel material annealing method according to an embodiment of the present invention is a steel material annealing method in which a steel material 1 is disposed in a heating furnace 2 and heat-treated as shown in FIG. The said annealing method can be performed as one process of manufacture of the steel plate used for a motor vehicle, a ship, a building material, a household appliance, etc., for example.

<Heating furnace>
In the annealing method, a batch type open coil annealing furnace as shown in the figure can be used as the heating furnace 2. More specifically, the annealing method can suitably employ a UAD (Unitized Annealing Department) annealing method. In this case, the steel material 1 annealed by the annealing method is typically a coil in which a long cold-rolled steel sheet is wound.

  The heating furnace 2 is disposed in the furnace body 3, the table 4 on which the steel material 1 is placed, the heater 5 disposed in the furnace body 3, and the furnace body 3. And a circulation fan 6 disposed on the side of the table 4. Further, the heating furnace 2 is provided with a gas supply device 7 for supplying a non-oxidizing gas into the furnace body 3.

(Furnace body)
The furnace body 3 includes an outer wall 8 and a heat insulating material 9 disposed on the inner side of the outer wall 8, and is configured to seal the inner space.

  The outer wall 8 is made of, for example, iron, and ensures the strength and confidentiality of the furnace body 3.

  The heat insulating material 9 is formed from a porous material such as brick or mortar, for example.

  The lower limit of the ratio of the total volume of the heat insulating material 9 to the inner volume of the outer wall 8 is preferably 5%, more preferably 7%. On the other hand, the upper limit of the ratio of the total volume of the heat insulating material 9 to the inner volume of the outer wall 8 is preferably 30%, more preferably 20%. When the total volume of the heat insulating material 9 is less than the said minimum, there exists a possibility that the heat insulation of the heating furnace 2 may be insufficient, and thermal efficiency may become inadequate. Conversely, when the total volume of the heat insulating material 9 exceeds the above upper limit, the amount of moisture released from the heat insulating material 9 becomes excessive, and the consumption of the non-oxidizing gas may increase unnecessarily.

  As a minimum of the porosity of the heat insulating material 9, 10% is preferable and 15% is more preferable. On the other hand, the upper limit of the porosity of the heat insulating material 9 is preferably 85% and more preferably 80%. When the porosity of the heat insulating material 9 is less than the lower limit, the heat insulating property of the heat insulating material may be insufficient and the thermal efficiency may be insufficient. Conversely, when the porosity of the heat insulating material 9 exceeds the upper limit, the strength of the heat insulating material may be insufficient.

(table)
The table 4 is configured so that the gas in the furnace can pass vertically.

(heater)
The heater 5 can be composed of, for example, an electric heater, a radiant tube burner, or the like.

(Circulation fan)
The circulation fan 6 sucks the gas in the furnace heated by the heater 5 through the gap between the steel plates 1 through the table 4 from the top to the bottom, and supplies the gas again to the heater 5. Circulate in the furnace.

(Gas supply device)
The gas supply device 7 includes a gas cylinder 10 filled with a high-pressure non-oxidizing gas, and an adjustment valve 11 that adjusts the flow rate of the non-oxidizing gas supplied from the gas cylinder 10 to the heating furnace 2. Can do.

As the non-oxidizing gas supplied by the gas supply device 7, any gas that does not oxidize the steel material 1 may be used, but a reducing gas is preferably used. Examples of such non-oxidizing gas include hydrogen gas (H ₂ ), nitrogen gas (N ₂ ), argon gas (Ar), a mixed gas of hydrogen and nitrogen, and ammonia decomposition gas.

