JP5200406B2 - Steel strip cooling method - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a steel strip cooling method and cooling equipment when quenching a steel strip after annealing in a continuous annealing step.

  In the continuous annealing process of the steel strip, it is important to control the heat treatment conditions of heating and cooling in order to impart the mechanical properties required for the steel strip to be treated. On the other hand, when producing a high-strength steel strip in the continuous annealing process, a treatment for rapidly cooling the heated steel strip is necessary from the viewpoint of increasing the strength of the steel strip, and from the viewpoint of press formability, cooling is required. A treatment (tempering treatment) for heating the steel strip again is necessary.

  By the way, in recent years, the demand for high-strength steel strips has increased, and the importance of rapid cooling technology essential for the production of high-tensile steel plates has increased. As the rapid cooling method, there are a water quenching method, a roll cooling method, an air-water mixing (mist) cooling method, a gas jet cooling method, and the like, and these methods are appropriately selected in order to obtain a necessary material.

  Among these, the water quenching method in which the steel strip is immersed in the cooling water has the fastest cooling rate and is suitable for the production of a high-strength steel sheet because the addition of alloy elements for increasing the strength can be reduced. As a water quenching method, a method is generally used in which a heated steel strip is immersed in water, and at the same time, cooling water is jetted from a quench nozzle provided in water to the steel strip to perform rapid cooling.

  However, in the water quenching method, it is difficult to control the cooling stop temperature because of its fast cooling rate, and the steel strip is cooled to the same temperature as the water temperature while passing through the immersion bath.

  In FIG. 7, the schematic of the temperature history of the steel strip at the time of using the water quenching method in the conventional continuous annealing equipment is shown. As shown in FIG. 7, in the method using the conventional water quenching method, after the steel strip surface is spray-cooled by the quench nozzle, it is further cooled in the immersion tank, so the water temperature in the immersion tank is 35 ° C. (Quench nozzle injection speed: 3 m / s, jet water temperature: 30 ° C.), the temperature of the steel strip at the time of reaching the induction heating device is also cooled to 35 ° C. For this reason, in the overaging process after rapid cooling, the load given to the heating capability with the induction heating apparatus which reheats a steel strip will become high. On the other hand, in order to reduce the heating capacity load in the induction heating apparatus, it is conceivable to increase the cooling water temperature. However, the increase in the water temperature results in a decrease in the cooling capacity, and the required predetermined material cannot be obtained. .

  Further, when the steel strip speed is increased, the load on the induction heating device becomes remarkably large, so it is difficult to improve the productivity by increasing the steel strip speed (plate feed speed in FIG. 7: 90 m / min, Steel strip thickness: 1.6 mm, TS (tensile strength): 990 MPa, El (elongation): 17.4%.

  As a method for reducing the load of the reheating capacity while maintaining the fast cooling rate of the water quenching method, there is a technique disclosed in Patent Document 1. In order to rapidly cool the steel strip, it is cooled by air spray, and immediately after that, it is immersed in water as in the prior art. Furthermore, a plurality of heating rolls are provided in the immersion tank, and heating is performed simultaneously with cooling, and the temperature of the steel strip on the exit side of the immersion tank is increased, aiming to reduce the load of the reheating capacity. .

In Patent Document 2, as a method for cooling a strip in a continuous annealing furnace, a rapid cooling nozzle and a nozzle capable of injecting hot water are installed, and the steel strip is rapidly cooled using the rapid cooling nozzle. Two types of methods are disclosed: a cooling method and a cooling method in which cooling of the steel strip is stopped at a predetermined temperature using the nozzle capable of injecting hot water. Moreover, when injecting hot water, the method of providing and collecting a shielding board so that the hot water injected from the nozzle may not be drained to the downstream side, and collect | recovering in a water receiving tank is disclosed.
Japanese Patent Publication No. 1-18974 JP-A 61-183415

  However, in the conventional method disclosed in Patent Document 1, since the cooling rate is ensured by air spray and cooling in the immersion tank, the cooling length is inevitably long, and the cooling required for the high-tensile steel plate. There is a problem that it is difficult to ensure speed. Moreover, since the heating roll in the immersion tank is in the aqueous solution in the immersion tank, it is heated inside the roll, but is cooled because it is in contact with the aqueous solution on the roll surface. For this reason, the roll temperature is very unstable and is not suitable for steady operation.

