KR20030054974A - Method for cooling a hot rolled steel strip - Google Patents

Abstract

PURPOSE: A method for cooling hot rolled coil is provided to cool strip of the hot rolled coil to secure retained austenite fraction uniformed in width and length directions of the strip in manufacturing transformation induced plasticity hot rolled coil. CONSTITUTION: In a method for manufacturing strip by rapidly cooling strip of hot rolled coil that is rolled to a certain thickness through hot rolling mill(2), primarily rapidly cooled in water cooling section(3) and coiled in a coil shape in down coiler(4), the method for cooling the hot rolled coil comprises a step of uncoiling the coiled hot rolled coil in a direction at uncoiler(5); a step of furnishing upper and lower cooling headers having a plural nozzles in a width direction of the strip at upper and lower sides of the strip proceeded in a direction after being uncoiled in the above step and forcibly rapidly cooling the strip to a temperature of 100 deg.C or less using cooling water sprayed from the headers one or more of which are installed in a length direction of the strip; a step of drying moisture remained on the surface of the strip rapidly cooled in the rapid cooling step in dryer(12) using a heat source of high temperature; and a step of recoiling the strip dried in the drying step at recoiler(7).

Description

Hot rolled coil cooling method {METHOD FOR COOLING A HOT ROLLED STEEL STRIP}

The present invention relates to a method for cooling a hot rolled coil for cooling a steel sheet of a hot rolled coil so as to secure a uniform residual austenite fraction in the width direction and the length direction in manufacturing the hot rolled modified organic plastic steel coil.

In general, transformed organic plastic or TRP (TRIPSformation Induced Plasticity) contains unstable residual austenite in the steel. When plastic deformation occurs, the residual austenite transforms to martensite and exhibits high strength and high ductility. . Therefore, it is a high-strength steel for processing suitable for the site that requires high strength while receiving a lot of processing, such as automotive structural parts.

The metamorphic organic plastic steel can be manufactured by two methods of hot rolling and cold rolling. In both cases, it is very important to appropriate the fraction of phase because the excellent machinability of the metamorphic organic plastic steel is due to the residual austenite contained in the steel. For this purpose, it is essential to control the phase transformation through the heat treatment process after rolling together with the selection of the appropriate alloy system.

However, there is a problem in that residual austenite is decomposed by the characteristic of being completely cooled after winding even when the coil is manufactured through the optimization of the material composition and the pattern cooling in producing the modified organic plastic steel sheet through the hot rolling process.

In particular, the coil wound in a roll form greatly varies the cooling rate for each part according to its characteristics as a bulk material, and the content of residual austenite is greatly reduced in the width center portion where cooling is relatively slow, causing unevenness of the material, thereby increasing the error rate. It is a problem.

On the other hand, high strength high ductility hot rolled steel sheet has been developed in a considerable number of manufacturing methods have been applied to the actual commercialization. The development of existing high strength, high ductility steels was carried out in the 80s in the dual phase steel (composite structure of ferrite and martensite), triphase steel (ferrite, bainite). ) And martensite complexes), ferrite-bainite composite tissue reinforcement and the like.

These steels have a tensile strength of about 60kg / mm ² , an elongation of about 30%, and high strength and high ductility.

TRIP steel has been developed in the 90's and is currently working for commercialization. The characteristic of this steel is that its tensile strength is about 80kg / mm ² and it shows high ductility.

In the case of Nippon Steel, it has been reported that it can be manufactured as dual phase steel by winding 0.06-0.10% C, 0.25-1.3% Si, 1.1-1.5% Mn steel below about 300 ℃. (鐵 と 鋼), Vol. 68 (1982) No. 9, p. 1306), and addition of 0.5-1.5% Cr to 0.04-0.06% C, 0.5-1.0% Si, 1.5% Mn steel in the signal steelmaking The steel was finished rolling at about 850 ° C. and cold wound at about 250 ° C. to form ferrite as a base structure and contained 10-20% bainite and 3-5% martensite. It has been announced that a triphase steel can be produced (Iron and Steel, Vol. 68 (1982) No. 9, p. 1185).

In the case of signal steel, Nb is added to 0.05-0.07% C, 0.5% Si, 1.1-1.5% Mn steel with 0.04% or less of tensile strength of 10-20% bainite in ferrite base structure. It was announced that it is possible to produce mm ² grade ferrite-bainite composite tissue steel (Trans. ISIJ, Vol. 23 (1983), p.303). A ferrite-bainite composite tissue steel with a tensile strength of 70 kg / mm ^{2 has} been developed by adding 0.06% complex (CAMP-ISIJ, Vol. 1 (1988, p.881).

