JP2983366B2 - Carburizing and nitriding equipment in continuous annealing furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous annealing furnace for annealing a continuously fed steel strip, and more particularly, to a method for continuously forming a cold-rolled steel strip between a heating zone and a soaking zone and a cooling zone. The present invention relates to a continuous annealing furnace provided with a carburizing / carburizing zone for carburizing or nitriding.

[0002]

2. Description of the Related Art Conventionally, cold-rolled steel sheets for press working have been produced by box annealing low carbon-rimmed steel or low carbon-aluminum killed steel having a carbon (C) amount of C ≧ 0.01% or more. In view of recent demands for energy saving and shortening of production delivery time, conversion to continuous annealing has been actively promoted.

The cold-rolled steel sheet for press working manufactured by the continuous annealing method is inferior in drawability because the heating and soaking time is extremely short as compared with the cold-rolled steel sheet obtained by the box annealing method. Therefore, in order to make the drawability at least equal to that of the box-annealed material, measures such as setting the hot-rolling winding temperature and the annealing temperature of the low-carbon steel higher than those of the box annealing method are taken. Furthermore, since the continuous annealing method has an extremely short cooling time, the carbon dissolved in the solid solution during the annealing is precipitated by performing an overaging treatment, but the solid solution carbon still remains. It was difficult to obtain late aging.

Therefore, as a means for obtaining aging resistance equivalent to that of a box-annealed low carbon-aluminum killed steel and higher workability, ultra-low carbon steel (C ≦ 0.01%, Al ≦ 0.20).
%) And the addition of carbide forming elements such as Ti, Nb, B, etc., as necessary, has become common, and is currently widely used as a steel sheet for press working. However, such ultra-low carbon steel has a somewhat higher reactivity in pressurized zinc phosphate treatment applied as a coating base treatment than conventional low carbon-rimmed steel and low carbon-aluminum killed steel. Inferior, the fineness of the generated zinc iron phosphate crystals and the stability when the chemical conversion treatment conditions fluctuated were disadvantageous.

[0005] With respect to weldability, in the case of ultra-low carbon steel, the structure of the heat-affected zone (HAZ) is coarsened and the strength tends to be lower than that of the welded portion or the base metal. It was disadvantageous in terms of fatigue properties over low carbon-aluminum killed steel. Furthermore, since ultra-low carbon steel is rich in ductility and very tenacious, when punching or shearing is performed under the same conditions as low carbon-aluminum killed steel, burrs are generated on the end face, and the burrs are peeled off in the subsequent pressing step Therefore, there is a problem of inducing a star defect, and it has been strongly desired to improve the punching property of ultra-low carbon steel.

[0006] Therefore, a cold-rolled steel sheet for press forming excellent in workability and punching property is provided, in which solid carbon or solid nitrogen is present only in the surface layer of the steel sheet by carburizing or nitriding the steel sheet surface. Prior art (Japanese Patent Publication No. 1-24331)
JP-A-63-38556).
However, this prior art does not propose a specific equipment configuration for continuously producing cold-rolled steel sheets for press forming in which solid solution carbon or solid solution nitrogen is present only in the surface layer.

Therefore, the present applicant has previously proposed a continuous annealing furnace capable of continuously producing a cold rolled steel sheet having a carburized or nitrided layer formed only on the surface layer. As shown in FIG. 4, for example, this continuous annealing furnace includes a heating zone 2 or a heating zone 2 and a soaking zone 3 for heating a continuously fed cold-rolled steel sheet, and cooling zones 6 and 7 for cooling the steel sheet. In the continuous annealing furnace having the heating zone 2 or the heating zone 2 and the soaking zone 3
And a cooling and nitriding zone 5 for continuously carburizing or nitriding the steel sheet between the cooling zones 6 and 7. According to this continuous annealing furnace, when continuously annealing a steel sheet continuously fed as a steel strip by welding,
After performing predetermined recrystallization by heating and soaking performed by the heating zone 2 and the soaking zone 3, carburizing and nitriding are controlled by controlling the steel sheet temperature, atmospheric conditions, transfer speed (furnace time), and cooling conditions. By doing so, the concentration and depth of the surface carburized / nitrided layer can be set to desired values while satisfying the material specifications of the steel sheet.

[0008] Carburizing at the original continuous annealing transfer speed
If the path length of the carburizing / nitriding zone is set so that the nitriding process can be performed, the carburizing / nitriding process can be added without changing the processing speed of the continuous annealing.

