JP2012036412A - Method for continuous-annealing steel sheet using continuous-annealing furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous-annealing method, with which in the method for continuous-annealing a steel sheet using a continuous-annealing furnace, a scratch developed in a P-additional extra-low carbon BH steel, containing 0.0014-0.0025% C, ≤0.5% Si, 0.03-1.0% Mn, 0.01-0.15% P, ≤0.015% S, 0.005-0.1% Al and ≤0.0040% N, can be made to disappear.SOLUTION: The continuous-annealing furnace is provided with a heating zone, a soaking zone, a primary cooling zone, an overaging zone and a secondary cooling zone, in this order. In the continuous-annealing furnace, the plurality kinds of steel, containing two kinds of a low carbon aluminum-killed steel and the P-additional extra-low carbon BH steel, are continuously annealed, and the outlet-side temperature of the overaging zone is made to be in the range of 250-300°C at the time of annealing the low carbon aluminum-killed steel and is made to be the temperature of >300°C at the time of annealing the P-additional extra-low carbon BH steel.

Description

  The present invention relates to a method for continuously annealing a steel sheet using a continuous annealing furnace.

  The cold-rolled steel sheet is work-hardened as it is and its workability is extremely poor, so that good workability is achieved through an annealing process. As an annealing method for such a cold-rolled steel sheet, a batch annealing method and a continuous annealing method are used. The present invention is directed to a continuous annealing method using a continuous annealing furnace.

  In the continuous annealing method, the cold-rolled steel strip unrolled from the coil is annealed when it passes through the continuous annealing furnace, and more usually, after coil cooling, it is continuously combined from temper rolling to inspection and refinement. The latter steel strip is wound on a coil. As shown in FIG. 1, in the continuous annealing furnace 1, the steel strip passes through the heating zone 5, the soaking zone 6, the primary cooling zone 7, the overaging zone 8, and the secondary cooling zone 9 in this order, and heat treatment is performed. Applied. On the entry side of the facility, two payoff reels 2, a welder 3, and an entry side looper 4 are installed. When the rewinding of the steel strip from one coil is completed, the steel strip from another coil is rewound. Then, the two steel strips are joined by the welding machine 3, and the other coil is continuously fed into the continuous annealing furnace 1.

  In the continuous annealing device, the steel strip length from the payoff reel 2 to the take-up reel 12 is close to 2000 m. Therefore, in order to pass the sheet normally, the roll horizontal, the orthogonal center adjustment, the roll shape and the roll crown Selection of quantity, introduction of automatic position control device, etc. are performed. In a furnace such as an overaged zone, a center position control (CPC) such as an intermediate guide type steering device is installed as an automatic position control device (Non-Patent Document 1, page 654).

  In continuous annealing, various types of steel plates are fed one after another, and continuous annealing is performed sequentially. Typical varieties subjected to continuous annealing include low-carbon aluminum killed steel and ultra-low-carbon aluminum killed steel, and ultra-low-carbon aluminum killed steel further includes Ti-added ultra-low-carbon aluminum killed steel and Nb-Ti-added ultra-low-carbon aluminum killed steel. Can be divided into steel. About the heating rate in the heating zone of the continuous annealing furnace and the annealing temperature in the soaking zone, optimum annealing conditions are set for each type.

  After recrystallization and crystal grain growth in the heating zone 5 and soaking zone 6, rapid cooling in the primary cooling zone 7 and overaging treatment in the overaging zone 8 are performed to precipitate supersaturated solid solution C. That is, after heating and soaking, a supersaturated solid solution state of solid solution C is formed in the primary cooling zone 7 by rapid cooling to 500 ° C. or less, and a driving force for precipitation is given. Next, heating and holding is performed at a predetermined temperature and time in the overaging zone 8 to precipitate the solid solution C almost completely (Non-Patent Document 1, page 638).

  In FIG. 2, the outline | summary of the equipment from the primary cooling zone 7 of the continuous annealing furnace to the secondary cooling zone 9 through the overaging zone 8 is shown. The steel strip temperature in the primary cooling zone is measured by the primary cooling zone thermometer 22, and the steel strip temperature on the overaged zone outgoing side is measured by the overaged zone outgoing side thermometer 21. CPC is installed in two places (20a, 20b) for automatic position control of the steel strip.

  In low-carbon aluminum killed steel, the plate temperature that enables the most efficient overaging treatment, that is, overaging with the overaging zone entry side temperature in the range of 380 to 460 ° C and the overaging band exit side temperature in the range of 250 to 300 ° C Processing is performed. On the other hand, for extremely low-carbon aluminum killed steel, the temperature in the overaging zone is not particularly specified.

  As described above, in the continuous annealing, various varieties are successively sent to the continuous annealing furnace. Regarding the overaging zone, the temperature range is specified for low-carbon aluminum killed steel as described above, and there is no special specification for extremely low-carbon aluminum killed steel. Therefore, as the temperature setting of the overaging zone, the temperature setting required for the low-carbon aluminum killed steel is set, and the same temperature setting is used when annealing the ultra-low-carbon aluminum killed steel.

