JPH05202425A - Carburizing and nitriding treatment equipment in continuous annealing furnace - Google Patents

Abstract

PURPOSE:To provide carburizing and nitriding treatment equipment in a continuous annealing furnace, in which the uniform surface carburizing and nitriding layer can be formed on a cold-rolled steel strip, by executing the decarburization and the nitriding to the steel strip heated and soaked in an annealing process after controlling the strip to the optimum strip temp. to the decarburization and denitriding. CONSTITUTION:The strip temp. adjusting zone 4 for adjusting the steel strip A fed between a heating zone 2 and a soaking zone 3 as the continuous annealing process and a carburizing zone 5 as the carburizing process in the vertical type continuous annealing furnace to the suitable strip temp. to the decarburization is arranged. Then, in an inlet side and an outlet side of this strip temp. adjusting zone 4, strip temp, distributing sensors 10, 11 are arranged to detect the temp. distribution in the width direction and the longitudinal direction of the steel strip A, and based on the detected signals, by controlling flow rate valves of gas jet nozzles divided into the strip width direction in gas jet cooling devices 12 arranged in plural sets so as to face to the steel strip fed in a tower of the strip temp. adjusting zone 4, the unevenness of the strip temp. in the width direction of the steel strip A is eliminated.

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 continuously fed steel strips, and more particularly to a continuous cold strip 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 / nitriding zone for carburizing or nitrifying.

[0002]

2. Description of the Related Art Cold-rolled steel sheets for press working have hitherto been manufactured by box-annealing low carbon-rimmed steel or low carbon-aluminum killed steel having a carbon content of C ≧ 0.01%. In view of recent demands for energy saving and shortening of production delivery time, conversion to continuous annealing is being actively promoted.

The cold-rolled steel sheet for press working produced by the continuous annealing method is inferior in drawability as compared with the cold-rolled steel sheet obtained by the box annealing method because its heating and soaking time is extremely short. Therefore, in order to make this drawability at least comparable to that of a box-annealed material, measures such as making the hot rolling coiling temperature and annealing temperature of the low carbon steel higher than those of the box-annealing method are taken. Further, since the continuous annealing method also has an extremely short cooling time, the solid solution carbon is precipitated during the annealing by performing the overaging treatment, but since the solid solution carbon still remains, the workability is not guaranteed at room temperature. It was difficult to obtain delayed aging.

Therefore, as a means of obtaining the aging resistance equivalent to that of the low carbon-aluminum killed steel annealed in a box and the higher workability thereof, a very low carbon steel (C ≦ 0.01%, Al ≦ 0.20).
%, And a technique of adding a carbide-forming element such as Ti, Nb, B or the like as needed is widely used, and is currently widely used as a steel sheet for press working. However, such an ultra-low carbon steel has a reactivity that is somewhat higher than that of conventional low carbon-rimmed steel and low carbon-aluminum killed steel in a zinc phosphate treatment performed as a coating base treatment after press forming. However, the fineness of the formed zinc iron phosphate crystals and the stability when the chemical conversion treatment conditions were changed were disadvantageous.

Regarding the weldability, in the case of ultra-low carbon steel, the structure of the heat-affected zone (HAZ) becomes coarse and the strength tends to be lower than that of the welded portion and the base metal. Fatigue characteristics were lower than those of low carbon-aluminum killed steel. Furthermore, since ultra-low carbon steel is rich in ductility and extremely tough, when punching or shearing is performed under the same conditions as low carbon-aluminum killed steel, burrs are generated on the end faces and these burrs are peeled off in the later pressing step. Therefore, there is a problem of inducing star-shaped defects, and it has been strongly desired to improve the punchability of ultra-low carbon steel.

Therefore, by carburizing or nitrifying the surface of the steel sheet, solid solution carbon or solid solution nitrogen is allowed to exist only in the surface layer of the steel sheet to provide a cold-rolled steel sheet for press forming excellent in workability and punchability. Conventional technology (Japanese Patent Publication No. 1-24231)
Japanese Patent Laid-Open No. 63-38556).
However, this prior art has not proposed a specific equipment configuration for continuously producing a cold-rolled steel sheet for press forming in which solid solution carbon or solid solution nitrogen is present only in the surface layer.