[0009]

Generally, under the processing conditions of a uniform thickness and a uniform feed rate, the carburizing depth h of the steel sheet is expressed by the following equation (1), the sheet temperature T and the diffusion of carbon progress. It is determined according to the elapsed time t of the temperature range. h = f (T, t) (1) In evaluating the surface carburized layer, focusing on the carbon concentration of the outermost surface layer and the carbon concentration distribution of the surface carburized layer, the carbon concentration of the outermost surface layer is as follows. The carbon concentration distribution of the surface carburized layer is determined by the carbon concentration of the outermost surface layer, the temperature of the steel sheet, the elapsed time at each temperature, the composition of the steel sheet material, etc. You. Of these conditions, since the components of the steel sheet material are determined in advance, it is important to set the atmospheric conditions such as the carbon potential. Above all, the temperature conditions, that is, the temperature of the steel sheet is controlled to be substantially constant at a predetermined temperature. Most importantly.

However, when a steel sheet is carburized and nitrided together with annealing in an actual continuous annealing furnace, since the steel sheets having different specifications are welded and continuously fed as a steel strip, the final material temperature is reduced by the annealing material. When the annealing process is performed at the maximum processing capacity, the annealing speed, that is, the elapsed time greatly changes within one coil. Further, for example, as shown in FIG. 5, when two adjacent steel plates welded have different thicknesses, the target plate temperature and the actual plate temperature may be different due to a time constant error of the plate temperature control before and after the two seams. Local changes occur.
Therefore, temperature variations occur in the longitudinal direction of the steel strip.

Further, in an actual continuous annealing furnace, there are heat transfer functions such as heat transfer between the steel strip and the roll, convection heat transfer in the furnace, and radiant heat transfer of the furnace wall. In many cases. Even if such a variation in the temperature of the steel strip is at a level that does not cause any problem with the annealing, it becomes a problem that cannot be ignored in order to form the surface carburized layer uniformly. This problem is exactly the same when performing nitriding.

Further, particularly in the case where nitriding is involved, since the eutectoid point of solute nitrogen is lower than the soaking temperature of ordinary annealing, the temperature in the furnace of the carburizing / nitriding zone is adjusted to the eutectoid point. Even if it is set to a value close to the temperature, it takes time for the temperature of the steel strip to reach that temperature, so that the actual nitriding time is shortened, and there is an irrationality that sufficient nitriding cannot be performed. The present invention has been developed in view of these problems.The steel strip heated and soaked in the annealing step is further controlled to a sheet temperature optimum for carburizing and nitriding, and then carburizing or nitriding is performed. An object of the present invention is to provide a carburizing and nitriding treatment facility in a continuous annealing furnace that can form a uniform surface carburized / nitrided layer by performing, and can provide a steel sheet having a stable outermost surface carbon and nitrogen concentration. Things.

[0013]

Means for Solving the Problems In the present invention, the carburizing and nitriding equipment in the continuous annealing furnace according to claim 1 is reduced.
Cold rolled steel spliced with steel plates with different specifications including thickness
And a band continuously fed pressurized heat heating zone or heating zone and the soaking zone, and then between the cooling zone for cooling the steel strip, continuously carburizing the heating or soaking heated steel strip Alternatively, in a carburizing and nitriding treatment facility in a continuous annealing furnace provided with a carburizing and carburizing zone to be carburized, the steel strip is placed in front of the carburizing and carburizing zone .
The temperature of the seam of the steel sheet is adjusted to a temperature suitable for carburizing or nitriding.

According to a second aspect of the present invention, in the carburizing and nitriding treatment equipment for a continuous annealing furnace according to the second aspect of the present invention, the sheet temperature adjusting zone includes a sheet temperature measuring device including a temperature distribution in a width direction of the steel strip; It is characterized by comprising a sheet width temperature adjusting device capable of dividing the temperature in the width direction and controlling the sheet width temperature adjusting apparatus based on a detection signal of the sheet temperature measuring device. It is.

[0015]

According to the present invention, in the carburizing and nitriding treatment equipment for a continuous annealing furnace according to the first aspect of the present invention, the temperature of the seam of the steel sheet in the steel strip before the carburizing / nitriding zone is controlled by the sheet temperature adjusting zone. Since it can be adjusted to a temperature suitable for nitriding, at least prevent the variation of the sheet temperature in the longitudinal direction of the steel strip, by controlling the sheet temperature according to the above one set, the carburizing and nitriding depth, The carbon concentration and the nitrogen concentration in the outermost surface layer can be stabilized at predetermined values .