In addition, in Nippon Steel, 0.03-0.30% Mo is added to steel containing 0.06-0.22% C, 0.05-1.0% SI, 0.5-2.0% Mn and 0.25-1.5% Al as necessary to add residual austenite. Patented for steel with excellent press workability, deep workability and bendability by showing high strength of 50kg / mm ² and elongation of more than 35% by containing -20% (Japanese Patent Laid-Open No. 6-145892) ). The amount of Al in this steel grade is adjusted in the range of 0.6% Si ≤% Al ≤ 3-12.5x% C, and the method of heat-treating the steel grade in a two-phase region (350-600 after holding for 10 seconds to 3 minutes at 650-900 ℃) After cooling to 4-200 ℃ / sec to the temperature range of ℃, it is maintained here for 5 seconds to 10 minutes and then cooled to a temperature below 250 ℃ at a cooling rate of 5 ℃ / sec or more). Patents are also submitted for the manufacturing method (Japanese Patent Laid-Open No. Hei 6-145788).

In steel mill, less than 0.18% C, 0.5-2.5% Si, 0.5-2.5% Mn, 0.05% or less P, 0.02% or less S, 0.01-0.1% Al steel, 0.02-0.5% Ti and 0.03-1.0 % Nb is added alone or in combination (in this case, the amount of C, Nb and Ti is adjusted within the range of% C> (% Ti / 4) + (% Nb / 8)), and the finishing rolling temperature is over 820 ℃ After finishing at, the patent has been filed for the case of winding at a temperature below 500 ℃ by maintaining at least 10 seconds in a temperature range of 820-720 ℃ and then cooling at a cooling rate of 10 ℃ / sec or more.

The steel has high strength of 70kg / mm ² or more at the amount of C below 0.18%, and the low amount of C also shows excellent advantages such as spot welding and fatigue characteristics (Japanese Patent Laid-Open No. 5-5). 179396). In addition, since the steel shows resistance yield ratio, the conventional precipitation-reinforced steel has a high yield ratio, which solves the problem of a large number of spring back after press processing, and has high strength, high ductility, and excellent stretch flange characteristics. It is said to show characteristics.

In the case of Zhuwoo Metal, the steel containing 0.05-0.25% C, 0.05-1.0% Si, 0.8-2.5% Mn, 0.8-2.5% Al and the precipitation strengthening elements such as Nb, Ti and V were added to the steel at 780-840 ℃. After rolling in the temperature range of and then cooled to a temperature of 600-700 ℃ at a cooling rate of more than 10 ℃ / sec, and then subjected to air cooling for 2-10 seconds and at a temperature of 300-450 ℃ at a cooling rate of 220 ℃ / sec A patent for a method for producing steel with 5% or more of retained austenite as a result of accelerated cooling was also filed (Japanese Patent Laid-Open No. Hei 5-112846).

This steel has a large amount of hard martensite, which leads to poor stretch flanges. Therefore, Si is less than 1.0%, which is required to solidify solid solution in precipitated polygonal ferrite. It is a limitation.

As described above, various transformation organic plastic steel manufacturing methods have been reported through the hot rolling process, but it can be seen that most of the techniques are to optimize the material components and to optimize the cooling control conditions in the water cooling zone. However, no matter how the components and cooling control conditions are optimized, the behavior of decomposition of residual austenite also varies according to the difference in the cooling rate of each coil part when manufacturing a large sized coil. You lose. Therefore, the determination of the cooling rate after winding in consideration of the decomposition behavior of the retained austenite after winding is necessary to ensure commercial quality, but as mentioned above, the existing publicly known technologies do not include such contents. .

As shown in FIG. 1, the process for producing a transformed organic-baked steel by a conventional hot rolling facility includes a roughing mill 2a and a finishing mill 2b that continuously connect slabs heated at a high temperature in the heating furnace 1. After rolling to a predetermined thickness through a hot rolling mill (2) consisting of quenching in a first step in the water cooling stage (ROT) (3), and by the ferrite transformation during the air cooling holding time to secure an appropriate amount of ferrite fraction After concentrating the carbon with transformation austenite, it is quenched to the proper temperature in two stages, and wound up in a coil (4) in the form of a roll. It is placed in the coil yard storage and cooled naturally by air cooling to room temperature. I am applying a pattern.

