JP4490789B2 - Continuous annealing method for steel sheet - Google Patents

Description

The present invention relates to a method for continuously annealing a steel sheet using a steel sheet continuous annealing facility for performing heat treatment of a high strength thin steel sheet.

  In order to improve workability, the thin steel sheet after cold rolling is a continuous annealing facility that is continuously provided with heating / soaking, primary and secondary cooling zones, reheating / overaging zones, and final cooling zones. Heat treatment is applied. In this heat treatment, the steel sheet is usually heated and soaked in a temperature range of 700 to 900 ° C. depending on the material, and a required cooling rate is obtained in the primary cooling zone on the front side and the secondary cooling zone on the rear side. Then, it is over-aged at a temperature of about 400 ° C. and finally cooled to a temperature range where it can be handled.

By the way, for example, unlike a soft thin steel plate, a high strength steel plate used for weight reduction of an automobile, for example, a high strength material having a strength of 600 MPa class, the heating rate is higher than a certain level after heating and soaking. Requires rapid adjustment cooling. As a cooling device for the primary cooling zone on the front stage side that enables such rapid adjustment cooling, a gas jet cooling method that has the advantage that non-oxidation and uniform cooling of the steel sheet surface is possible and the equipment cost is relatively low Has been disclosed (for example, see Patent Documents 1 and 2). Moreover, as a cooling device for the secondary cooling zone on the rear stage side, a roll cooling device that cools an internally cooled multi-stage roll while pressing the steel plate while pressing it, or a high cooling ability immersion cooling device that uses a cooling medium such as water. Etc. are known.
JP-A-4-160120 (pages 2 to 4) JP 2002-3156 A ([0006] to [0018])

  However, mechanical properties such as strength and elongation required for high-strength steel sheets are becoming higher year by year, and the manufacturing methods are becoming more complicated. In particular, in a high-strength steel plate (super high-tensile material) having a strength of 1000 MPa class or more, it is necessary to more strictly manage the quenching start temperature and quenching end temperature after heating and soaking, and the cooling rate therebetween. Table 1 shows an example of cooling conditions for the high-tensile and ultra-high-tensile materials when a roll cooling device is used for secondary cooling. The cooling end temperature in Table 1 indicates the roll cooling end temperature of the secondary cooling.

  From Table 1, in order to produce a high-strength steel sheet whose structure is composed of austenite and bainite, the ferrite transformation region in the temperature range of about 800 to 500 ° C. is quenched in order to prevent the ferrite structure from appearing, and the martensite structure In order not to appear, it is necessary to gradually cool to an overaging temperature of about 400 ° C. after rapid cooling. In the gas jet cooling devices disclosed in Patent Documents 1 and 2, since only the same cooling unit is arranged in series, generally, the soaking end temperature of the high-strength steel plate is equalized and quenched. If the temperature is set to about 850 to 900 ° C. in order to ensure the heat resistance, and the rapid cooling is performed at a cooling rate necessary for preventing the ferrite structure from appearing in the temperature range of 800 to 500 ° C. from the soaking end temperature, the cooling end temperature is lowered. In addition, there is a risk that the cooling device will be rapidly cooled to a temperature range that does not require rapid cooling at 900 to 800 ° C. in the initial stage of cooling, and the cooling device becomes larger than necessary, resulting in a reduction in cooling efficiency.