In the carburizing and nitriding equipment for a continuous annealing furnace according to claim 2 of the present invention, a sheet temperature measuring device including a temperature distribution in a width direction of a steel strip provided in the sheet temperature adjusting zone;
A sheet width temperature control device capable of dividing the steel strip in the width direction to adjust the temperature, and an instrumentation control device that controls the sheet width temperature control device based on a detection signal of the sheet temperature measurement device, thereby forming a steel strip plate. Variations in the sheet temperature in the width direction can be prevented, and the surface carburized / nitrided layer can be made more uniform.

[0017]

FIG. 1 shows an embodiment of a carburizing and nitriding treatment equipment in a continuous annealing furnace according to the present invention. FIG. 1 shows the configuration of a vertical continuous annealing furnace for continuously annealing cold-rolled steel sheets. The continuous annealing furnace includes a coil unwinder, a welding machine, a washing machine, and the like (not shown). Equipment, pre-tropical 1,
It comprises a heating zone 2, a leveling zone 3, a plate temperature control zone 4, a carburizing zone 5, a first cooling zone 6, a second cooling zone 7, and a not-shown outlet equipment such as a shearing machine and a winder. All of these facilities are constructed in a tower-like vertical type in order to reduce the installation area.

The ultra-low carbon cold rolled steel sheet is welded in the longitudinal direction regardless of the specifications such as the sheet thickness and the material, and is continuously fed as a steel strip A from the entrance facility. , Heating zone 2,
After passing through the soaking zone 3, the plate temperature control zone 4, the carburizing zone 5, the first and second cooling zones 5 and 6 in order, it is finally cooled to room temperature.
The heating zone 2 is continuously fed from an inlet facility, and heats a preheated cold-rolled steel sheet to, for example, a recrystallization temperature or higher. Specifically, the furnace temperature is 900 to 950 ° C,
The steel sheet is heated so that the temperature of the steel strip becomes 700 to 800 ° C. The heated cold-rolled steel sheet is held in the soaking zone 3 for a required time, and then falls into the sheet temperature control zone 4.

The steel sheet A which has been heated and soaked to the sheet temperature necessary for annealing in the heating zone 2 and the soaking zone 3 is converted into a sheet most suitable for carburizing in the following carburizing zone 5. The temperature is adjusted. In this plate temperature control zone 4,
In order to suppress the variation in the sheet temperature in the longitudinal direction of the steel strip due to the time constant error of the furnace temperature control described above, and to further suppress the variation in the sheet temperature in the sheet width direction of the steel strip, a gas jet cooling device is used. 12 are used to control these sheet temperatures. Specifically, as shown in FIGS. 1 and 2, sheet temperature distribution sensors 10 and 11 provided on the entrance side and the exit side of the sheet temperature adjustment zone 4 allow the steel strip A to move in the sheet width direction and the longitudinal direction. The temperature distribution is detected, and a detection signal is sent to a host computer 17 provided in, for example, a central control room. On the other hand, the gas jet cooling device 12 is configured as a set of a plurality of gas jet nozzles 13 arranged side by side in the plate width direction of the steel strip A. , A plurality of sets are provided to face the steel strip fed. Each of these gas jet nozzles 13 has a flow valve 14.
The gas supply source 1 is provided through the flow valve 14.
5 is connected to a gas temperature controller 16 for controlling the inert cooling gas supplied from 5 to a predetermined temperature. While tracking the steel strip A, the host computer 17 determines which part of the steel strip A including the sheet width direction is cooled based on the detection signal from the entrance side sheet temperature distribution sensor 10, that is, the cooling amount of the part. Is calculated, and control amounts such as a flow rate, a flow velocity, and an injection time of the cooling gas for satisfying the cooling amount are calculated, and the throttle opening of the flow valve 14 is adjusted according to the control amounts to each gas jet nozzle 13. Perform each time. Further, the host computer 17 performs feedback control for constantly correcting the cooling amount based on a detection signal from the delivery-side plate temperature distribution sensor 11. Thus, the steel strip A controlled to the sheet temperature most suitable for carburizing reaches the carburizing zone 5.