  In the case of the method disclosed in Patent Document 2, when cooling is performed using a rapid cooling nozzle, the steel strip is cooled to the water temperature, and then the steel strip is reheated. It is difficult to reduce the load of the heating capacity. In addition, when a nozzle capable of hot water injection is used, the cooling capacity is lowered, so that it is difficult to ensure a sufficient cooling rate for ensuring desired characteristics for all steel strips to be manufactured. Furthermore, the shielding plate installed on the downstream side of the cooling nozzle is installed as a device for collecting droplets when performing hot water injection, and has no effect on the function during rapid cooling. Not done.

  Therefore, the present invention provides a steel strip cooling method and cooling equipment capable of ensuring a high cooling rate comparable to that of the conventional water quenching method and stably ensuring a high production efficiency in the continuous annealing process. The purpose is to provide.

In order to solve the above problems, the present invention has the following features.
[1] have contact to the continuous annealing process, after heating the steel strip gas jet cooling, a cooling method of a steel strip to be performed before the over-aging treatment,
After being heated, water is sprayed onto the steel strip cooled by gas jet and cooled, and then the cooled steel strip is cooled by immersing it in an immersion bath having a water temperature of 60 to 90 ° C. A method for cooling a steel strip, wherein the temperature of water to be injected is 25 ° C. or higher and 45 ° C. or lower .
[2] have contact to the continuous annealing process, after heating the steel strip gas jet cooling, a cooling method of a steel strip to be performed before the over-aging treatment,
After the heating, the steel strip cooled by gas jet is cooled by jetting water, and the cooled steel strip is used as a cooling medium, and a liquid containing an ionic liquid at least in part is used. A method for cooling a steel strip, characterized in that the temperature of water immersed in the immersion bath and sprayed onto the steel strip is lower than the liquid temperature in the immersion bath.
[3] ionic liquid, the following equation (1) to (5) of a salt represented by the chemical formula of the strip according to the above [2], which consists of one or two or more thereof Cooling method.

Wherein R ^{1 to} R ⁴ are selected from the group consisting of linear or branched, substituted or unsubstituted alkyl, aryl, alkoxyalkyl, alkylenearyl, hydroxyalkyl and haloalkyl, where N is a nitrogen atom or A phosphorus atom, X is an anion selected from methyl sulfate, PF ₆ ⁻ , BF ₄ ⁻ and a halogen compound, Y is a cation containing a thero atom such as nitrogen or phosphate, Z is glycerol, citric acid, urea, It is a neutral molecule capable of hydrogen bonding selected from other neutral proton donors or acceptors. M and n are integers selected for imparting electrical neutrality, and q is an integer of 0 to 1000.
[ 4 ] In the above [ 2 ] or [ 3 ], the temperature of water sprayed on the steel strip is 25 ° C. or more and less than 60 ° C., and the temperature of the cooling medium in the immersion bath is 150 to 300 ° C. To cool the steel strip.

  According to the present invention, in a continuous annealing process, a steel strip cooling method and a cooling facility capable of ensuring a high cooling rate comparable to that of a conventional water quenching method and stably ensuring a high production efficiency are provided. Provided.

  Hereinafter, an example of the best mode for carrying out the present invention will be described.

  FIG. 1 shows an example of the equipment configuration of a continuous annealing equipment to which the cooling equipment according to the present invention is applied. 2 and 3 show an example of the equipment configuration of the cooling equipment according to the present invention.

  As shown in FIG. 1, the steel strip continuous annealing equipment includes a heating zone 2, a soaking zone 3, a gas jet cooling zone 4, a cooling facility 5, induction for continuously heating or cooling the steel strip 1 that is continuously conveyed. It consists of a heating device 6 and an overaging zone 7.