Therefore, the present applicant previously proposed a continuous annealing furnace capable of continuously producing a cold-rolled steel sheet in which a carburized or nitrided layer is formed only on the surface layer. This continuous annealing furnace is, for example, as shown in FIG. 4, 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, 7 for cooling the steel sheet. In a continuous annealing furnace having and, the heating zone 2 or the heating zone 2 and the soaking zone 3
And the cooling zones 6 and 7 are provided with a carburizing / nitriding zone 5 for continuously carburizing or nitrifying the steel sheet. 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 / nitriding is performed by controlling the steel plate temperature, atmosphere conditions, transfer speed (in-reactor time), and cooling conditions. By doing so, the concentration and depth of the surface carburizing / nitriding layer can be set to desired values while satisfying the material specifications of the steel sheet.

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

[0009]

Generally, under the processing conditions of uniform plate thickness and uniform feed rate, the carburizing depth h of the steel plate is as follows: It is determined according to the elapsed time t of the temperature range. h = f (T, t) (1) In evaluating this 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 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. It Of these conditions, the composition of the steel plate material is determined in advance, so it is important to set the atmospheric conditions such as carbon potential, but above all, the temperature condition, that is, the temperature of the steel plate is controlled to be almost constant at a predetermined temperature. To do is most important.

However, when a steel sheet is annealed and carburized / nitrided in an actual continuous annealing furnace, steel sheets having different specifications are welded and continuously fed as a steel strip. In addition, when the annealing process is produced with the maximum processing capacity, the annealing speed, that is, the elapsed time largely changes within one coil. Further, for example, when the plate thicknesses of two adjacent steel plates welded as shown in FIG. 5 are different from each other, the target plate temperature and the actual plate temperature differ between the two seams before and after the joint due to the time constant error of the plate temperature control. Local changes occur.
Therefore, temperature variations occur in the longitudinal direction of the steel strip.

In an actual continuous annealing furnace, there are heat transfer functions such as heat transfer between the steel strip and rolls, convective heat transfer in the furnace, and radiant heat transfer in the furnace wall. There are often variations. Such a variation in the temperature of the steel strip is a problem that cannot be ignored in order to uniformly form the surface carburized layer, even if there is no problem with annealing. This problem is exactly the same when performing nitrification.

Further, particularly in the case of accompanying nitrification, since the eutectoid point of solid solution nitrogen is lower than the soaking temperature of ordinary annealing, the temperature in the furnace of the carburizing / nitriding zone is set to the eutectoid point. Even if it is set in the vicinity, it takes time for the temperature of the steel strip to reach that temperature, so the actual nitriding time is shortened, and there is also an irrational reason that sufficient nitriding cannot be performed. The present invention has been developed in view of these various problems, and the steel strip heated and soaked in the annealing step is further carburized or nitrified after controlling the plate temperature to be optimal for carburizing / nitriding. An object of the present invention is to provide a carburizing / nitriding treatment facility in a continuous annealing furnace capable of forming a uniform surface carburizing / nitriding layer by performing and providing a steel sheet having a stable outermost surface carbon concentration and nitrogen concentration. It is a thing.

[0013]

A carburizing and nitriding treatment facility in a continuous annealing furnace according to claim 1 of the present invention is a heating zone for heating a cold-rolled steel strip continuously fed or a heating zone and a uniform zone. Carburizing and nitriding in a continuous annealing furnace provided with a carburizing / nitriding zone for continuously carburizing or nitrifying the heated or soaked steel strip between a tropical zone and a cooling zone for cooling the steel strip. In the processing equipment, a plate temperature adjusting zone for adjusting the temperature of the steel strip to a temperature suitable for carburizing or nitriding is provided in front of the carburizing / nitriding zone.