However, as described above, the cooling rate of the wound coil is very slow, so that sufficient temperature and time for the residual austenite obtained in the water cooling target to decompose is maintained, making it difficult to secure a predetermined fraction of the retained austenite. In particular, the cooling history of the winding coil surface and the inner winding portion is very different, causing a material deviation, that is, a deviation of the residual austenite fraction.

For this reason, a method of immersing the wound hot rolled coil in water, spraying water spray on the surface of the hot rolled coil, or forced air cooling using a blower to cool the hot rolled coil at a high speed may be used. It is not a solution to fundamentally solve the variation of materials caused by the difference in cooling history between the outer and outer parts.

Therefore, the present invention has been proposed to solve the conventional problems as described above, the object is to maintain the residual austerity by uniformly rapidly cooling the metamorphic organic plastic steel hot rolled coil produced through a hot rolling process using a separate water cooling equipment By preventing the decomposition of nitrates, ensuring uniform residual austenite throughout the width and length directions of the steel sheet, shortening the time required for cooling the hot rolled coil, and suppressing the growth and transformation of the oxide film on the surface of the steel sheet through rapid cooling The aim of the present invention is to provide a hot rolled coil cooling method capable of improving the properties, minimizing material deviations and significantly improving the error rate.

1 is a schematic view showing a conventional hot rolled coil cooling method;

2 is a schematic view showing a hot rolled coil cooling method of the present invention;

Figure 3 is a side view showing a pipe lamina cooling method employed in the hot rolled coil cooling method of the present invention,

Figure 4 is a schematic diagram showing the change of the fraction of the retained austenite according to the cooling rate after the hot rolled steel coil winding method in the present invention,

Figure 5 is a graph showing the change in the temperature of the steel sheet according to the flow rate of the cooling water according to the water cooling time in the cooling step of the hot rolled coil cooling method of the present invention.

Explanation of symbols on the main parts of the drawings

1 .... Furnace 2 .... Hot Rolling Mill

3 .... water cooler 5 .... uncoiler

6 .... Calibration Mill 7 .... Recoiler

11 .... Supply Cooling Table 12 .... Dryer

As a technical configuration for achieving the above object, the present invention,

In a method of cooling a hot rolled coil rolled to a predetermined plate thickness through a hot rolling mill and then quenched in one step in a water cooling stand, and wound in a roll form in a winder,

Uncoiling the wound hot rolled coil in one direction in an uncoiler;

The upper and lower cooling headers having a plurality of nozzles in the width direction on the upper and lower sides of the steel sheet which is uncoiled in the step and proceeds in one direction, are sprayed from the header installed in one or more in the longitudinal direction of the steel sheet. Forcibly quenching the steel sheet to 100 ° C. or less as cooling water;

Drying the moisture remaining on the surface of the steel sheet quenched in the quenching step as a high temperature heat source;

By recoiling the dried steel sheet in the recoil in a coil form of the recoil in the step by providing a hot rolled coil cooling method characterized in that it comprises.

Hereinafter, the present invention will be described in more detail.

As shown in FIG. 2, the present invention comprises a series of conventional uncoilers 5, a cold pass mill 6, and a recoiler 7. Steel plate released from hot hot rolled coil by installing supply and demand cooling stand 11 and dryer 12 at the inlet side of the cold correction facility 6 between the uncoiler 5 and the recoiler 7 in the equipment configuration. Rapid cooling and simultaneous calibration.

That is, the hot rolled coil wound by a general continuous hot rolling facility is loaded into the uncoiler 5 to be fed to the straightening mill 6, which is the correction facility, by using a coil transfer cart (not shown). The losing steel sheet is cooled to 100 ° C. or lower while passing through the supply / cooling stage 11.

In this cooling process, it is possible to prevent decomposition of the retained austenite obtained through the control of the cooling pattern in the water supply cooling zone 11, thereby securing a fraction of the target retained austenite, and the width direction and the longitudinal direction. You can also eliminate the deviation.

Subsequently, the hot-rolled steel sheet which has completed cooling in the supply / cooling chiller 11 is subjected to a hot air dryer 12 for removing moisture remaining on the surface, and then, a straightening mill 6 which is a cold correction facility for shape correction. While passing through the hot rolling in the rolling mill (2) and the bending of the hot-rolled steel sheet generated during forced water cooling to the water cooling stand 11, the corrected steel sheet is a recoiler (7) installed on the exit side of the straightening mill ) Will be shipped in coil form again. Through this process, it is possible to achieve a side effect of drastically reducing the cooling process of the hot rolled coil, which usually takes about 3 to 5 days, within a few minutes.