  On the other hand, when the steel plate after the soaking is quenched with the roll cooling device in the secondary cooling zone without being cooled in the primary cooling zone, the cooling capacity is high, but the steel plate is not used for roll cooling from a high temperature. Since the heat shrinkage is large, warping and corrugation occur, and uniform contact with the cooling roll in the width direction is difficult to obtain, making uniform cooling difficult. In addition, when a high-strength steel sheet is produced by water quenching with an immersion cooling device in the secondary cooling zone after the soaking is completed, it is preferable to increase the rapid cooling start temperature, that is, the soaking end temperature. Since the temperature drop of the steel sheet during quenching is fast, the shape in the width direction of the steel sheet is distorted, making it difficult to obtain a flat high-strength steel sheet. Further, when the steel plate after the soaking is quenched with a gas jet cooling device to a temperature range of 400 ° C. or lower, which is lower than the conventional water quenching start temperature, In the cooling device, the cooling capacity is insufficient and the rapid cooling condition is not satisfied, and it is difficult to manufacture a desired high strength steel plate. Even if the desired high-strength steel sheet can be produced by this water quenching method, in order to eliminate the lack of cooling capacity of the gas jet cooling device and lower the water quenching start temperature than before, a gas jet It is necessary to set the cooling device to the maximum cooling capacity state, to slow down the line speed, that is, the steel plate passage speed, and to secure a long cooling time, and the productivity is lowered. In this way, high-strength steel sheets that require strict control of the cooling rate in the transformation temperature range from soft steel sheets that do not have a specified cooling rate after soaking by heat treatment in a continuous annealing facility using a conventional cooling device. It is very difficult to manufacture a wide variety of thin steel sheet products up to (super high tensile material).

The present invention has been made in view of the above-mentioned problems, and the problem is that a heat treatment necessary for producing a high-strength steel plate (super high-tensile material) having a structure of austenite and bainite and having a thickness of 1000 MPa or more is provided. An object of the present invention is to provide a method for continuously annealing a steel sheet using a continuous annealing facility that can be performed without impairing productivity.

  In order to solve the above problems, the present invention employs the following configuration.

That is, at least a heating zone, a soaking zone, a cooling unit on the front side that cools the steel plate at a cooling rate of 10 ° C./s or less, and a rear stage side that cools the steel plate at a cooling rate of 30 ° C./s or more. The structure is austenite and bainite using a continuous annealing facility having a primary cooling zone, a secondary cooling zone, and an overaging zone provided with a gas jet cooling device comprising a cooling unit capable of rapid cooling. In the continuous annealing treatment method of a steel plate for producing a high-strength steel plate of 1000 MPa or more, the steel plate is heated to 850 to 900 ° C. by the heating zone, soaked by the soaking zone, and then the gas jet in the primary cooling zone the front side of the cooling unit of the cooling device, between 500 ° C. from 800 ° C. in the case of performing the roll cooling and between 400 ° C. from 800 ° C. in the case of performing the liquid immersion cooling, 30 ° C. / s High strength of 1000 MPa or more, wherein the structure is composed of austenite and bainite, characterized by rapid cooling at the above cooling rate, secondary cooling by the secondary cooling zone, and subsequent overaging treatment by the overaging zone It is the continuous annealing processing method of the steel plate which manufactures a steel plate.

Here, the front stage side of the gas jet cooling apparatus in the primary cooling zone refers to the middle side to the upstream side of the cooling apparatus, and the rear stage side refers to the middle stage to the downstream side. Also, the mildly in order to obtain the desired cooling rate, each cooling unit of the cooling device, for example, to equalize the coolant gas flow, and a small cooling capacity than if cooling in the same cooling capacity It means cooling, and conversely with rapid cooling, it means cooling with a large cooling capacity in order to obtain the required cooling rate .

  In this way, when manufacturing a high-strength steel plate (high-tensile material, ultra-high-tensile material) having a steel sheet structure composed of austenite and bainite, the cooling unit on the front side of the gas jet cooling device in the primary cooling zone is used. In order to realize the above steel sheet structure, it is possible to cool slowly from a soaking temperature of about 900 ° C. to about 800 ° C., and to cool rapidly in a temperature range from about 800 to 500 ° C. in a subsequent cooling unit. Primary cooling can be performed at the required cooling rate. In addition, the temperature range from 500 degreeC to the overaging temperature of about 400 degreeC after completion | finish of primary cooling can be cooled with the internal water-cooled roll cooling apparatus normally installed in a secondary cooling zone. Thus, the necessary steel sheet structure can be obtained, and the primary cooling end temperature can be lowered to about 500 ° C., so that deformation of the steel sheet due to roll cooling in the secondary cooling zone hardly occurs, and the steel sheet structure is stable. It is possible to perform heat treatment that satisfies the steel plate shape at the same time.

