JPS6254035A - Continuous heat treatment of steel strip - Google Patents

Abstract

PURPOSE:To effect continuous heat treatment of a steel strip without forming an oxide film, etc., by heating the steel strip respectively by non-reduction type heating burners in the heating region on the inlet side of each burner of a direct firing heating zone and by reduction type heating burners which can form non-equil. regions in flames. CONSTITUTION:The steel strip S is subjected to the continuous heat treatment consisting in heating in the direct firing heating zone, holding for >=5sec in a radiation tube type holding zone, then quick cooling at a cooling rate of >=40 deg.C/min. Part of the heating in the direct firing heating zone is executed by the non-reduction type heating burners, i.e., the heating burners capable of forming the non-equil. regions where an intermediate product of combustion exists and free O2 does not exist, in the direct firing heating zone. The steel strip is heated by the reduction type heating burners in the heating region on the outlet side of the direct firing heating zone. The steel strip is partly oxidized by the heating in the heating region on the inlet side of each region by adopting such direct firing heating method, but the oxide films are reduced by the reduction heating in the heating regions on the outlet side. The steel strip is thus delivered in a non-oxidizing state from each pass to the holding zone of the next pass or succeeding holding zone.

Description

[Detailed description of the invention] [Industrial application fields] The present invention relates to a continuous heat treatment method for 11 years.

[Prior art and its problems] Conventionally, in the heating furnace of continuous annealing equipment, an indirect heating method using a radiant tube has been generally adopted in order to prevent steel strip oxidation. However, although this indirect heating method does not have the problem of oxidation of the steel strip unlike the general direct heating method, it does have the following specific problems.

b) Because the furnace temperature is low, at a maximum of 900°C, the furnace length must be increased to ensure the necessary heating capacity, making equipment expensive.

口) Due to indirect heating, the thermal response is poor, and therefore it is not possible to respond quickly to changes in the size of the steel strip or changes in the processing heat cycle.

c) It is necessary to wash and remove cold rolling oil and the like adhering to the surface of the cold rolled steel strip before heat treatment, which increases equipment costs and operating costs.

In contrast to this indirect heating method, a so-called non-oxidizing direct flame heating method is proposed in Japanese Patent Publication No. 58-44133 and Japanese Patent Publication No. 59-1989.
No. 29651 etc., it was proposed for continuous heat treatment equipment for cold rolled steel strips, and it is also widely used in hot-dip galvanizing lines and continuous annealing lines for electrical steel sheets. This method is based on the strip temperature (Max.

As the temperature rises (900℃), the air ratio in each combustion control zone is reduced (from less than 1.4 to 0.6) to suppress oxidation of the steel strip while heating.

Since it is an open flame method, it has the advantage of avoiding the above-mentioned problems (a) to (c).

However, although this method is said to be non-oxidizing, it is actually a slight oxidizing method, and even in the combustion generated gas with an air ratio of less than 1.0, there is CO2, which is an oxidizing gas. Because it contains a large amount of H2O, the thickness of the oxide film after heating varies from less than 50A on the original plate to 500A on the original plate.
-The number of people will increase to 1,000. Therefore, this method also has the following drawbacks.

b) A phenomenon in which the oxide film moves and grows on the roll surface (roll pickup) occurs, damaging the roll or strip surface.

After heating with direct flame, it is necessary to completely reduce the surface with H2-containing atmospheric gas, but since the oxide film is thick, it is necessary to reduce the surface with a high concentration of H2
2 is required, and the atmosphere is significantly contaminated.

c) Since the emissivity of the strip surface fluctuates greatly due to the oxide film, it is difficult to measure the heating temperature using a radiation thermometer and control combustion in the direct-fired furnace based on this measurement.

[Means for Solving the Problems] In view of these conventional problems, the present invention aims to provide a method that can appropriately and continuously heat-treat a steel strip without causing problems due to oxide films or the like. It is something.

For this reason, the present invention provides continuous heat treatment in which the steel strip is heated in a direct flame heating zone consisting of one pass or two or more passes, then soaked in a soaking zone for 5 seconds or more, and then rapidly cooled at a cooling rate of 40°C/second or more. At the same time, in the direct-fired heating zone, the inlet heating area of each pass is heated by a non-reducing heating burner, and the outlet heating area of each pass contains combustion intermediate products and does not contain free oxygen. Its basic feature is that the non-equilibrium region is heated by a reduction type heating burner that can be formed in a flame.