In the carburizing and nitriding treatment facility in the continuous annealing furnace according to claim 2 of the present invention, the plate temperature adjusting zone includes a plate temperature measuring device including a temperature distribution in the width direction of the steel strip, and the steel strip. It is characterized by comprising a plate width temperature adjusting device capable of adjusting the temperature by dividing in the width direction, and an instrumentation control device for controlling the plate width temperature adjusting device based on a detection signal of the plate temperature measuring device. Is.

[0015]

In the carburizing and nitriding treatment facility in the continuous annealing furnace according to claim 1 of the present invention, the temperature of the steel strip is suitable for carburizing or nitriding before the carburizing / nitriding zone by the plate temperature adjusting zone. Since the temperature can be adjusted, it is possible to prevent the plate temperature from fluctuating at least in the longitudinal direction of the steel strip, and control the plate temperature according to the above formula 1 to thereby determine the carburizing / nitriding depth and the outermost surface layer. It is possible to stabilize the carbon concentration and the nitrogen concentration to predetermined values. Especially when the heating / soaking temperature and the carburizing / nitriding temperature in the annealing step are different, it is possible to perform carburizing / nitriding promptly from the time when the steel strip is fed to the carburizing / nitriding furnace. The elapsed time according to the above equation 1 can be sufficiently obtained, and at the same time, the path length of the carburizing / nitriding furnace can be effectively used.

In the carburizing and nitriding treatment facility in the continuous annealing furnace according to claim 2 of the present invention, a plate temperature measuring device including a temperature distribution in the width direction of the steel band provided in the plate temperature adjusting zone,
The strip width of the steel strip is controlled by a strip width temperature control device capable of adjusting the temperature by dividing the steel strip in the width direction, and an instrumentation control device controlling the strip width temperature control device based on a detection signal of the strip temperature measurement device. It is possible to prevent the plate temperature from varying in the width direction and further homogenize the surface carburized / nitrided layer.

[0017]

EXAMPLE FIG. 1 shows an example of a carburizing and nitriding treatment facility in a continuous annealing furnace according to the present invention. The figure shows the structure of a vertical continuous annealing furnace for continuously annealing cold-rolled steel sheets. This continuous annealing furnace has a coil rewinding machine, a welding machine, a washing machine, etc. Equipment, pre-tropical zone 1,
The heating zone 2, the soaking zone 3, the plate temperature adjusting zone 4, the carburizing zone 5, the first cooling zone 6, the second cooling zone 7, a shearing machine, a winding machine and the like, which are not shown in the drawing, are provided. All of these facilities are constructed in a tower-type vertical type in order to reduce the installation area.

The ultra-low carbon cold-rolled steel sheet is welded in the longitudinal direction irrespective of specifications such as thickness and material, and is continuously fed as a steel strip A from the inlet side equipment, and then the preheat zone 1 , Heating zone 2,
After passing through the soaking zone 3, the plate temperature adjusting zone 4, the carburizing zone 5, and the first and second cooling zones 5 and 6 in that order, they are finally cooled to room temperature.
The heating zone 2 is continuously fed from the inlet side equipment to heat the 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 is 700 to 800 ° C. Then, the heated cold-rolled steel sheet is held in the soaking zone 3 for a necessary time, and then reaches the sheet temperature adjusting zone 4.