  In addition, when manufacturing a high-strength steel sheet using an immersion cooling device that uses water or the like as a cooling medium that is usually arranged in parallel with the roll cooling device, the cooling unit on the rear stage side can be rapidly cooled. Therefore, since it can cool to a low temperature range of about 500 ° C. in the primary cooling zone, immersion cooling can be performed from the low temperature range, and deformation of the steel sheet is suppressed. Therefore, a desired high-strength steel plate (super high-tensile material) can be manufactured also by performing immersion cooling as secondary cooling. Furthermore, since the cooling device itself does not need to have an excessive cooling capacity particularly on the front side, the cooling device as a whole does not need to be unnecessarily large.

Gas jet cooling apparatus continuous annealing equipment is quenchable cooling unit of the latter stage side, with respect to cooling capacity of mildly possible cooling unit of the preceding stage, it is preferred to have at least 2 times the cooling capacity .

  Here, strictly speaking, the double cooling capacity means that the heat removal flux in the temperature range of 800 to 500 ° C. is twice that of the steel sheet having the same thickness within the range to be heat-treated, that is, When the temperature of the cooling medium gas in each cooling unit is the same, it means that the ratio of the maximum average jet heat transfer coefficient in the temperature range of 800 to 500 ° C. obtained at the maximum output gas flow rate of each cooling unit is twice. To do.

  If it does in this way, the required cooling in a temperature range is satisfied to about 800-500 degreeC required for structure | tissue formation of a high strength steel plate also to the steel plate of the upper limit thickness of object of a continuous annealing process, and it is secondary cooling. Even when immersion cooling is performed, since the cooling start temperature can be further lowered without significantly reducing the plate passing speed from its upper limit value, a high-strength steel plate having a stable shape with suppressed cooling deformation of the steel plate (ultra-high tensile steel material) ) Can be manufactured.

The gas jet cooling device for a continuous annealing facility, the inlet side and outlet side of the cooling device, as well as to an intermediate position Rukoto provided surface thermometer of the steel sheet between the outlet side cooling unit and the inlet side cooling unit preferable.

  Thus, by installing the surface thermometer not only at the entrance / exit side of the cooling device but also at the intermediate position, it becomes possible to control the cooling rate necessary for the formation of the structure of the high-strength steel sheet with higher accuracy.

According to this invention , efficient cooling adapted to the cooling rate necessary for realizing a structure of a high-strength steel sheet of 1000 MPa or more is possible. Also, the cooling unit capable of rapid cooling on the rear stage side can lower the primary cooling end temperature even for the steel plate of the maximum thickness to be processed, so that the steel plate is used during roll cooling or immersion cooling in the secondary cooling zone. Therefore, heat treatment that satisfies the steel sheet structure and the stable steel sheet shape at the same time can be performed. Thereby, the continuous annealing treatment required for producing a high-strength steel plate (super high-tensile material) having a structure of austenite and bainite and having 1000 MPa or more can be performed without impairing productivity.

  Embodiments of the present invention will be described below with reference to the accompanying FIGS.

  FIG. 1 shows a main part of a continuous annealing furnace 1 for a thin steel plate. A steel plate that has been subjected to a pretreatment such as a surface cleaning treatment from an entry-side facility has a temperature range above the recrystallization temperature in the heating zone 2a, For example, in the case of a high-strength steel plate, after being heated to a temperature range of 850 to 900 ° C., it is soaked in the soaking zone 2b for uniforming the structure and ensuring hardenability, etc. The primary cooling zone 3a and the secondary cooling zone 3b are rapidly cooled at a required cooling rate. And the steel plate when rapidly cooled to room temperature by the liquid immersion cooling device in the secondary cooling zone 3b is reheated to the overaging temperature in the reheating zone 4 and then held at this temperature in the overaging zone 5. Then, it is cooled to a temperature that can be handled in the final cooling zone 6.

