JPH06192747A - Continuous annealing equipment for metal strip - Google Patents

Abstract

PURPOSE:To provide continuous annealing equipment, in which a high tensile strength steel strip having ultra deep drawability while keeping the tensile strength can be manufactured, by tempering the steel strip after decarburize- cooling. CONSTITUTION:By annealing at the recrystallizing temp. or higher to the strip A in the annealing zone constituted with a heating zone 6 and soaking zone 3, the ductility El and the deep drawability (r) value are improved, and the carburization is applied at the recrystallizing temp. or lower in a carburizing zone 4. Further, the strip A is cooled to the prescribed cooling temp. at the prescribed cooling velocity in a first cooling zone 5, and by existence of the solid solution C to only the surface layer of the strip A, the tensile strength TS is improved. By applying tempering at 250-550 deg.C holding temp. in the strip temp. holding zone 6, in the condition of holding the lowering quantity in tensile strength TS of the strip A to a little, the ductility El and the deep drawability (r) value lowered by the carburize-cooling is greatly recovered to constitute the whole process.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous annealing equipment for metal strips, for example, a strip made of ultra-low carbon steel is passed from an annealing furnace to a carburizing furnace to continuously perform annealing carburization. It is suitable for producing a high-tensile steel plate having an ultra deep drawability.

[0002]

2. Description of the Related Art In the metal secondary processing industry such as the automobile industry, it is required for a metal plate to be processed to have both high workability and strength. Specifically, in the automobile industry, it is necessary to reduce the weight of the vehicle body in order to pursue low fuel consumption due to the global environmental problems that have recently become a problem, so a steel plate with higher strength while maintaining the conventional deep drawability Is required.

As an evaluation index of such a metal plate, for example, ductility, deep drawability, aging property, strength, secondary work brittleness, bake hardenability, spot weldability and the like are considered. Therefore, when the deep drawability is evaluated with a Rank Ford value (hereinafter r value: metal plate width strain / plate thickness strain) with particular emphasis on the deep drawability, carbon in steel (hereinafter referred to as C) ) Is known to be most advantageous, and in addition, due to this low carbonization, ductility (Elongation: El) and aging (Agin
g Index: AI) is also improved. On the other hand, on the other hand, as the amount of C in the steel decreases, the other evaluation indices largely deteriorate. For example, the tensile strength (tensile strength, Tensile Strength: T
S) is lowered and the grain boundary strength is lowered, so that the secondary work embrittlement is deteriorated, and the amount of solid solution C is lowered, so that the bake hardenability is deteriorated. Further, when the C content in the steel is 50 ppm or less, the grain growth rate is accelerated by heating by welding, and the heat affected zone (Heat Affecte)
Spot weldability deteriorates due to the coarsening of the d Zone (HAZ).

Therefore, as shown in FIG. 1, a metal strip made of an ultra-low carbon steel is recrystallized by a continuous annealing treatment to obtain the above-mentioned ductility, deep drawing property and aging property. By the presence of solid solution C in the surface layer by the treatment, the tensile strength, secondary work embrittlement, BH property,
In order to improve the spot weldability, the present applicant has developed a continuous annealing carburizing equipment described in JP-A-4-88126 as shown in FIG.

According to this continuous annealing carburizing equipment, the heating zone 2
Alternatively, after performing a predetermined recrystallization annealing on the metal strip (strip A) in the soaking zone 3, in the carburizing zone 4, the steel plate temperature, atmosphere specifications, transfer speed (in-furnace time), and cooling conditions are controlled. By carrying out the carburizing treatment, it is possible to continuously manufacture the metal strip having the desired surface carburizing depth and concentration distribution while satisfying the material specifications of the metal strip.

The first and second cooling zones 5 and 7 for cooling the metal zone carburized in the carburizing zone 4 are very important,
It is essential for the actual continuous carburizing operation of such strips. In these cooling zones 5 and 7, diffusion of the solid solution C carburized by quenching the metal zone is stopped, and the solid solution C is fixed only in the extremely thin area of the surface of the surface zone of the metal zone. Therefore, for example, in the continuous annealing carburizing equipment described in JP-A-4-88126, the strip after carburizing has a plate temperature of 600 ° C. or less, preferably about 500 to 400 ° C. in the first cooling zone 5. It is rapidly cooled at a cooling rate of 20 ° C./sec or more, and the steel plate temperature is 2 in the second cooling zone 7.
Gas cooling is performed until the temperature reaches about 50 to 200 ° C. On the other hand, by controlling the cooling rate and the temperature reached by cooling,
High bake hardening type steel plate with low aging at room temperature (low AI-high B)
It has been found that H steel plates can also be manufactured.