Here, the surface of the steel sheet during hot rolling in the hot rolling mill (2) is combined with oxygen in the air to form an oxide film, which is mainly wistite (FeO). In the case of slow cooling for 3 to 5 days by the conventional method, the thickness of the oxide film is increased by being combined with oxygen in the air while being maintained at a high temperature for a long time. At the same time, the oxide film of the visite phase formed at high temperature gradually becomes hematite (hematite, Fe). ₂ O ₃ ) will be transformed.

At this time, the hematite has a finer structure and higher fracture strength than visite, and is difficult to pickle because it is poorly soluble to acid during pickling, whereas when quenched immediately after winding by the method of the present invention, an increase in oxide film thickness is increased. It suppresses the transformation into hematite and improves pickling property in the pickling process.

On the other hand, since the cooling rate is about several tens to several hundred ℃ per second, many cracks occur in the oxide film during the quenching process.

That is, the instant the oxide film is preferentially cooled when the coolant in the cooling water contact with the steel sheet generates a high temperature deviation between the oxide film and the base metal. In this process, the tensile film has a high tensile stress due to the difference in thermal expansion coefficient. Is formed and many microcracks occur in the oxide film whose toughness is reduced by cooling.

Such microcracks become a penetration path of the acid solution in the pickling process, which acts as a factor for greatly improving acidity.

In addition, in the case of the uncoiler 5, the equipment and technology used in the usual correction process may be applied as it is. In order to continue the present invention, the wire between the coil and the trailing hot rolled steel sheet are welded to each other at the inlet side of the uncoiler 5. A welding machine (not shown) is provided, and a coil split shearing device is installed on the exit side to weld the steel sheet end portion of the preceding coil and the steel sheet tip portion of the next coil to undergo cooling and correction process, followed by dividing and winding by coil unit. Serialization can be achieved.

Hereinafter, the configuration and operation of each facility will be described in detail.

Water-cooling equipment for rapid cooling during the transfer of steel sheet is a technique such as pipe laminar method, water curtain method, water spray method or immersion cooling method used in the ordinary hot rolling process This can be used. However, since the entrance temperature of the material is usually 500 ° C or less, water cooling is mainly performed in the nucleation boiling region rather than film boiling, so that rapid cooling is possible through the process of water directly contacting the steel sheet. In this case, the cooling capacity depends on the cooling water flow rate rather than the cooling method.

3 is a schematic view showing a cross section of a pipe lamina type cooling system as an example of a preferred cooling method. As shown in FIG. The upper and lower cooling headers 20 and 30 having a plurality of nozzles 21 and 31 in the width direction of the steel sheet to quench the upper and lower surfaces of the steel sheet running in one direction. Install multiple pieces at regular intervals. Accordingly, the upper and lower cooling headers 20 and 30 spray the upper and lower cooling water forcibly spraying the nozzles 21 and 31 onto the steel plate surface to forcibly quench the steel plate below 100 ° C. It is. Then, the steel sheet is transferred to the exit side by the table roller 40.

The steel sheet cooled to 100 ° C. or lower while passing through the supply / cooling stage 11 passes through a dryer 12 for removing residual moisture from the surface. The dryer 12 preferably has a hot air method. Moisture as well as moisture penetrated between the cracks of the oxide film formed during quenching should be completely removed to prevent rust caused by residual moisture.

Therefore, when the quenching in the water supply cooling zone 11 as a heat of the steel sheet itself, the temperature of the steel sheet is quenched to about 50 ~ 80 ℃ not too low so as to evaporate and remove the residual moisture penetrated between the cracks of the oxide film formed on the surface It is desirable to.

The steel sheet dried as the temperature remaining in the dryer 12 or on the steel sheet itself is then passed through a calibration mill 6, which is a correction facility for shape correction, and such shape correction is performed at a temperature of 100 ° C. or lower. Equipment and techniques such as skin pass, tension leveler, etc. may be used. If a cold calibration is not required, a roll gap may be passed and a series of equipment except for a cold calibration rolling mill may be used.

Subsequently, the shape-corrected steel sheet is wound again in the form of a coil by the recoiler 7, and when the wire between the coils and the trailing steel plate are welded for continuity, the steel sheet may be divided in coil units on the entire surface of the recoiler 7. Shearing equipment is required.

<Example>

Hereinafter, the method of the present invention will be described through examples.

Inhibition of residual austenite decomposition behavior by rapid quenching The slab was prepared by dissolving the steel as shown in Table 1 below.