  The gas supply device 7 supplies the non-oxidizing gas to the heating furnace 2 at a constant flow rate. As a lower limit of the supply amount of the non-oxidizing gas per hour from the gas supply device 7 to the heating furnace 2, 15% of the inner volume of the outer wall 8 of the heating furnace 2 is preferable, and 20% is more preferable. On the other hand, the upper limit of the supply amount of the non-oxidizing gas per hour from the gas supply device 7 to the heating furnace 2 is preferably 60% of the inner volume of the outer wall 8 of the heating furnace 2, and more preferably 50%. When the supply amount of the non-oxidizing gas per hour from the gas supply device 7 to the heating furnace 2 is less than the lower limit, the water vapor inside the heating furnace 2 cannot be sufficiently replaced with the non-oxidizing gas, There is a possibility that oxidation of the steel material 1 cannot be sufficiently suppressed. On the contrary, when the supply amount of the non-oxidizing gas per hour from the gas supply device 7 to the heating furnace 2 exceeds the above upper limit, the consumption amount of the non-oxidizing gas and the cost of the heat treatment of the steel material 1 is unnecessarily increased. There is a risk.

  As shown in FIG. 2, the annealing method includes a step of placing the steel material 1 in the heating furnace 2 <step S1: steel material placement step>, and a step of replacing the inside of the heating furnace 2 with an oxidizing gas <step S2: Replacement step> and a step <step S3: heat treatment step> of heat-treating the steel material 1 by heating and cooling so as to obtain a preset temperature profile while supplying the non-oxidizing gas into the heating furnace 2. It can be a method.

<Steel material placement process>
In the steel material arrangement step of step S <b> 1, the steel material 1 is arranged on the table 4 of the heating furnace 2 so that the central axis of the coil of the steel material 1 is perpendicular to the table 4.

<Replacement process>
In the replacement process of step S2, an inert gas is supplied from the gas supply device 7 to the heating furnace 2, and the interior of the heating furnace 2 is replaced with the inert gas.

  By providing this replacement step, the amount of oxygen and moisture in the heating furnace 2 before heating can be reduced, and oxidation of the steel material 1 can be more reliably suppressed.

<Heat treatment process>
In the heat treatment process of step S3, for example, while supplying the non-oxidizing gas from the gas supply device 7 to the heating furnace 2, for example, a heating process in which the furnace temperature is heated to an annealing temperature equal to or higher than the austenitizing temperature, and the furnace temperature is set to the annealing temperature. A soaking step for maintaining the temperature in the furnace and a cooling step for slowly lowering the furnace temperature are performed.

Further, the amount of the non-oxidizing gas supplied to the heating furnace 2 per hour is F [Nm ³ / h], the inner volume of the outer wall 8 is V [m ³ ], and the heat insulating material 9 occupies the inner volume V of the outer wall 8. When the ratio is X _m [%] and the porosity of the heat insulating material 9 is X _c [%], the relationship of the following formula (1) is satisfied.
F / V × 100 ≧ (1/30 × X _m × X _c ) + (58/3) (1)

The left side of the above formula (1) represents the percentage obtained by dividing the supply amount F of the non-oxidizing gas per hour to the heating furnace 2 by the internal volume of the outer wall 8 of the heating furnace 2. This is an index representing the ventilation (substitution) ratio of the atmospheric gas in the heating furnace 2 with the non-oxidizing gas. On the other hand, “X _m × X _c ” on the right side of the above formula (1) is an index representing the ratio of the pores of the heat insulating material 9 to the internal volume of the outer wall 8, that is, the moisture releasing ability of the heat insulating material 9. Therefore, by setting the value on the left side of the above formula (1) to be equal to or greater than the value on the right side, the moisture released from the heat insulating material 9 can be replaced with the non-oxidizing gas in a relatively short time. Surface oxidation can be sufficiently suppressed.

Here, the moisture index X (t) indicating the moisture content at the time t [h] of the moisture content in the heating furnace 2 can be represented by a residence time distribution function represented by the following equation (2).
X (t) = ( _Xm * _Xc ) * exp (-F / V * t) (2)

  That is, in the heating furnace 2, the dew point of the atmospheric gas in the furnace rises due to the release of water vapor from the heat insulating material 9, but the furnace is discharged by supplying the non-oxidizing gas to the heating furnace 2. The dew point of the internal atmosphere gas can be lowered. More specifically, a value obtained by multiplying the moisture index X (t) by 0.0075 is the moisture content [% by mass] contained in the air.