  As shown in FIG. 2, in the primary cooling zone 3a accommodated in the furnace shell F, four stages of cooling units 7a, 7b, 7c, 7d are arranged along the plate passing direction, The cooling units 7a and 7b and the rear cooling units 7c and 7d have the same nozzle density (nozzle number / steel plate cooling area), for example, so that the nozzle for gas injection can be uniformly cooled over the entire width of the steel plate. Many are arranged in a lattice or zigzag so as to be cooled from both sides of the steel plate. From each nozzle of each cooling unit 7a to 7d, for example, HNX gas (composition: H2 gas 5 to 10%, N2 gas 95 to 90%) which is a general non-oxidizing atmosphere gas of the annealing furnace is used as a cooling medium. The nozzles are connected to the fans 8a to 8d through headers so that the refrigerant gas is injected evenly. Since the non-oxidizing gas of the cooling medium is circulated, the return gas sprayed on the steel sheet is cooled to a predetermined cooling medium temperature by the gas cooling device 9 and then cooled from the fans 8a to 8d. It is supplied to each of the units 7a to 7d. A flow rate regulator (not shown) is provided on the cooling medium gas discharge side of each of the fans 8a to 8d. Steel plate surface thermometers 10a, 10b, and 10c are installed on the inlet side of the cooling unit 7a, the outlet side of the cooling unit 7b, and the outlet side of the cooling unit 7d, respectively. The secondary cooling zone 3b provided subsequent to the primary cooling zone 3a includes a roll cooling device 11 in which rolls 11a whose interior is cooled by water cooling or the like are arranged in multiple stages, and a liquid dipping device using water as a cooling medium. 12 are juxtaposed and can be used according to the required characteristics of the high-strength steel sheet.

  The cooling units 7a and 7b on the front stage side are formed so as to be slowly cooled, and the cooling units 7c and 7d on the rear stage side are formed so as to be capable of rapid cooling. That is, the maximum cooling capacity of the cooling units 7a and 7b is a temperature range (900 to 500 ° C.) that requires a cooling rate control in order to produce a high-strength steel plate (super high-tensile material) in a predetermined cooling time. The cooling units 7a to 7d are designed to have a cooling capacity smaller than that in the case of cooling with the same cooling capacity (average cooling capacity). Conversely, the maximum cooling capacity of the cooling units 7c and 7d is designed to be greater than the average cooling capacity. Specifically, the nozzle configuration of each of the cooling units 7a to 7d is the same, and the fans 8c of the cooling units 7c and 7d with respect to the capacity (maximum output gas flow rate) of the fans 8a and 8b of the cooling units 7a and 7b, By increasing the capacity (maximum output gas flow rate) of 8d, it is possible to perform slow cooling on the front side and rapid cooling on the rear side. Further, on the rear stage side, the gas cooling device 9, the temperature after cooling by the gas cooling devices 9, 9 of the refrigerant gas circulated, that is, the refrigerant gas discharged to the cooling units 7 c, 7 d becomes lower. If the apparatus configuration in which the heat exchange capacity of 9 is increased, more effective rapid cooling can be realized by the cooling units 7c and 7d on the rear stage side.

  Temperature measurement data from the steel plate surface thermometers 10a, 10b, and 10c installed on the inlet side of the cooling unit 7a, the outlet side of the cooling unit 7b, and the outlet side of the cooling unit 7d, respectively, It is taken into the computing unit together with the plate speed, and compared with the respective target temperatures of the outlet side of the cooling unit 7b (front side cooling unit outlet side) and the outlet side of the cooling unit 7d (outside side of the rear stage cooling unit), Based on the deviation, the refrigerant gas discharge flow rates of the fans 8a to 8d of the cooling units 7a to 7d are controlled so that the deviation from the target outlet temperature is eliminated, and the cooling capacity is adjusted. The temperature deviation and the discharge flow rate of the refrigerant gas to be controlled can be calculated in advance in correspondence with the steel plate dimensions (plate thickness) and the plate passing speed.