This strip temperature adjusting zone 4 is a strip most suitable for carburizing in the following carburizing zone 5 of the steel strip A heated and soaked to the strip temperature necessary for annealing in the heating zone 2 and the soaking zone 3. The temperature is adjusted. In this plate temperature adjustment zone 4,
In order to suppress the variation in the strip temperature in the longitudinal direction of the steel strip due to the time constant error in the furnace temperature control described above, and also to suppress the variation in the strip temperature in the strip width direction of the steel strip, a gas jet cooling device. 12 is used to control these plate temperatures. Specifically, as shown in FIG. 1 and FIG. 2, the plate temperature distribution sensors 10 and 11 provided on the inlet side and the outlet side of the plate temperature adjusting zone 4 move the steel strip A in the width direction and the longitudinal direction. The temperature distribution is detected and the detection signal is sent to, for example, the host computer 17 provided in the centralized control room. On the other hand, the gas jet cooling device 12 is constituted by a set of a plurality of gas jet nozzles 13 arranged side by side in the strip width direction of the steel strip A. A plurality of sets are provided so as to face the steel strip fed in. The gas jet nozzle 13 has a flow valve 14
And a gas supply source 1 via the flow valve 14.
5 is connected to a gas temperature control device 16 that controls the inert cooling gas supplied from the device 5 to a predetermined temperature. The host computer 17 tracks which part of the steel strip A, including the width direction of the steel strip A, based on the detection signal from the inlet side plate temperature distribution sensor 10. Is calculated, and control amounts such as the flow rate, flow velocity, and injection time of the cooling gas for satisfying the cooling amount are calculated, and the throttle opening of the flow valve 14 is adjusted to each gas jet nozzle 13 according to the control amount. Do each time. Further, the host computer 17 performs feedback control for constantly correcting the cooling amount based on the detection signal from the outlet side plate temperature distribution sensor 11. In this way, the steel strip A controlled to the plate temperature most suitable for carburizing reaches the carburizing strip 5.

The carburized zone 5 has a carbon content C ≧ 0.0 at 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 temperature in the furnace 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 carburizing zone 5, if the steel plate temperature is lower than 650 ° C., the carburizing rate is lowered and the heat treatment productivity is lowered. On the other hand, when the temperature in the furnace exceeds 900 ° C., the solute carbon diffuses, which is a so-called diffusion annealing state, 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 may cause deterioration of chemical conversion treatment, quality deterioration, and adverse effects of post-processing. The composition of the carburizing gas supplied into the carburizing furnace is, for example, CO = 5 to 10 vol%, H ₂ = _{2 to 4} v.
ol%, CO / CO ₂ = 15 to 20 and the balance = N ₂ , and this 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, a seal member is provided at the inlet and the outlet of the steel strip into the carburizing zone.

The steel strip A exiting the carburizing zone 5 reaches the first cooling zone 6. In this first cooling zone 6, solid solution carbon is fixed only in an extremely thin area on the surface of the cold rolled steel sheet, so that the steel strip A after carburization has a steel sheet temperature of 600 ° C. or less, preferably 50
It is rapidly cooled at a cooling rate of 20 ° C / sec. Or more until it reaches 0 to 400 ° C. In the first cooling zone 6, the cooling gas flow rate, the flow velocity and the cooling roll temperature, which are blown to the steel strip conveyed in the cooling zone, so that this cooling condition can be achieved,
The wrapping angle and the like are controlled.

The steel strip A exiting the first cooling zone 6 then reaches the second cooling zone 7. In the second cooling zone 7, gas cooling is performed up to a steel plate temperature of about 250 to 200 ° C. In this way, finally, an extremely low carbon cold-rolled steel sheet for press forming, in which solid solution carbon is present only in the surface layer, can be obtained.
This cold-rolled steel sheet for press forming has excellent press formability and chemical conversion treatability as described in JP-B-1-42331. Then, such a cold-rolled steel sheet for press working also has excellent weldability, punchability, and slidability.

Next, specific examples will be described. An extremely low carbon steel sheet having an amount of carbon in steel C = 20 ppm and a sheet width W = 1200 mm was subjected to continuous annealing and carburization in the continuous annealing furnace of FIG. 1 according to the temperature history of FIG. (A), (b), (c) and (d) in the temperature history of FIG. 3 are (a), (b) and (b) of FIG. 1, respectively.
It corresponds to the steel strip temperature at each point in (c) and (d). Specifically, FIG. 3A is a temperature range in the plate temperature control zone, FIG. 3B is a temperature zone in the carburizing zone, FIG. 3C is a temperature zone in the first cooling zone, and FIG.
Indicate the first cooling zone outlet side temperature regions, respectively.