It is known that the combustion gas generated by burning a heating burner at an air ratio of 0.8 to 0.95 is oxidizing to the surface of the steel strip in equilibrium theory. However, after repeated studies by the present inventors, we found that if rapid combustion is performed by selecting a burner structure, the generated combustion gas will remain between 430 and 90% for a short time after combustion ends.
It has been found that it exhibits a strong reducing power on the surface of the steel strip at 0°C. The cause of this reducing power is due to intermediate reaction products (intermediate ions, radicals, etc.) that exist for a very short time after combustion before becoming an equilibrium mixed gas such as H2, Go, H20°CO□, N2, etc., and are caused by rapid combustion. As a result, a non-equilibrium region in which no free oxygen remains in the flame and in which the intermediate reaction products are present is formed in a manner that is clearly distinguishable from the equilibrium gas region.

For this reason, in the method of the present invention, when heating the steel strip in the direct-fired heating zone, part of the heating is performed using the reduction type heating burner, that is, in the non-equilibrium region where combustion intermediate products exist and free oxygen does not exist. It is carried out with a heating burner that can form.

In such a heating burner, completely burned Co, H2O, N2, . While the region containing H2CO, etc., that is, the quasi-equilibrium region, is oxidizing, the non-equilibrium region is reducing, and by colliding this flame with the rA band in the non-equilibrium region, the steel strip can be heated without oxidizing it. Can be heated. This region has enough reducing power to remove the oxide film of about 300 people within one second. FIG. 3 shows an example of measurement using an ion detection probe of the non-equilibrium region in the flame formed by such a heating burner.
A high current value measured by the probe means that the ionic strength is high and therefore a large amount of combustion intermediate products are present. According to this, a non-equilibrium region is formed over a predetermined range outside the burner outlet, and the outside of the non-equilibrium region is composed of Co2. It is a quasi-equilibrium region containing H2O, N2, etc.

The present invention will be explained in more detail below. A steel strip is directly heated to a predetermined temperature in one or more direct-fired heating zones, but in the present invention, in one or more passes, the heating area on the entrance side of the pass (the entire heating area of the pass is A conventionally commonly used non-reducing heating burner (wave number type burner) is used in the input side heating area when divided into two at an arbitrary position, and on the other hand, the exit side heating area of the pass (the entire heating area of the pass is divided into two at an arbitrary position) is used. In the exit side heating area (divided into two), the steel strip is heated using the reduction type heating burner.

When the direct fire heating zone consists of multiple passes, heating is performed in the manner described above for each pass. By adopting such a direct flame heating method, the steel strip is oxidized to a certain extent by heating in the inlet heating area of each pass, but the oxide film is reduced by reductive heating in the outlet heating area, and the steel strip is heated from each pass to the next. pass or subsequent soaking zone in an unoxidized state.

Heating with a reduction type heating burner as described above can heat the steel strip in a non-oxidizing state from beginning to end by performing heating in the entire heating region of the direct heating method pass, but this type of burner is generally not used. It has a smaller heat capacity than a non-reducing burner (diffusion type burner), and if the entire heating area is to be heated with this burner, a sufficient amount of heat cannot be secured unless a large number of burners are used. On the other hand, roll pickup due to steel f oxidation can be prevented if the steel strip is in a non-oxidized state immediately before the exit-side passing roll of the pass that constitutes the direct-fired heating zone. For this reason, in the present invention, each pass constituting the direct-fired heating zone is heated by a non-reducing heating burner with a large heat capacity in the inlet side heating area, thereby ensuring a sufficient amount of heat, and in the outlet side heating area. The oxide film formed on the surface of the steel strip in the inlet heating region is reduced by a reduction heating burner, and the steel strip is sent to the next pass or soaking zone in an unoxidized state.

The steel strip is maintained in a predetermined temperature range for 5 seconds or more in the subsequent soaking zone. In steel strips, grain nuclei are generated and grain growth begins when the recrystallization temperature is exceeded in the latter half of the heating zone, but the soaking time described above is long enough for these grains to grow to a predetermined grain size. This is the minimum time required.