[0007]

By the way, in the continuous annealing and carburizing equipment in which the continuous carburizing equipment is added to the continuous annealing equipment, it is certainly possible to manufacture a high-strength steel sheet having a deep drawability effective for press working. On the other hand, however, in order to further reduce the weight and reduce the cost associated with the reduction of materials, stable supply of high-tensile steel sheet having ultra-deep drawability in pursuit of deep drawability is required.

However, in order to increase the strength typified by tensile strength (TS), for example, the amount of increase in solid solution C due to carburization may be increased. However, if the amount of solid solution C is increased, deep drawability (r value) is obtained. And press workability represented by ductility (El) deteriorates. Therefore, there is an urgent need in the industry for the development of a continuous annealing carburizing equipment that solves these conflicting problems.

Further, in the actual setting of various conditions of such a continuous annealing and carburizing equipment, the following problems have been found out. (1) Regarding the carburizing rate, according to a report by Ha et al. (Youn Ha, Shiro Haruyama et al .: Journal of Japan Institute of Metals 49 (1985) 7,529),
As shown in Fig. 3, when the amount of C in the metal surface layer is high to some extent and the carburizing time is long, the carburizing speed is such that after the C concentration in the surface layer reaches the equilibrium concentration (that is, the equilibrium concentration), C is the metal structure. Since it is proportional to the rate of diffusion inside, it is usually proportional to the square root of time, and this time carburizing gain area is called the diffusion rate-controlling area. On the other hand, as described above, the amount of C in the metal surface layer is When the carburizing time is extremely low and the carburizing time is extremely short, the C concentration in the surface layer portion does not reach the equilibrium concentration, so that the carburizing rate is proportional to the rate at which the metal surface layer portion and carbon directly react. It is known that the time carburization gain region is called the surface reaction rate-determining region.

Therefore, for example, from the specifications required for the metal for improving the secondary processing brittleness resistance (Japanese Patent Laid-Open No.
When the carburizing conditions for the metal band are determined, both the carburizing concentration and the carburizing depth are extremely small. In this case, it is necessary to perform carburizing treatment in the surface reaction rate-controlling region.
It was found that the carbon potential (C potential) control based on the conventional management of CO / CO ₂ etc. cannot control the amount of carburizing to the metal strip, which is considered to be in the state where the equilibrium carbon concentration in the steel of the metal strip surface is in the same state. did. (2) Further, generally, the atmospheric gas composition under the carburizing condition can be obtained by chemical equilibrium. In the conventional solution method, all possible reactions are listed, and the gas composition is obtained by solving the nonlinear simultaneous equations from the equilibrium relationship of these reactions. However, it is extremely difficult to find the exact limit of sooting (sooting) from the reaction equation of the gas phase system. (3) Further, regarding the above-mentioned surface reaction rate, there is a report by Leaf et al., But in this report, only the carburizing rate in CO gas is discussed. The actual deployment of operations cannot be done.

The present invention was developed in view of these problems, and the metal strip after carburizing and cooling is heated to 250 to 550 ° C.
The same tensile strength (T
It is an object of the present invention to provide a continuous annealing equipment capable of producing a high-strength steel sheet having S) and a super deep drawability having a high deep drawability (El, r value).

[0012]

Means for Solving the Problems The inventors of the present invention have made extensive studies on the above problems, and as a result, have developed the present invention based on the following findings. That is, while extensively studying the carburizing conditions of the metal zone in the surface reaction rate-controlling region, it is possible to control the carburizing state represented by the state of solid solution C also by the temperature history (history) after carburizing. Focused on what can be done. Then, after the cooling step of specifically controlling the diffusion of the solid solution C and fixing the solid solution C, by holding the metal strip at a predetermined plate temperature, the effect of tempering occurs, and the carburizing cooling reduces the effect. It has been found that it is possible to recover the ductility (El) and deep drawability (r value).

Thus, the continuous annealing equipment for metal strips of the present invention is
Of a metal strip having a carburizing zone for carburizing a continuously fed metal strip and a cooling zone for cooling the metal strip continuously delivered from the carburizing zone at a predetermined cooling rate to a predetermined cooling temperature In the continuous annealing equipment, a plate temperature holding zone for holding the metal zone continuously fed from the cooling zone at a holding temperature of 250 ° C. to 550 ° C. is provided.