In this continuous annealing furnace, the annealing target plate temperature in the heating zone 2 and the soaking zone 3 is 780 ° C., and the carburizing target plate temperature in the plate temperature adjusting zone 4 and the carburizing zone 5 is 78.
C and the carburizing gas composition in the carburizing zone 5 is CO =
5 vol%, H ₂ = 3 vol%, N ₂ = 91 vol%,
The balance = 1 vol% and continuous annealing and carburization were performed at a steel strip feeding rate of 150 mpm.

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

As described above, the continuous annealing furnace according to the present embodiment
Therefore, cold-rolled steel sheets with excellent press formability and chemical conversion treatability are connected.
It can be provided continuously. In the above example, carburization
The case has been explained.
A nitrification zone may be provided. Also, by changing the atmosphere
The same furnace can be used for carburizing and nitrifying separately. Nitrifying
As the atmosphere, for example, NH₃Containing (N ₂+
H₂) It is sufficient to use gas or other mixed gas
It The carburized zone of the present invention is not only carburized but also carbonitrided.
May be performed.

Further, although continuous annealing of the cold-rolled steel sheet of ultra-low carbon steel has been described in the above-mentioned embodiments, the present invention is not limited to this, and other low-carbon steel such as low-carbon-rimmed steel and low-carbon-aluminum-killed steel can be used. It can also be applied to steel types. Furthermore, in this embodiment, the plate temperature adjusting zone is provided between the soaking zone and the carburizing zone, but the soaking zone and the sheet temperature adjusting zone are formed in the same furnace, and the soaking zone is omitted. It is also possible to respectively provide a plate temperature adjusting zone between the tropical zone 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 made into one cooling zone.

[0030]

As described above, according to the carburizing and nitriding treatment facility in the continuous annealing furnace of the present invention, the temperature of the steel strip is carburized or nitrided before the carburizing / nitriding zone by the plate temperature adjusting zone. Since it can be adjusted to a temperature suitable for the above, it stabilizes the carburizing / nitriding depth and the carbon concentration and nitrogen concentration in the outermost surface layer to a predetermined value by preventing the variation of the plate temperature at least in the longitudinal direction of the steel strip. Further, by providing a temperature adjusting mechanism in the plate width direction in the plate temperature adjusting band, it is possible to prevent the plate temperature from varying in the plate width direction and to make the surface carburized layer uniform. ..

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an example of a carburizing / nitriding treatment facility in a continuous annealing furnace of the present invention.

FIG. 2 is a functional configuration diagram of a plate temperature adjusting zone in a carburizing / nitriding treatment facility in the continuous annealing furnace of FIG.

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

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

FIG. 5 is an explanatory diagram of plate temperature variation in a steel strip longitudinal direction at a steel plate seam portion of a steel strip in continuous annealing.

[Explanation of symbols]

 2 is heating zone 3 is soaking zone 4 is plate temperature adjusting zone 5 is carburizing zone 10 is inlet side plate temperature distribution sensor 11 is outlet side plate temperature distribution sensor 12 is gas jet cooling device A is steel strip

Claims (2)

[Claims] 1. A heating zone or heating zone for heating a cold-rolled steel strip that is continuously fed and a soaking zone, and a cooling zone for cooling the steel strip. In a carburizing / nitriding treatment facility in a continuous annealing furnace equipped with a carburizing / nitriding zone for continuously carburizing or nitrifying a steel strip, the temperature of the steel strip is changed to carburizing or nitrifying in front of the carburizing / nitriding zone. A carburizing and nitriding treatment facility in a continuous annealing furnace, which is provided with a plate temperature adjusting zone for adjusting to an appropriate temperature. 2. The plate temperature adjusting zone is a plate temperature measuring device including a temperature distribution in the width direction of the steel strip, a plate width temperature adjusting device capable of adjusting the temperature by dividing the steel strip in the width direction, and the plate. A carburizing and nitriding treatment facility 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.