  For this reason, for example, when the dew point is lowered to −20 ° C. in 10 hours, the amount of water contained in the air having the dew point of −20 ° C. is 0.102% by mass. 6 may be set.

  The lower limit of the ratio of the left side of the formula (1) to the right side is 1.0, preferably 1.1, and more preferably 1.2. On the other hand, the upper limit of the ratio of the left side to the right side of the formula (1) is preferably 2.0 and more preferably 1.9.

(Heating process)
In the heating step, the moisture (water vapor) adsorbed on the heat insulating material 9 is released as the temperature in the heating furnace 2 rises, and the moisture content (dew point) in the atmospheric gas in the furnace is increased. On the other hand, since the non-oxidizing gas is blown into the heating furnace 2 from the gas supply device 7, the atmosphere containing moisture released from the heat insulating material 9 is replaced with the non-oxidizing gas. For this reason, after the furnace temperature rises sufficiently and the heat insulating material 9 finishes releasing moisture, the moisture content in the atmospheric gas gradually decreases.

  Although the maximum temperature of the heat treatment in the annealing method is selected according to the type of steel material 1 and the purpose of use, it is set to 700 ° C. or higher and 1000 ° C. or lower, for example.

(Soaking process)
In the soaking process, the temperature inside the heating furnace 2 is maintained at the annealing temperature, whereby the steel material 1 is heated to the annealing temperature.

  The holding time of the soaking step may be, for example, 1 hour or more and 36 hours or less, depending on the size of the steel material 1 or the like.

(Cooling process)
In the cooling step, the furnace temperature is gradually lowered. The average cooling rate in the cooling step can be, for example, 20 ° C./h or more and 100 ° C./h or less.

<Advantages>
In the annealing method of the steel material, by satisfying the above formula (1), the water vapor released from the heat insulating material 9 is driven out of the heating furnace 2 by the non-oxidizing gas, and the dew point of the atmospheric gas in the furnace is kept low. Thus, the oxidation of the surface of the steel material due to the oxidation of the surface of the steel material 1 can be suppressed. Further, the steel material annealing method can be performed relatively inexpensively because it does not require a special control device, and can be performed relatively easily because the burden on the operator is small.

[Other Embodiments]
The said embodiment does not limit the structure of this invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description and common general knowledge of the present specification, and they are all interpreted as belonging to the scope of the present invention. Should.

  In the steel material annealing method, the heating furnace to be used may be a continuous type.

  The shape of the steel material annealed by the steel material annealing method is not limited to a strip-like shape wound in a coil shape, and may be an arbitrary shape such as a block shape.

  In the annealing method for the steel material, the replacement step may be omitted.

  EXAMPLES Hereinafter, although this invention is explained in full detail based on an Example, this invention is not interpreted limitedly based on description of this Example.

  Test No. 1 in which a cold-rolled coil was produced as a steel material to be annealed and annealed while blowing an inert gas using a heating furnace in which a heat insulating material was disposed inside the outer wall. 1-9 were performed.

(Steel)
A cold rolled coil was prepared as the steel material to be annealed. Specifically, first, a raw material is melted and cast, and a slab containing 0.5% by mass of carbon, 0.2% by mass of silicon and 0.8% by mass of manganese, with the balance being iron and inevitable impurities. The slab was produced and hot rolled. Next, the obtained rolled material is pickled to completely remove the oxidized scale on the surface, and further cold-rolled to obtain a steel plate having a plate pressure of 2 mm, a plate width of 950 mm, and a length of 20 m, an inner diameter of 610 mm. Was coiled into a cold rolled coil.

(heating furnace)
Test No. 1 to 3, a batch type UAD annealing furnace in which the inner volume of the outer wall is 100 m ³ , the volume ratio of the heat insulating material to the inner volume of the outer wall is 7%, and the porosity of the heat insulating material is 20% is used as the heating furnace. . Test No. In 4 to 7, a batch type UAD annealing furnace in which the inner volume of the outer wall was 100 m ³ , the volume ratio of the heat insulating material occupying the inner volume of the outer wall was 7%, and the porosity of the heat insulating material was 80% was used as the heating furnace. . Test No. 8-9, as a heating furnace, a batch type UAD annealing furnace in which the inner volume of the outer wall is 100 m ³ , the volume ratio of the heat insulating material to the inner volume of the outer wall is 20%, and the porosity of the heat insulating material is 17.4%. Using.