  As shown in FIGS. 2 and 3, the cooling facility 5 includes a rapid cooling means 51 that injects water into the steel strip 1 to perform rapid cooling, and a dipping tank 52 disposed on the downstream side of the rapid cooling means 51. And partition means 53 for draining water disposed between the rapid cooling means 51 and the immersion tank 52. A pinch roll 8 is installed immediately above the steel strip 1 entry side of the rapid cooling means 51, and a ringer roll 9 and a dryer 10 are installed downstream of the immersion tank 52. In addition, in the case where an ionic liquid or a liquid containing an ionic liquid in at least a part of the cooling medium is used instead of water as the cooling medium in the immersion tank 52, as shown in FIG. A heater 11 for keeping the temperature is attached.

  Here, the ionic liquid exists as a stable liquid at a high temperature of 100 ° C. or higher. Further, the ionic liquid is non-volatile, does not evaporate, and does not easily deteriorate, so that it can be used repeatedly. Furthermore, it has a large heat capacity and high heat retention.

  Thus, since the ionic liquid can exist as a liquid at a high temperature of 100 ° C. or higher, the cooling stop temperature can be kept higher than that of water when placed in the immersion tank 52, and the induction heating device 6 on the rear stage side can be maintained. Can be reduced.

  In order not to impair the properties of the ionic liquid, it is desirable that the content ratio of the ionic liquid contained in the cooling medium in the immersion bath 52 is high, and it is more desirable that the cooling medium is composed of only the ionic liquid. However, the ionic liquid may contain water, other ions, other mixtures, or the like.

  As the ionic liquid exhibiting the effects as described above, those represented by the following chemical formulas (1) to (5) are preferably used. In addition, as this ionic liquid, what is represented by the following chemical formula (1)-(5) may be used independently, and 2 or more types may be mixed and used for it.

Wherein R ^{1 to} R ⁴ are selected from the group consisting of linear or branched, substituted or unsubstituted alkyl, aryl, alkoxyalkyl, alkylenearyl, hydroxyalkyl and haloalkyl, where N is a nitrogen atom or A phosphorus atom, X is an anion selected from methyl sulfate, PF ₆ ⁻ , BF ₄ ⁻ and a halogen compound, Y is a cation containing a thero atom such as nitrogen or phosphate, Z is glycerol, citric acid, urea, It is a neutral molecule capable of hydrogen bonding selected from other neutral proton donors or acceptors. M and n are integers selected for imparting electrical neutrality, and q is an integer of 0 to 1000.

  The rapid cooling means 51 may be configured by slit nozzles 54 arranged in a plurality of stages on both sides of the front and back surfaces of the steel strip 1 so that cooling water can be sprayed on each of the front and back surfaces of the steel strip 1. it can. By injecting cooling water from the slit nozzle 54 onto the entire front and back surfaces of the steel strip 1 that has entered the rapid cooling zone in which the rapid cooling means 51 is disposed, the heated steel strip 1 can be rapidly cooled. Become.

  Here, between the rapid cooling means 51 and the immersion tank 52, the cooling water sprayed by the rapid cooling means 51 is drained so as not to flow into the immersion tank 52 disposed on the rear stage side. The partition means 53 is arranged. Without the partitioning means 53, a large amount of cooling water sprayed by the rapid cooling means 51 flows into the immersion tank 52. As a result, the water temperature in the immersion tank decreases, and after cooling, the heating load in the induction heating device 6 is increased. Increase. For this reason, the partition means 53 which suppresses the fall of the water temperature in an immersion tank is needed. As the partition means 53, a shielding plate can be used, but is not particularly limited to the shielding plate, and for example, a roll or the like may be used. By using the roll, it is possible to install the steel strip 1 closer to the steel strip 1, and the amount of cooling water flowing into the immersion tank 52 can be further suppressed. It is also effective to prevent cooling water from flowing into the immersion tank 52 by installing a gas nozzle in the partitioning means 53, injecting gas from the gas nozzle, and purging.