[0014]

In the continuous annealing equipment for metal strips of the present invention, for example, a metal strip continuously fed into a heating zone or an annealing zone having a soaking zone is annealed under a plate temperature condition of a recrystallization temperature or higher. By doing so, for example, the crystal orientation of the strip made of ultra-low carbon steel is developed to exhibit sufficient ductility and deep drawability. Then, for example, when the metal strip is continuously fed from the annealing zone into the carburizing zone, the metal strip delivered from the annealing zone is forcibly cooled or allowed to cool and heated as necessary. Alternatively, when the annealed metal strip is completely cooled by winding, etc., it may be heated as necessary to reheat the metal strip continuously fed into the carburized strip. Carburizing under the plate temperature condition below the crystallization temperature suppresses the diffusion of solid solution C into the inner zone of the metal zone and increases only the concentration of the surface zone of the metal zone. A large amount of solid solution C is allowed to exist at the grain boundaries by suppressing the incorporation of solid solution C. Further, by continuously feeding the metal strip sent from the carburizing zone into the cooling zone, and cooling the metal zone to a predetermined ultimate cooling temperature at a predetermined cooling rate in the cooling zone, the solid solution C The diffusion is stopped and a large amount of solid solution C is fixed only to the surface portion of the metal strip, and the strength represented by the tensile strength TS is improved. Then, for example, when the metal strip is continuously fed from the cooling zone to the plate temperature holding zone, the metal strip delivered from the cooling zone is forcibly cooled or allowed to cool, and if necessary. After heating, the metal strip is heated to 250 ° C in the plate temperature holding zone.
By holding at a holding temperature of 550 ° C, substantial tempering is performed, and the ductility and deep drawability reduced by carburizing in the carburizing zone or cooling in the cooling zone are recovered with the tensile strength as it is or with a small reduction amount. It is possible to manufacture a high-strength steel sheet having an ultra deep drawability.

[0015]

EXAMPLE FIG. 4 shows an example of the continuous annealing equipment for metal strips according to the present invention, which is an example of continuous annealing carburizing equipment for strips made of ultra-low carbon steel. In the figure, the ultra-low carbon steel strip A is a coil rewinding machine, a welding machine, a washing machine, etc.
The heating zone 2, the soaking zone 3, the carburizing zone 4, the first cooling zone 5, the plate temperature retaining zone 6, the second cooling zone 7, the shearing machine, the winding machine, etc. Thus, the plate temperature history / history shown in FIG. 5 is performed.

As shown in FIG. 5, the heating zone 2 is for continuously heating the strip A preheated in the preheat zone 1 by continuously feeding it from the inlet side equipment to a temperature higher than the recrystallization temperature. Heats the strip so that the temperature in the furnace is 850 to 1000 ° C and the temperature of the strip A is 700 to 950 ° C. Then, the heated strip A is maintained at the recrystallization temperature or higher for the required time in the soaking zone 3 to develop a {1,1,1} texture advantageous for deep drawability.

A large number of radiant tubes are arranged in the vicinity of the strip passage of the strip which is vertically passed through the heating zone 2 and the soaking zone 3 through a hearth roll. The temperature of the furnace (furnace temperature) is controlled by burning the fuel gas fed to the furnace. In the present invention, the setting of the supply flow rate of the fuel gas is required for the furnace determined from the heat balance in the furnace, which is obtained by adding the exhaust gas loss heat, the heat dissipated into the furnace, etc. (Necessary) It is equivalent to the amount of heat, and is performed by a host computer (not shown) according to the control logic of the entire line described later.