Furthermore, the steel strip heated and soaked in this way is maintained at a predetermined temperature as necessary, and then placed in a quenching zone for 40 minutes.
It is rapidly cooled at a cooling rate of ℃/second or more. In order to improve the aging properties of products, it is necessary to precipitate [C] dissolved in the soaking zone as quickly as possible in the overaging zone following rapid cooling, and the above cooling rate is set to a supersaturated state in order to achieve this. This is necessary to create a state in which [C] is dissolved in solid solution.

That is, the faster the cooling rate is, the higher the degree of supersaturation of the solid solution EC is, and the shorter the overaging treatment time is required, so the minimum cooling rate is regulated.

The steel strip that has undergone a series of heat treatments as described above is then subjected to overaging treatment, final cooling, etc., as required, and is made into a product. Generally, overaging treatment is not required when Ti and Nb are added, but it goes without saying that the present invention can also be applied to heat treatment in such cases.

In addition, heating with a reduction type heating burner in a direct-fired heating furnace is
The purpose is to heat the steel strip by a non-equilibrium region formed in the longitudinal middle of the burner flame, and for this purpose, the flame is applied almost perpendicularly to the surface of the steel strip, and so as to collide in the non-equilibrium region. .

[Example] An embodiment of the present invention will be described below with reference to FIG.

Figure 1 shows the equipment used for carrying out the present invention.
a) Preheating zone, (b) Direct fire heating zone (1 pass), (C)
is a radiation tube type heating soaking zone, (d,') is a gas jet cooling zone, (e) is a roll cooling zone, (f) is an overaging treatment zone,
(g) is the final cooling zone, and in this example, the steel strip consists of preheating, direct flame heating, radiation tube type (heating) soaking, slow cooling by gas jet cooling, rapid cooling by roll cooling, overaging treatment, and final cooling. A series of heat treatments called cooling are performed. The specific contents are as follows.

(1) Preheating zone: 1200 to 1400 from the direct fire preheating zone
The cooled steel strip is preheated to 250-330°C using high temperature combustion exhaust gas at 250°C to 330°C.

(2) Direct-fired heating zone: After preheating, the cooling zone is reductively heated to 430 to 800°C using a direct-fired heating burner. The direct heating burner is divided into 3 to 6 groups, and the first and second groups on the outlet side are heated by the above-mentioned reducing burners, and the remaining inlet zones are heated by conventional non-reducing burners. The temperature of the gas produced after combustion in all burners is 1300 to 1600°C, and this high temperature gas is blown perpendicularly to the steel strip from a distance of 200 to 500 mm at a speed of 5 to 20 m/s.

Figure 2 shows the preheating area and direct heat heating. Oxide film thickness on the surface of the steel strip passing through the strip This shows the transition of the steel strip. At the outlet of the open heating zone consisting of It can be seen that it has been reduced to minutes.

(3) Radiant tube heating soaking area ; 900°C for heating in an open heating zone Since there is an upper limit of If it is necessary to heat the conduct. On the other hand, the steel strip reaches its heating limit. After that, it is heated in a weakly reducing atmosphere for 5~ Perform chest heat for about 120 seconds.

(4) Gas jet cooling zone : The steel strip in the soaking zone is cooled by subsequent rolls. Immediate rapid cooling start temperature (550 -750°C).

(5) Roll cooling zone: The steel strip is brought into contact with a water-cooled roll and quenched from 250 to 400°C at a high speed of 40°C/S or higher to perform quenching.

(6) Overaging treatment zone: Overaging treatment is performed by holding the temperature in the range of 400°C to 150°C for 30 seconds or more.

(7) Final cooling zone : Steel strip after overaging treatment is heated to 150℃ or higher. It is cooled to the bottom and released into the atmosphere.

Next, an example of the reduction type burner used in the present invention shown in FIGS. 4 and 5 will be described.

The illustrated heating burner has a plurality of combustion air discharge holes (2) provided at intervals in the circumferential direction on the inner wall (6) of a cylindrical burner tile (1), and a fuel gas Discharge hole (3) An angle θ of 60°C or less is attached to the tangent to the inner circumference of the burner tile.

(-) indicates the burner axial inner path 1 female N of the fuel gas discharge hole (3) and the air discharge hole (2). Reverse (+
), set it to -0.1D to +0.25D (D: burner inner diameter).

c) Distance from air discharge hole (2) to burner tile outlet (5): 4L is 0.6D to 3D.