  Most of the cooling water jetted from the slit nozzle 54 of the rapid cooling means 51 is dammed by the partition means 53 and is collected in the circulation tank 56 from the drain outlet 55 provided in the rapid cooling means 51. The cooling water collected in the circulation tank 56 is cooled to a predetermined temperature by the cooling device 57, returned to the circulation tank 56, and sprayed from the slit nozzle 54 again.

  A part of the cooling water sprayed from the slit nozzle 54 may flow into the rear immersion tank 52 from the partition means 53 and the steel strip 1 or the gap between the partition means 53. However, the amount flowing into the immersion tank 52 is smaller than the amount of water or liquid in the immersion tank 52, and there is almost no influence on the water temperature or liquid temperature in the immersion tank 52.

  In general, the strength of the steel strip depends on the cooling rate at the time of rapid cooling, and the strength becomes insufficient when the cooling rate becomes slow. In the conventional water quenching method in which water is spray-cooled in water, a boundary layer on the surface of the steel strip is broken by a jet to achieve a faster cooling rate than simply immersing the steel strip in water. On the other hand, in the present invention, even when the rapid cooling means 51 and the immersion tank 52 are separated, the rapid cooling means 51 is equivalent to spraying in water by ejecting a sufficient amount of water from the slit nozzle 54. Cooling capacity is obtained.

  In the case of the same steel strip size, the cooling rate depends on the jet flow rate from the slit nozzle 54 and the coolant temperature. The faster the jet flow rate from the nozzle, the higher the cooling rate. However, the cooling effect is substantially the same if the jet flow rate is at a certain level (eg, 2 m / s or more). On the other hand, the cooling rate of the water temperature decreases as the temperature increases. Therefore, in order to maintain a high cooling rate, the temperature of the cooling water in the rapid cooling means 51 needs to be lower than the water temperature in the immersion tank 52. Here, the temperature of the cooling water is preferably as low as possible, and is preferably 25 ° C. or more and less than 60 ° C. in consideration of the material of the steel strip.

  In consideration of the quenching start temperature, which is the temperature of the steel strip 1 entering the rapid cooling means 51, variations in the cooling water temperature, fluctuations in the line speed of the continuous annealing equipment, etc., the cooling water injected from the slit nozzle 54 The upper limit of the temperature is more preferably 45 ° C. Thereby, stable operation becomes possible.

  The steel strip 1 cooled by the rapid cooling means 51 is then immersed in water or ionic liquid which is a cooling medium in the immersion tank 52. Here, the water temperature or the liquid temperature in the immersion bath 52 is 60 to 90 ° C. for water and 150 to 300 ° C. for ionic liquid in order to reduce the heating load of the induction heating device 6 on the rear stage side. It is preferable to do.

  In FIG. 4, the schematic of the temperature history of the steel strip at the time of using the cooling method of the steel strip which concerns on this invention is shown. As shown in FIG. 4, for example, when the water temperature of the cooling water in the immersion tank 52 is maintained at 80 ° C. (quenching nozzle injection speed: 3 m / s, injection water temperature: 35 ° C.), the induction heating device 6 It becomes possible to keep the temperature of the steel strip at the time of reaching about 80 ° C. Therefore, the load given to the heating capacity of the induction heating device 6 can be reduced, and the steel strip speed can be increased. In FIG. 4, the sheet passing speed was 130 m / min, the steel strip thickness: 1.6 mm, TS (tensile strength): 998 MPa, and El (elongation): 17.6%.

  Furthermore, as another example, FIG. 5 shows a schematic diagram of the temperature history of the steel strip when the steel strip cooling method according to the present invention is used. As shown in FIG. 5, for example, when the liquid temperature of the ionic liquid in the immersion tank 52 is maintained at 250 ° C. (quenching nozzle injection speed: 3 m / s, injection water temperature: 35 ° C.), the induction heating device The temperature of the steel strip when reaching 6 can be maintained at about 250 ° C. Therefore, the load given to the heating capacity of the induction heating device 6 can be reduced, and the steel strip speed can be increased. In addition, the sheet | seat speed | rate in FIG. 5: 180 m / min, steel strip board thickness: 1.6 mm, TS (tensile strength): 998 MPa, El (elongation): 17.6%.