In order to form a carburized layer in which solid solution carbon (C) exists in the extremely thin portion (surface layer portion) of the surface of the strip A, the carburized zone 4 is formed in the carburized zone 4 as shown in FIG. The carburizing furnace is 700-9 by a host computer (not shown).
The strip temperature (plate temperature) is controlled by controlling the temperature in the furnace at 50 ° C.
Is 700 ° C. or higher, preferably the recrystallization temperature or lower, and the strip running speed is controlled so that the strip passes through the carburizing furnace in 10 to 120 seconds. As the atmosphere plasticity in the carburizing furnace, for example, a mixed atmosphere of CO + H ₂ + N _{2 having} a CO concentration of 25% or less and an H ₂ concentration of 30% or less is used. By the way, the furnace temperature control is performed in order to keep the carburizing amount (carburizing reaction rate) constant in the strip passing direction to suppress variations in materials. Also, as is known, if sooting, that is, if free carbon [C] is attached to the surface of the steel sheet, it becomes a factor of deteriorating the surface quality and adversely affecting the subsequent steps. At the same time, the reaction in the furnace is accelerated in a predetermined direction, for example, the carburizing reaction direction. As a result, if the dew point rises, the carburizing reaction will be hindered, or the strip surface will be oxidized to cause temper coloration. The temperature inside the furnace and the temperature inside the furnace are importantly controlled based on the carburizing condition setting logic described in Japanese Patent Application No. 4-320891 previously proposed by the applicant.

In the present embodiment, the strip in the carburizing furnace is passed through the hearth rolls 10 while moving up and down in the furnace. These hearth rolls 10 maintain their rotatability and roll crown in a predetermined state. Therefore, for example, the vicinity of the bearing is cooled. Also, in order to maintain the strength and abrasion resistance of the roll itself, chromium is used for the hearth roll.
Alloys are used. However, when the carburizing atmosphere gas reaches the vicinity of the hearth roll and is cooled, sooting proceeds, so that C adheres to the hearth roll and then C diffuses inside the hearth roll. In this case, C
When r and C are combined, Cr carbide is precipitated, which destroys or expands the crystal grains of the heat-resistant alloy used for the hearth roll, while the solid solution Cr decreases, so the hearth roll becomes brittle. Porous corrosion progresses by being oxidized. When the hearth roll is exposed to the carburizing atmosphere gas in this manner, experiments by the present inventors have revealed that the hearth roll must be replaced within two years. Therefore, in this embodiment, the hearth roll chamber is separated from the carburizing atmosphere by the non-contact sealing device 11 so as to prevent the deterioration of the hearth roll, and the hearth roll chamber is so weak that the deterioration of the hearth roll does not proceed. By making the carburized state, it has succeeded in preventing so-called decarburization, in which C is diffused from the carburized surface layer portion while the strip passes through the separated hearth roll chamber. In addition, the time that the strip passes through the hearth roll chamber is extremely short,
When decarburization from the steel sheet surface layer portion for the time concerned does not pose a problem, the hearth roll chamber may be set to a non-carburizing atmosphere.

Although the structure of the sealing device 11 will not be described in detail here, for example, the sealing layer interposed between the hearth roll chamber and the carburizing atmosphere chamber has a three-layer structure, of which the seal on the hearth roll chamber side is formed. The weak carburizing atmosphere gas is ejected to the layer, the carburizing atmosphere gas is ejected to the seal layer on the carburizing atmosphere chamber side, and exhaust is performed from the intermediate seal layer. The flow rate is controlled so that the flow of each atmospheric gas is directed to the intermediate seal layer side, and the circulation flow generated by the plate laminar flow accompanying the passage of the strip is formed on the end face in the width direction of the strip of the seal layer. The air is exhausted from the exhaust port.

Strip A delivered from this carburizing zone 4
Is fed to the first cooling zone 5. In this first cooling zone 5, the solid solution C is fixed only in the extremely thin area of the surface layer of the strip, so that the strip after carburizing has a steel sheet temperature of 600 ° C. or lower, preferably 500 ° C. as shown in FIG. ~
It is rapidly cooled at a cooling rate of 20 ° C / sec. Or more until it reaches about 400 ° C. In order to achieve this cooling condition in the first cooling zone 5, the flow rate, flow velocity and cooling roll of the sprayed cooling gas blown from the cooling gas jet to the strip conveyed in the cooling zone by the host computer. The temperature, winding angle, etc. are controlled.

The metal strip delivered from the first cooling zone 5 is delivered to the plate temperature holding zone 6. This plate temperature holding zone 6 is shown in FIG.
In order to recover the ductility (El) and deep drawability (r value) that have been reduced by the carburizing and cooling in the carburizing zone 4 and the first cooling zone 5, as shown in Fig. This is for improving the deep drawability while maintaining the temperature and performing so-called tempering, while maintaining the high tension obtained by the carburizing and cooling.