C Si Mn P S Al N TRIP steel 0.18 1.7 1.5 0.0012 0.002 0.031 0.008

The slab, which is a rolled material, was reheated at 1200 ° C. and then hot rolled to prepare a hot rolled steel sheet having a final thickness of 3.0 mm. At this time, the hot rolling finish temperature (FDT) is 850 ~ 900 ℃, hot rolling immediately after the hot rolling, and then quenched to the intermediate air cooling temperature (680 ℃) and then air cooled for about 6 seconds.

After air cooling, hot-rolled steel sheet was manufactured by applying pattern cooling to the set winding temperature of 420 ℃ again, and then cooled to room temperature at various cooling rates, and then observed the effect of cooling rate after the winding process on the decomposition of residual austenite. It was.

Figure 4 shows the result of measuring the amount of residual austenite according to the cooling rate, it can be seen that when the cooling rate is 200 ℃ / min or less decomposition of the residual austenite occurs, the decomposition behavior is completely suppressed at a higher rate. .

Therefore, it can be seen that the cooling rate of several tens to several hundred degrees Celsius per second can be completely suppressed when water is cooled while unwinding the coiling coil as in the method of the present invention.

A minimum cooling rate to suppress residual austenite decomposition is sufficient as a cooling rate of 200 ° C. per minute as in the previous Example 1. However, the slower the cooling rate is, the longer the length of the equipment, so it is desirable to cool as fast as possible to increase the cooling efficiency.

The length of the water cooling plant is determined by the cooling water flow rate, the plate speed and the thickness of the steel sheet. For example, the result of calculating the case where the plate | board speed is 300mpm for the steel plate whose entrance temperature is 600 degreeC and the steel plate thickness is 6 mm as an upper limit of cooling conditions is shown in FIG. 5, and flow volume is 1000 l / m <2> / min In the case of about 2.5 seconds, 2000 l / ㎡ / min in about 1.7 seconds the temperature drops to less than 100 ℃, so if this cooling time is required, the length of the water cooling zone is assuming that the mailing speed is 300mpm It can be seen that they are within about 12.5m and 8.5m respectively.

Therefore, it can be seen that if the flow rate has a flow rate of at least 1000 l / m 2 / min or more, a highly concentrated cooling system can be configured within 20 m.

According to the present invention as described above, after the formation and winding of the transformation organic plastic steel containing a predetermined residual austenite through the hot rolling process, while unwinding the hot hot rolled coil wound by 100 while using water cooling in a few seconds of a few seconds By recooling after the rapid cooling to below C, the shape is corrected, a product having a uniform fraction of retained austenite over the entire length and width of the coil can be produced.

In addition, by eliminating the natural cooling process, which takes several days, the logistics flow can be improved, inventory costs corresponding to the air-cooling period can be shortened, delivery time for demand can be shortened, and large coil yards for natural air cooling can be reduced. It can be economically effective.

In addition, by quenching high temperature hot rolled coil, microcracks can be formed on the surface oxide film, the oxide film growth can be suppressed, and the phase transformation of the oxide film to hematite can be suppressed to greatly improve pickling in the pickling process, which is the next step. And improving the surface quality.

While the invention has been shown and described with respect to specific embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit or scope of the invention as set forth in the claims below. I would like to clarify that knowledge is easy to know.

Claims (3)

In the method of manufacturing a steel sheet by quenching the steel sheet of the hot rolled coil rolled to a predetermined plate thickness through a hot rolling mill and then quenched in a water cooling stage in one step, and wound in a roll form in a winder, Uncoiling the wound hot rolled coil in one direction in an uncoiler; The upper and lower cooling headers having a plurality of nozzles in the width direction on the upper and lower sides of the steel sheet which is uncoiled in the step and proceeds in one direction, are sprayed from the header installed in one or more in the longitudinal direction of the steel sheet. Forcibly quenching the steel sheet to 100 ° C. or less as cooling water; Drying the moisture remaining on the surface of the steel sheet quenched in the quenching step as a high temperature heat source; Hot-rolled coil cooling method comprising the step of recoiling the steel sheet dried in the step of the recoil in a coil form. The method of claim 1, The forced quenching step is a hot rolled coil cooling method characterized in that the cooling rate is made at 200 ℃ or more per minute. The method of claim 1, The forced quenching step is a hot rolled coil cooling method characterized in that the steel sheet is cooled to about 50 ~ 80 ℃ to evaporate and remove the residual moisture penetrated between the cracks of the steel sheet surface oxide as the heat of the steel sheet itself when quenching.