When the heating burner configured in this way is used at an air ratio of 1.0 or less, a non-equilibrium region is formed in the flame within a predetermined range. In other words, in such a heating burner, rapid combustion is achieved by the swirling flow of combustion air from the air discharge hole (2) and the fuel gas discharged from the center of the burner, and the combustion is carried out over a predetermined range outside the burner outlet. , a region containing a large amount of combustion intermediate products and no unreacted free oxygen, that is, a non-equilibrium region is formed.

FIG. 6 shows the reductive heating characteristics of such a heating burner, that is, the limit temperature at which it can be heated without oxidation (the limit temperature for a thin plate of ordinary steel), and shows the reduction heating characteristics of such a heating burner.
It has been shown that the steel strip can be heated up to about 900° C. in the range of 95°C.

The configuration of the heating burner will be explained in more detail.

In the figure, (7) is a fuel gas nozzle protruding from the burner tile inner end wall (4), and in this embodiment, fuel gas discharge holes (3) are spaced apart in the circumferential direction of this fuel gas nozzle (7). is formed.

In such a heating burner, its air discharge hole (2)
The reason for having an air supply angle θ is to create a swirling flow in the combustion air within the burner tile.This swirling flow forms a negative pressure area inside the burner, and this negative pressure recirculates the gas. This promotes the fuel flow and allows the formation of a suitable non-equilibrium region. By setting this air supply angle i to a maximum of 60°C, preferably 20 to 40°C, a stable swirling property of the airflow can be obtained.

When the burner axial distance N between the fuel gas discharge hole (3) and the air discharge hole (2) is on the (-) side, the gas temperature is high and combustion intermediate products are also highly distributed over a wide range. , On the other hand, free 02 (unreacted 02) tends to be distributed long in the axial direction. In order to appropriately form the non-equilibrium region, it is necessary to minimize the remaining distance of the free 02 in the burner axial direction, and its limit is -0, 1D.

If N is on the (+) side, a proper non-equilibrium region will be formed, but if it becomes too large, the inner end wall of the burner tile will be heated to over 1400°C, which is undesirable, and it will be difficult to protect the SiC on the inner end wall of the burner tile. The limit is +0.25D.

FIG. 7 shows the burner axial distance from the burner outlet and the gas temperature in the burner tile when the burner axial direction N of the fuel gas discharge hole (3) and the air discharge hole (2) is -0.25D. The relationship between the 02 concentration and the ionic strength was investigated, and according to this, when N is on the (-) side, the remaining distance Mi of free 02 in the axial direction. It has been shown that there is a large presence of

Figure 8 shows the burner axial distance N between the fuel gas hole and the air discharge hole.
and the remaining distance in the axial direction of the free 02.

According to this, when N becomes larger than 10 and ID on the (-) side, Lo increases rapidly, so on the (-) side, -0 and 1D are the limits. becomes.

On the other hand, FIG. 9 shows the relationship between the burner axial distance from the burner outlet, the 0□ concentration, the ion strength, and the gas temperature when N is +0.1D. According to Figs. 8 and 9, if N is on the (+) side, there is no problem with the 0□ concentration, and there is a proper non-equilibrium region at a distance of 0.5D or more from the burner outlet. It is formed.

However, if N is increased to the (+) side, the burner tile inner end wall (4) is heated, so the relationship graph between the distance N and the temperature Tb of the burner tile inner end wall (4) in Fig. 1O. As shown in , Tb becomes 1400°C or more at +0.25D, and therefore, considering that the material of the inner end wall is SiC, it is preferable to set it to +0.25D or less in terms of the heat resistance limit. From the above, the burner center distance N between the combustion gas discharge hole and the air discharge hole is preferably in the range of -0.1D to 0.25D.

The distance giL from the air discharge hole (2) to the burner tile outlet (5) has a close relationship with the formation range of the non-equilibrium region. That is, if L exceeds 3D, the non-equilibrium region will be formed only in the portion immediately after the burner tile exit, which is not preferable. On the other hand, if L is less than 0.6D, the flame becomes a petal-shaped flame immediately after the exit of the burner tile, and an appropriate non-equilibrium region cannot be stably obtained on the burner central axis. Therefore 0.6D~3.
It is preferable to set L in the range of 0D.

When continuously heating a thin Sr4 slope, the burner tile outlet (5
) and the steel plate beyond a certain distance to the north (usually about 100 mm or more), there is a risk that ah will come into contact with the burner during threading. Therefore, it is preferable that the non-equilibrium region in the flame be formed in as wide a range as possible, including the steel strip passing position located at a predetermined distance from the burner outlet side.