  3A and 3B show, as described above, the steel plate cooling pattern in the case where the front side of the gas jet cooling device in the primary cooling zone is mildly cooled and the rear side is rapidly cooled, as shown in the secondary cooling zone. The case where a roll cooling device is used and the case where a liquid immersion cooling device is used are schematically shown. FIGS. 4 (a) and 4 (b) schematically show steel plate cooling patterns in the case where the entire cooling unit of the gas jet cooling is cooled with the same cooling capacity without being moderately slowed. From these schematic diagrams, in the case where the secondary cooling zone is either a roll cooling device or a liquid immersion device, a high-strength steel plate (high-tensile steel) is more likely to perform two-stage cooling of gentle cooling on the front side and rapid cooling on the rear side. , Ultra-high-tensile material), it is possible to control the cooling rate strictly, that is, to increase the cooling rate in the temperature range (800-500 ° C or 800-400 ° C) that requires rapid cooling, and the steel plate after soaking is efficient It can be seen that it can be cooled. This slow and rapid two-stage cooling is effective for an ultra-high tensile material that requires a quick cooling of 30 ° C./s, particularly in the temperature range.

  The primary cooling zone of the quenching zone 1 of the continuous annealing equipment shown in FIG. 1 is the same as that of the cooling zone 7a, 7b, the rear side shown in FIG. Two types of high-strength steel plates with a four-stage gas jet cooling device of the cooling units 7c and 7d, each having a maximum thickness of 2.6 mm to be processed and components designed for the cooling path RQ and the cooling path WQ, respectively. All the materials to be processed (ultra-high tensile material) are cooled by gas jet at a cooling start temperature of 900 ° C., and then roll cooling (cooling path RQ) and immersion cooling in water (cooling path WQ) are performed in the secondary cooling zone. Each was applied to produce a high-strength steel plate (super high-tensile material). Details of gas jet cooling at that time are shown in Table 1. The cooling start temperature, that is, the steel plate temperature on the inlet side of the cooling unit 7a is 900 ° C. in both the cooling paths RQ and WQ, and the maximum plate passing speed is 60 m / min in any cooling path. The cooling capacity ratio between 7a and 7b and the rear cooling units 7c and 7d was 1: 1.5 in the cooling path RQ and 1: 2 in the cooling path WQ.

  From Table 2, by the rapid cooling in the cooling units 7c and 7d on the rear stage side, the temperature range of 800 to 500 ° C. is exceeded in the case of the cooling path RQ, and the temperature range of 800 to 400 ° C. is exceeded in the case of the cooling path WQ. It can be seen that the cooling rate of 30 ° C./s necessary for the production of the high-tensile material is realized. In the case of the cooling path RQ, the cooling units 7a and 7b on the front stage side pass the intermediate temperature that is the outlet side temperature of the cooling unit 7b, that is, the cooling start temperature on the rear stage side by slow cooling from the cooling start temperature 900 ° C. It is possible to adjust the plate speed to 800 ° C. without reducing the plate speed from the maximum plate passing speed. On the other hand, in the case of the cooling pass WQ, in order to suppress deformation of the steel sheet due to immersion cooling in water, it is necessary to lower the gas jet cooling end temperature to 400 ° C. than in the case of the cooling pass RQ. Due to the restrictions on the facility capacity of the gas jet cooling apparatus of the example, the plate passing speed is reduced by about 25% from the maximum plate passing speed. Also in this case, the cooling capacity ratio of the first-stage cooling units 7a and 7b and the second-stage cooling units 7c and 7d is 1: 2, and the first-stage cooling starts from the cooling start temperature of 900 ° C. by the first-stage cooling. The temperature can be adjusted to 800 ° C. That is, by increasing the cooling capacity of the rear-stage cooling units 7c and 7d to twice the cooling capacity of the front-stage cooling units 7a and 7b, the cooling rate required for the above-mentioned ultra-high tensile material can be realized. It has become. It should be noted that by maintaining the cooling capacity ratio at 1: 2 and increasing the facility capacity of the gas jet cooling capacity to be higher than the facility capacity in the case of the embodiment, even in the case of the cooling path WQ, the plate passing speed can be maximized. It is possible to realize the above-mentioned cooling rate necessary for the production of the super high tensile material without lowering the plate speed.