The carburized zone 5 has a carbon content C ≧ 0.0 in an extremely thin portion (0.5 μm to 100 μm or less) on the surface of the cold-rolled steel sheet.
In order to form a 1% carburized layer, the furnace temperature is controlled to 650 to 900 ° C., and the transport speed is controlled so that the cold-rolled steel sheet passes through the carburized zone in 10 to 30 seconds. In such a carburized zone 5, when the steel sheet temperature is lower than 650 ° C., the carburizing speed is reduced, and the heat treatment productivity is reduced. On the other hand, when the temperature in the furnace exceeds 900 ° C., the solute carbon diffuses, that is, a so-called diffusion annealing state occurs, and the solute carbon cannot be fixed.

The temperature distribution in the carburizing zone is preferably within 50 ° C. in order to prevent sooting on the surface of the cold-rolled steel sheet. If free carbon adheres to the surface of the steel sheet, it causes deterioration in quality such as deterioration of chemical conversion treatment and adverse factors in post-processes. As the composition of the carburizing gas supplied into the carburizing furnace, for example, CO = 5 to 10 vol%, H ₂ = _{2 to 4} v
ol%, CO / CO ₂ = 15 to 20, balance = N _2. The carburizing gas is supplied into the carburizing zone 5 at a rate of 1000 Nm ³ / hr or more. In order to prevent the carburizing gas supplied to the carburizing zone from leaking to the outside, seal members are provided at the inlet and the outlet of the steel strip into the carburizing zone.

The steel strip A that has left the carburized zone 5 reaches the first cooling zone 6. In the first cooling zone 6, in order to fix solid-solution carbon only in an extremely thin range on the surface of the cold-rolled steel sheet, the steel strip A after carburizing is heated at a steel sheet temperature of 600 ° C. or less, preferably 50 ° C. or less.
Cool rapidly at a cooling rate of 20 ° C./sec. Or more until the temperature becomes about 0 to 400 ° C. In the first cooling zone 6, the flow rate, the flow rate and the cooling roll temperature of the cooling gas blown to the steel strip conveyed in the cooling zone are set so that the cooling condition can be achieved.
The winding angle and the like are controlled.

The steel strip A that has exited the first cooling zone 6 then reaches the second cooling zone 7. In the second cooling zone 7, gas cooling is performed to a steel sheet temperature of about 250 to 200 ° C. Thus, finally, an extremely low carbon cold-rolled steel sheet for press forming in which solid solution carbon exists only in the surface layer can be obtained.
This cold-rolled steel sheet for press forming has excellent press formability and chemical conversion treatment, as described in Japanese Patent Publication No. 42331/1994. Such a cold-rolled steel sheet for press working also has excellent weldability, punchability, and slidability.

Next, a specific embodiment will be described. An ultra-low carbon steel sheet having a carbon content of steel C of 20 ppm and a sheet width W of 1200 mm was subjected to continuous annealing and carburizing in the continuous annealing furnace of FIG. 1 according to the temperature history of FIG. (A), (b), (c), (d) in the temperature history of FIG. 3 are (a), (b),
This corresponds to the steel strip temperature at each point of (c) and (d). Specifically, FIG. 3A shows a temperature range in the plate temperature control zone, FIG. 3B shows a temperature range in the carburizing zone, FIG. 3C shows a temperature range in the first cooling zone, and FIG.
Indicates the first cooling zone exit side temperature range, respectively.

In this continuous annealing furnace, the target sheet temperature of the annealing treatment in the heating zone 2 and the soaking zone 3 is 780 ° C., and the target sheet temperature of the carburizing treatment in the sheet temperature control zone 4 and the carburizing zone 5 is 78.
0 ° C., and the carburizing gas composition in the carburizing zone 5 was CO
_{= 5vol%, H 2 = 3vol} %, N 2 = 91vol
%, Balance = 1vol%, steel strip feeding speed 150mp
m, continuous annealing and carburizing were performed.

As a comparative example, a cold rolled steel sheet which was subjected to only the carburizing treatment without adjusting the sheet temperature in the continuous annealing was also prepared. The thus obtained cold-rolled cold-rolled steel sheet according to the embodiment of the present invention, which has been subjected to the carburizing treatment, is applied to the entire length of the steel strip, that is, the entire inside of one coil, and the carbon concentration depth of the surface carburized layer (carbon content C ≧ 30 ppm). ) And the carbon concentration of the outermost layer were examined, and the former variation was about 10 μm.
m, the dispersion of the latter was about 5 ppm. Similarly, in the cold-rolled steel sheet for press working obtained as a comparative example, the variation of the carbon concentration depth of the surface carburized layer is about 30 μm, and the variation of the carbon concentration of the outermost surface layer is about 20 ppm. It became clear how uniform the surface carburized layer of the cold rolled steel sheet was.