FIG. 11 shows an investigation of the relationship between the distance Aft L and the distance MLR from the burner outlet to the end of the nonequilibrium region (the end on the anti-burner side, for example, point A in FIG. 9). According to this, when L exceeds 3D, the non-equilibrium region is formed only immediately after the exit of the burner tile, and is hardly formed in front of it. As L becomes smaller, the range of non-equilibrium region formation expands, but the region where L is less than 0.6D (
In X), the flame becomes a petal-shaped radial flame immediately after the exit of the burner tile, and an appropriate non-equilibrium region is not stably formed on the burner axis.

From the above, the distance F, It L from the air discharge hole (2) to the burner tile outlet (5) is 0.6D to 3. O.D.
It is desirable that it be within the range of .

Further, in the present invention, in addition to the heating burner described above, for example, a so-called radiant cup burner can be used.

In order to perform a rapid combustion reaction, this burner rapidly burns a pre-mixed gas mixture of air and fuel gas in the hemispherical recess of the burner tile, raising the temperature of the inner surface of the burner tile and heating mainly through radiant heat transfer. It has the characteristic that a high heat flux can be obtained in a region where the temperature of the heated object is high. By performing combustion in this burner at an air ratio of 1.0 or less, a non-equilibrium region is formed in the flame.

However, since this radiant burner uses a premixing method for combustion air and fuel gas, it is not possible to preheat the combustion air, and since the air cannot be preheated in this way, non-oxidation heating is limited to about 750°C. However, there are drawbacks such as inapplicability to cases where heating in a higher temperature range is required. In this regard, in the heating burner shown in Fig. 4, since preheated air can be used, it is possible to heat up to about 900°C without oxidation, and by using the preheated air in this way, the flame temperature can be increased. Compared to a radiant burner, the reduction action itself by the intermediate reaction product can also be improved in terms of effective C.

[Effects of the Invention] According to the present invention described above, a steel strip can be appropriately and economically continuously heat-treated using a direct flame heating method without causing problems such as steel strip oxidation and roll pickup caused by this. There is an effect.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of a continuous heat treatment line used in an embodiment of the present invention. FIG. 2 shows the transition of the oxide film in the preheating zone and the direct heating zone in the above example. FIG. 3 shows an example of the non-equilibrium region forming range of the reduction type heating burner used in direct fire heating according to the present invention. 4 and 5 show an example of a heating burner to be applied to the heating furnace of the present invention. FIG. 4 is a longitudinal sectional view, and FIG. 5 is a sectional view taken along the line V-■ in FIG. 4. be. FIG. 6 shows the reduction heating characteristics of the heating burner shown in FIGS. 4 and 5. FIG. 7 to 11 show the characteristics of the heating burner shown in FIGS. 4 and 5, and FIG. 7 shows the distance aN between the fuel gas 2 discharge hole and the air discharge hole in the burner axial direction. The relationship between the distance from the burner outlet and gas temperature, 0□ concentration, and ionic strength when 0.25D is shown. Figure 8 shows the relationship between the distance N between the fuel gas discharge hole and the air discharge hole in the burner axial direction and the free 02 burner. The relationship with the remaining axial distance L0 is shown in FIG. 9 when the distance 4N is +0. Distance from burner outlet and gas temperature ρ2 concentration when ID is used,
Figure 10 shows the relationship between the distance N between the fuel gas discharge hole and the air discharge hole and the burner tile rear wall temperature Tb, and Figure 11 shows the relationship between the distance from the air discharge hole and the burner outlet and the non-equilibrium region. Each shows the relationship with the distance LR to the end. Figure 2 f43 Figure g5 Zuko Hachiru Uta Figure 7 m8 Figure

Claims (1)

[Claims] After heating the steel strip in a direct flame heating zone, soak it in a soaking zone for more than 5 seconds,
Next, a continuous heat treatment is performed in which rapid cooling is performed at a cooling rate of 40 ° C./sec or more, and in the direct heating zone, heating is performed using a non-reducing heating burner in the inlet heating region of the pass, and in the outlet heating region of the pass, 1. A method for continuous heat treatment of a steel strip, characterized in that heating is performed by a reducing heating burner capable of forming a non-equilibrium region in a flame that contains combustion intermediate products and does not contain free oxygen.