  Thus, in the present invention, the cooling in the rapid cooling means 51 and the cooling in the immersion tank 52 are separated, the rapid cooling means 51 performs rapid cooling at a relatively low water temperature, and the immersion tank 52 has a relatively high temperature. By using water or an ionic liquid and maintaining the steel strip temperature at the exit side of the immersion tank 52 as high as possible, both the quality and the production efficiency could be improved.

  As an example of the present invention, a high-tensile cold-rolled steel sheet of 980 MPa class having a plate thickness of 1.6 mm and a plate width of 1000 mm was manufactured using the continuous annealing equipment and the cooling equipment shown in FIGS. The quenching temperature here was 680 ° C.

  The slit nozzle 54 of the rapid cooling means 51 in the cooling facility 5 is installed in 9 stages on both the front and back surfaces of the steel strip, and the cooling length is 900 mm. The amount of water sprayed from the slit nozzle 54 was 1000 T / Hr. Moreover, since the water temperature of the cooling water sprayed from the slit nozzle 54 rises due to the amount of heat brought in from the steel strip, the circulating water cooling device 57 was cooled so as to keep it at 30 ° C.

  The water temperature or liquid temperature in the immersion tank 52 rises due to the amount of heat taken from the steel strip because the outlet side plate temperature of the rapid cooling means 51 is 100 ° C. or higher. Here, the water temperature was controlled by changing the line speed condition. When the line speed was 105 mpm, the water temperature in the immersion tank 52 was 60 ° C., and when the line speed was 130 mpm, the water temperature in the immersion tank 52 was 80 ° C.

  When the line speed was 180 mpm, the liquid temperature of the ionic liquid in the immersion tank 52 was 250 ° C. The ionic liquid used here is ionic liquid IL-A2 (manufactured by Guangei Chemical Industry Co., Ltd.). This is an aliphatic amine-based ionic liquid, the cation is trimethylhexylammonium ion, and the structural formula is represented by the following formula (6).

Furthermore, _{anion, (CF 3 SO 2) 2} N - is.

The decomposition point of this ionic liquid is 484.6 ° C., the viscosity is 634.9 cPs at 25 ° C., and the ionic conductivity is 6.1 × 10 ⁻⁵ S / cm at 25 ° C.

  Steel strips were manufactured under the above line speed conditions, and the materials were evaluated. As a result, when the line speed was 105 mpm and the water temperature in the immersion tank 52 was 60 ° C. (Invention Example 1), TS (tensile strength): 995 MPa, El (elongation): 17.4%, and line speed 130 mpm, When the water temperature in the immersion tank 52 is 80 ° C. (Example 2 of the present invention), TS (tensile strength): 999 MPa, El (elongation): 17.5%, line speed 180 mpm, ionicity in the immersion tank 52 When the liquid temperature was 250 ° C. (Example 3 of the present invention), TS (tensile strength) was 998 MPa, and El (elongation) was 17.6%. Thus, also in the example of this invention, it was the intensity level equivalent to the time of the conventional manufacture. In addition, the method for evaluating the material is that a JIS No. 5 test piece is taken in the rolling direction at the center of the width of the manufactured steel strip, a tensile test is performed in accordance with the provisions of JIS Z 2241, and tensile properties (tensile Strength (TS) and elongation (El) were determined.

  As a comparative example, a high-tensile cold-rolled steel sheet having the same tensile properties as the example of the present invention was produced by a conventional method (steel sheet thickness: 1.6 mm, TS: 994 MPa, El: 17.4%). In this comparative example, the quench nozzle was installed in the immersion tank, and the cooling water sprayed from the quench nozzle and the water temperature in the immersion tank were set to 30 ° C. In the case of this comparative example, since the temperature of the steel strip is cooled to 30 ° C. in the immersion tank, the heating load in the induction heating device during reheating increases, and the line speed is increased to raise the temperature to the reheating temperature. Had to be 90 mpm.