As described above, according to the continuous annealing furnace of the present embodiment,
Cold rolled steel sheets with excellent press formability and chemical conversion
Can be provided continuously. In the above embodiment, the carburizing
The case was explained, but the nitriding was performed instead of the carburizing zone.
A sinter may be provided. Also, by changing the atmosphere
The same furnace can be used for carburizing and nitriding. Nitrifying
As the atmosphere, for example, NH_ThreeContaining (N _Two+
H_Two) It is sufficient to use gas or other mixed gas.
You. The carburized zone of the present invention is not only carburized but also carbo-nitrided.
May be performed.

In the above embodiment, continuous annealing of cold rolled ultra low carbon steel sheet was described. However, the present invention is not limited to this, and other low carbon steel such as low carbon rimmed steel and low carbon aluminum quenched steel can be used. Applicable to steel grades. Further, in the present embodiment, a plate temperature control zone is provided between the soaking zone and the carburized zone. However, the soaking zone and the plate temperature adjusting zone are formed in the same furnace, It is also possible to provide a plate temperature control zone between the tropics and the carburizing zone, and to provide a diffusion zone for adjusting the carburizing depth between the carburizing zone and the first cooling zone.

Further, two cooling zones can be combined into one cooling zone.

[0030]

As described above, according to the carburizing and nitriding treatment equipment in the continuous annealing furnace of the present invention, the seam of the steel sheet in the steel strip before the carburizing and nitriding zone is controlled by the sheet temperature control zone. > Temperature can be adjusted to a temperature suitable for carburizing or nitriding, so that at least the sheet temperature variation in the longitudinal direction of the steel strip is prevented to prevent carburizing and nitriding depth, carbon concentration and nitrogen in the outermost surface layer. The concentration can be stabilized at a predetermined value, and furthermore, by providing a temperature control mechanism in the width direction of the steel temperature control zone, it is possible to prevent variations in the temperature of the steel strip in the width direction and to perform surface carburization. The layer can be made uniform.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing one embodiment of a carburizing and nitriding treatment equipment in a continuous annealing furnace of the present invention.

FIG. 2 is a functional configuration diagram of a sheet temperature control zone in a carburizing and nitriding treatment facility in the continuous annealing furnace of FIG.

FIG. 3 is an explanatory diagram of a temperature history of continuous annealing performed by the continuous annealing furnace of FIG. 1;

FIG. 4 is an overall configuration diagram showing one example of a conventional carburizing and nitriding equipment in a continuous annealing furnace.

FIG. 5 is an explanatory diagram of a sheet temperature variation in a steel strip longitudinal direction in a steel sheet joint portion of the steel strip in continuous annealing.

[Explanation of symbols]

 2 is a heating zone 3 is a solitary zone 4 is a plate temperature regulation zone 5 is a carburizing zone 10 is an inlet side plate temperature distribution sensor 11 is an exit side plate temperature distribution sensor 12 is a gas jet cooling device A is a steel strip

Continued on the front page (58) Fields surveyed (Int.Cl. ⁶ , DB name) C21D 9/56 101 C23C 8/06 C23C 8/22 C23C 8/26 C21D 1/74 C21D 1/06

Claims (2)

(57) [Claims] 1. A steel plate having different specifications including at least a thickness.
And seamed cold rolled steel strip continuously fed to the pressurized heat heating zone or heating zone and the soaking zone, and then between the cooling zone for cooling the steel strip was heated the heating or soaking In a carburizing and nitriding treatment facility in a continuous annealing furnace provided with a carburizing / nitriding zone for continuously carburizing or nitriding a steel strip, the temperature of a seam of a steel sheet in the steel strip before the carburizing / nitriding zone is carburized. Alternatively, a carburizing and nitriding treatment facility in a continuous annealing furnace, wherein a plate temperature adjusting zone for adjusting a temperature suitable for nitriding is provided. 2. The apparatus according to claim 1, wherein the temperature control zone includes a temperature measuring device including a temperature distribution in a width direction of the steel strip, a temperature control device configured to divide the steel strip in the width direction to control the temperature, and The carburizing and carbonitriding equipment in a continuous annealing furnace according to claim 1, further comprising an instrumentation control device that controls the plate width temperature adjusting device based on a detection signal of a temperature measuring device.