Inside the plate temperature holding zone 6, the hearth roll 12
A large number of radiant tubes and cooling gas jet nozzles are arranged in the vicinity of the strip passage that is passed through the strip while moving up and down through the radiant tube. The supplied fuel gas is burned to control the temperature in the furnace (furnace temperature) to enable heating or soaking control of the plate temperature, and to control the flow rate of the cooling gas blown from the cooling gas jet nozzle to the strip surface. It is possible to control and cool the plate temperature. In the present invention, when the ultimate cooling temperature in the first cooling zone 5 is equal to or higher than the predetermined holding temperature, the metal strip is set to the predetermined holding temperature by cooling or forced cooling by the cooling gas jet nozzle, When the ultimate cooling temperature in the first cooling zone 5 is equal to or lower than the predetermined holding temperature, the radiant tube 1
It is applicable to any case where the metal strip is heated by 2 to the predetermined holding temperature.

The strip A fed from the plate temperature holding zone 6 is then fed to 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, an extremely low carbon cold-rolled steel sheet for press forming can be finally obtained in which solid solution C exists only in the surface layer portion. Next, the case where only the carburizing was performed on the metal strip by the continuous annealing equipment of the present example (including the cooling step),
The difference in material properties of the obtained steel sheet between the case of tempering by holding the sheet at the predetermined temperature T ° C. for the predetermined time tsec after the carburizing cooling will be described with reference to FIG. 6. As is clear from the figure, the tensile strength TS decreases when tempered, but the decrease amount is relatively small.
The ductility El and the deep drawability r value greatly increase, and the tendency becomes remarkable as the carburizing amount increases. This means that, conversely, while a high carburizing amount obtained in the carburizing step is fixed in the cooling step to obtain a large tensile strength, a large recovery of ductility and deep drawability can be obtained in the tempering step. It is possible to manufacture a high-strength steel sheet having an ultra-deep drawability, which is an object of the present invention, and to stably supply the steel sheet.

In this embodiment, only the case where the history of the plate temperature is changed as shown by the solid line in FIG. 5 has been described in detail. However, for example, as shown by the phantom line in FIG. The same can be applied to the case where the strip is reheated to carry out the carburizing or tempering.

[0026]

As described above, according to the continuous annealing equipment for metal strips of the present invention, the metal strip delivered from the cooling zone continuous from the carburizing zone is heated to a predetermined temperature of 250 ° C to 550 ° C in the plate temperature holding zone. Since it was possible to temper the metal strip while maintaining the temperature, the tensile strength obtained in the carburizing process and the cooling process remained unchanged, and the ductility and deep drawability decreased in those processes were greatly recovered. It enables production of high-tensile steel sheet having ultra-deep drawability and stable supply thereof.

[Brief description of drawings]

FIG. 1 is a conceptual explanatory view of a heat treatment process performed in a continuous annealing carburizing facility.

FIG. 2 is a schematic configuration diagram showing an example of continuous annealing equipment for implementing the heat treatment step of FIG.

FIG. 3 is an explanatory diagram of a diffusion rate-determining region after the carbon concentration in the surface zone of the metal band reaches the equilibrium concentration and a surface reaction-limiting region before reaching the equilibrium concentration.

FIG. 4 is a schematic diagram showing an embodiment of the continuous annealing equipment of the present invention.

5 is an explanatory diagram of temperature history / history of plate temperature control performed in the continuous annealing equipment of FIG.

6 is a characteristic explanatory view of tensile strength, ductility, and deep drawability of the metal strip or steel plate obtained by the continuous annealing equipment of FIG.

[Explanation of symbols]

 1 is pre-tropical zone 2 is heating zone 3 is soaking zone 4 is carburizing zone 5 is first cooling zone 6 is plate temperature keeping zone 7 is second cooling zone A is strip

(72) Inventor Hiroshi Kuramoto 1-chome, Mizushima Kawasaki-dori, Kurashiki-shi, Okayama (without street number) Inside the Mizushima Steel Works, Kawasaki Steel Co., Ltd. No address) Mizusaki Steel Works, Kawasaki Steel Co., Ltd. (72) Inventor Susumu Okada 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Co., Ltd.

Claims (1)

[Claims] 1. A carburizing zone for carburizing a continuously fed metal strip, and a cooling zone for cooling a metal strip continuously fed from the carburizing zone at a predetermined cooling rate to a predetermined cooling temperature. In the continuous annealing equipment for metal strips, a continuous metal strip is provided, which is provided with a plate temperature holding zone for holding the metal strip continuously fed from the cooling zone at a holding temperature of 250 ° C to 550 ° C. Annealing equipment.