  FIG. 6 shows a comparison of line speeds when producing high-tensile cold-rolled steel sheets in the inventive examples and comparative examples. As shown in FIG. 6, compared with the method of spraying in the conventional immersion tank as a comparative example, in the present invention example, the heating temperature in the induction heating device is reduced due to the increase in the immersion tank outlet side plate temperature, About 20% when the water temperature in the immersion tank is 60 ° C, about 40% when the water temperature in the immersion tank is 80 ° C, and about twice as fast as the liquid temperature in the immersion tank is 250 ° C. .

It is a figure which shows an example of the equipment structure of the continuous annealing equipment to which the cooling equipment which concerns on this invention is applied. It is a figure which shows an example of the equipment structure of the cooling equipment which concerns on this invention. It is a figure which shows another example of the equipment structure of the cooling equipment which concerns on this invention. It is the schematic of the temperature history of a steel strip at the time of using the cooling method of the steel strip which concerns on this invention. It is the schematic of the temperature history of the steel strip at the time of using the cooling method of the steel strip which concerns on this invention which is another example. It is the figure which showed the comparison of the line speed at the time of manufacturing a high tension cold-rolled steel plate by the example of this invention and a comparative example. It is the schematic of the temperature history of the steel strip at the time of using the water quenching method in the conventional continuous annealing equipment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel strip 2 Heating zone 3 Soaking zone 4 Gas jet cooling zone 5 Cooling equipment 51 Rapid cooling means 52 Immersion tank 53 Partition means 54 Slit nozzle 55 Drain outlet 56 Circulation tank 57 Cooling device 6 Induction heating device 7 Overaging zone 8 Pinch roll 9 Ringer roll 10 Hair dryer 11 Heater

Claims (4)

Continuously have you to annealing process, after heating the steel strip gas jet cooling, a cooling method of a steel strip to be performed before the over-aging treatment,
After being heated, water is sprayed onto the steel strip cooled by gas jet and cooled, and then the cooled steel strip is cooled by immersing it in an immersion bath having a water temperature of 60 to 90 ° C. A method for cooling a steel strip, wherein the temperature of water to be injected is 25 ° C. or higher and 45 ° C. or lower . Continuously have you to annealing process, after heating the steel strip gas jet cooling, a cooling method of a steel strip to be performed before the over-aging treatment,
After the heating, the steel strip cooled by gas jet is cooled by jetting water, and the cooled steel strip is used as a cooling medium, and a liquid containing an ionic liquid at least in part is used. A method for cooling a steel strip, characterized in that the temperature of water immersed in the immersion bath and sprayed onto the steel strip is lower than the liquid temperature in the immersion bath. The method for cooling a steel strip according to claim 2 , wherein the ionic liquid comprises one or a mixture of two or more salts represented by chemical formulas of the following formulas (1) to (5).

Wherein R ^{1 to} R ⁴ are selected from the group consisting of linear or branched, substituted or unsubstituted alkyl, aryl, alkoxyalkyl, alkylenearyl, hydroxyalkyl and haloalkyl, where N is a nitrogen atom or A phosphorus atom, X is an anion selected from methyl sulfate, PF ₆ ⁻ , BF ₄ ⁻ and a halogen compound, Y is a cation containing a thero atom such as nitrogen or phosphate, Z is glycerol, citric acid, urea, It is a neutral molecule capable of hydrogen bonding selected from other neutral proton donors or acceptors. M and n are integers selected for imparting electrical neutrality, and q is an integer of 0 to 1000. The temperature of the water injected into the steel strip is 25 ° C or more and less than 60 ° C, and the temperature of the cooling medium in the immersion bath is 150 to 300 ° C, cooling the steel strip according to claim 2 or 3 Method.

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