(Heat treatment)
While supplying 100 mass% concentration of hydrogen gas as the non-oxidizing gas, the temperature was raised to the annealing temperature (700 ° C.) over about 10 hours, kept for about 20 hours, and then gradually cooled. During this time, the dew point of the atmosphere in the furnace was measured.

Test No. 1 to 9, the amount of the non-oxidizing gas supplied to the heating furnace per hour is 23 [Nm ³ / h], 24 [Nm ³ / h], 45 [Nm ³ / h], and 24 [Nm, respectively. ³ [/ m], 30 [Nm ³ / h], 38 [Nm ³ / h], 45 [Nm ³ / h], 24 [Nm ³ / h], and 33 [Nm ³ / h].

(Evaluation)
Test No. above. About the cold-rolled coil annealed by 1-9, the square test piece of 1 side 15mm was cut out by plane view including the mill edge of the steel plate width direction end in the position of 20 m from the front-end | tip of a coil, respectively. These test pieces were photographed with an actual microscope at a color temperature of 3500 K and an illuminance of 10,000 lux, and the L * a * b * values on the surface of the steel sheet were measured from the photographed images.

  In Table 1 below, the test No. The volume ratio and porosity of the heat insulating materials of 1 to 9, the flow rate of the non-oxidizing gas, the ratio of the left side of the above formula (1) to the right side (if less than 1, the formula (1) is not satisfied), and processing The dew point of the furnace atmosphere 10 hours after the start and the L * value of the test piece are shown.

  As shown in the table, it can be seen that the larger the left side of the formula (1) is, the larger the L * value is and the less discoloration due to oxidation of the steel material. In the above table, when the L * value is 60 or more, it can be said that the discoloration due to oxidation is within the allowable range.

In FIG. The product (X _m × X _c ) of the ratio (F / V) of the non-oxidizing gas supply amount per hour to the heating furnace of 1 to 9 to the inner volume of the outer wall and the volume ratio of the heat insulating material and the porosity The relationship between the L * value and the oxidation of the steel material is plotted as “◯” when the L * value is 60 or more and “×” when the L * value is less than 60. In the figure, a straight line indicating that the left side and the right side of Equation (1) are equal is also shown.

  As described above, from the test results, it is possible to sufficiently suppress the formation of an oxide film on the steel material by performing heat treatment while blowing the non-oxidizing gas into the heating furnace so that the left side of the formula (1) is equal to or greater than the right side. It could be confirmed.

  The steel material annealing method according to the present invention can be suitably used for bright annealing of cold rolled coils.

DESCRIPTION OF SYMBOLS 1 Steel material 2 Heating furnace 3 Furnace body 4 Table 5 Heater 6 Circulation fan 7 Gas supply apparatus 8 Outer wall 9 Heat insulating material 10 Gas cylinder 11 Adjustment valve S1 Steel material arrangement process S2 Replacement process S3 Heat treatment process

Claims (2)

A method for annealing a steel material comprising a step of heat-treating a steel material while blowing a non-oxidizing gas into the heating furnace using a heating furnace in which a heat insulating material is disposed inside the outer wall,
In the heat treatment step, the non-oxidizing gas supply amount per hour to the heating furnace is F [Nm ³ / h], the inner volume of the outer wall is V [m ³ ], and the heat insulation occupies the inner volume of the outer wall. A steel material annealing method characterized by satisfying the relationship of the following formula (1), wherein the volume ratio of the material is X _m [%] and the porosity of the heat insulating material is X _c [%].
F / V × 100 ≧ (1/30 × X _m × X _c ) + (58/3) (1)   The method for annealing a steel material according to claim 1, wherein the heating furnace is a batch annealing furnace, and includes a step of replacing the atmosphere in the heating furnace with an inert gas before raising the temperature.

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