  On the other hand, when the cooling is performed uniformly by the cooling units 7a to 7d without performing the two-stage cooling of the first-stage-side gentle cooling and the second-stage-side rapid cooling, that is, the cooling capacity ratio between the first-stage and the second-stage is 1: 1. In this case, in the cooling path RQ, the cooling rate is about 20 ° C./s in the gas jet cooling temperature range of 900 to 500 ° C., and the required cooling rate of 30 ° C./s cannot be obtained. Further, in the cooling path WQ, the cooling rate is about 15 ° C./s in the same cooling temperature range of 900 to 400 ° C., and does not reach the required cooling rate of 30 ° C./s when performing immersion cooling as secondary cooling. In addition, if the cooling capacity of each cooling unit is increased uniformly to achieve the required cooling rate, the gas jet cooling end temperature becomes too low, and there is a risk that harmful quenching structures will be generated in the ultra-high tensile material. It becomes high and the structure necessary for ultra-high tensile materials cannot be obtained.

  The first-stage cooling units 7a and 7b that perform slow cooling and the second-stage cooling units 7c and 7d that perform rapid cooling do not necessarily have to be set to the same cooling capacity in two cooling units, respectively. It is also possible to achieve the required cooling rate by individually adjusting the cooling capacity of the cooling unit on both the rear stage side.

The present invention can be used as a continuous annealing treatment method for a high-strength steel sheet, particularly a thin steel sheet that enables the manufacture of a super high-tensile material having a strength of approximately 1000 MPa or more .

It is explanatory drawing which shows the principal part of a steel plate continuous annealing furnace. It is explanatory drawing of the gas jet cooling device of a steel plate continuous annealing furnace . It is explanatory drawing of the cooling pattern of the steel plate by the gas jet cooling device of FIG. It is explanatory drawing of the cooling pattern of the steel plate by the gas jet cooling device of a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Continuous annealing furnace 2a ... Heating zone 2b ... Soaking zone 3 ... Quenching zone 3a ... Primary cooling zone 3b ... Secondary cooling zone 4 ... Reheating zone 5 .... Over-aging zone 6 ... Final cooling zones 7a-7d ... Cooling units 8a-8d ... Fan 9 ... Gas cooling devices 10a-10c ... Steel plate surface thermometer 11 ... Roll cooling Device 11a ... Roll 12 ... Liquid immersion device D ... Duct F ... Furnace shell GA, GB ... Gas jet cooling device P ... Steel plate

Claims (1)

At least a heating zone, a soaking zone, a cooling unit that can cool the steel plate at a cooling rate of 10 ° C./s or less, and a quenching unit that cools the steel plate at a cooling rate of 30 ° C./s or more. The structure is composed of austenite and bainite using a continuous annealing facility having a primary cooling zone, a secondary cooling zone, and an overaging zone provided with a gas jet cooling device composed of a possible cooling unit. In the continuous annealing method of a steel plate for producing a high-strength steel plate of 1000 MPa or more, after heating the steel plate to 850 to 900 ° C. by the heating zone and soaking by the soaking zone, the gas jet cooling device in the primary cooling zone The cooling unit on the front side of the gas is slowly cooled to 800 ° C. at a cooling rate of 10 ° C./s or less, and then the cooling unit on the rear side of the gas jet cooling device in the primary cooling zone. Between 500 ° C. from 800 ° C. in the case of performing the roll cooling and between 400 ° C. from 800 ° C. in the case of performing the liquid immersion cooling, quenched with a cooling rate higher than 30 ° C. / s, further the secondary cooling zone A method of continuous annealing of a steel sheet for producing a high-strength steel sheet having a structure of austenite and bainite having a structure of austenite and bainite, wherein the structure is subjected to over-aging treatment by the above-described over-aging band.

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