  Patent Document 1 discloses a method for producing an alloyed hot-dip galvanized steel sheet having paint bake hardenability (BH property), room temperature slow aging property, and formability. In mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% , Al: 0.005 to 0.1%, N ≦ 0.0040%, a slab having a composition composed of the balance Fe and inevitable impurities is subjected to hot rolling and cold rolling under predetermined conditions, and continuous annealing. After annealing at 650 ° C to Ac3 transformation point in the line, temper rolling was performed to produce a cold-rolled steel sheet that had been annealed once, and subsequently heated to the galvanizing bath temperature at the continuous hot dip galvanizing line and plated The alloying heat treatment is performed under predetermined conditions, and then temper rolling is performed again. Using an annealed cold-rolled steel sheet, this is rapidly heated, and alloyed hot-dip galvanized steel sheet is manufactured by alloying heat treatment after plating immersion. Furthermore, the manufacturing method of the alloyed hot-dip galvanized steel plate which has BH property and normal temperature slow aging property, and was excellent in the paint bake hardening performance can be provided. Hereinafter, the steel having the component is referred to as “P-added extremely low carbon BH steel”.

  About the continuous annealing method of the steel plate using a continuous annealing furnace, it is also possible to carry out by directly connecting cold rolling and continuous annealing. Patent Document 2 discloses a continuous cold-rolled steel sheet manufacturing facility in which a continuous cold rolling facility and a continuous annealing facility are directly connected.

JP 2009-249715 A Japanese Examined Patent Publication No. 2-47282

Edited by the Japan Iron and Steel Institute, “Third Edition Steel Handbook III (1) Rolled Foundation / Steel”, published by Maruzen Co., Ltd. in May 1980

  In a continuous annealing furnace, continuous annealing was performed on varieties including low-carbon aluminum killed steel and ultra-low-carbon aluminum killed steel. As a phenomenon peculiar to the P-added ultra-low-carbon BH steel among ultra-low-carbon aluminum killed steel, continuous annealing was performed. The symptom that a crack was generated on the surface of the steel strip after completion was observed. On the surface in the vicinity of the width end of the steel strip, there is generated a soot that is inclined by several tens of degrees from the longitudinal direction of the steel strip and has a length of less than 1 mm. Such generation of soot was a phenomenon observed only in the P-added extra low-carbon BH steel.

  An object of the present invention is to provide a continuous annealing method capable of eliminating the soot generated in the P-added ultra low carbon BH steel in a continuous annealing method of a steel sheet using a continuous annealing furnace.

That is, the gist of the present invention is as follows.
(1) A method for continuous annealing of a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order,
In mass%, C: 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, S ≦ 0.01%, Al: 0 0.005 to 0.1% steel, balance Fe and inevitable impurities (hereinafter referred to as “low-carbon aluminum killed steel”),
In mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% , Al: 0.005 to 0.1%, N ≦ 0.0040%, including two types of steel (hereinafter referred to as “P-added ultra-low carbon BH steel”) composed of Fe and inevitable impurities. Continuously anneal multiple types of steel plates,
The outlet temperature of the overaging zone is in the range of 250 to 300 ° C. during the annealing of the low-carbon aluminum killed steel, and the temperature exceeds 300 ° C. during the annealing of the P-added extremely low-carbon BH steel. Continuous annealing method for steel sheet.
(2) The P-added ultra low carbon BH steel is in mass%, C: 0.0014 to 0.0030%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.00 01 to 0.15%, S ≦ 0.015%, Al: 0.005 to 0.1%, N ≦ 0.0040%, Ti: 0.002 to 0.015% and Nb: 0 In the above (1), one or two of 0.002 to 0.015% are contained so that Ti + Nb = 0.002 to 0.015%, and the balance is Fe and unavoidable impurities. The continuous annealing method of the described steel plate.
(3) The P-added ultra-low charcoal BH steel further contains B: 0.0001 to 0.0040% in mass%, and the steel sheet according to (1) or (2) above Continuous annealing method.
(4) The steel sheet according to any one of (1) to (3), wherein the low-carbon aluminum killed steel further contains B: 0.0020 to 0.0035% by mass. Continuous annealing method.

  In the continuous annealing method of the steel sheet using the continuous annealing furnace, the present invention changes the temperature pattern of the overaging zone depending on the type, and the exit temperature of the overaging zone is 250 to 300 ° C. when annealing the low-carbon aluminum killed steel. When the P-added ultra-low carbon BH steel is annealed, the temperature exceeding 300 ° C. can be used to significantly reduce the soot generated in the P-added ultra-low carbon BH steel.

It is a conceptual diagram of a continuous annealing apparatus. It is a conceptual diagram which shows the overaging zone of a continuous annealing furnace. It is a figure which shows a stress-strain diagram for every temperature about Nb-Ti addition ultra-low carbon steel. It is a figure which shows a stress-strain diagram for every temperature about P addition ultra low carbon BH steel. It is a figure which shows a stress-strain diagram for every temperature about low carbon aluminum killed steel.

  As described above, among the temperature patterns in the continuous annealing furnace, the temperature pattern in the overaging zone is specified only for the low-carbon aluminum killed steel, and the ultra-low-carbon aluminum killed steel is not particularly specified. The same applies to P-added ultra-low-carbon BH steel, which is a kind of ultra-low-carbon aluminum killed steel.

  In the present invention, P-added ultra-low carbon BH steel is, first, in mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015%, Al: 0.005 to 0.1%, N ≦ 0.0040%, steel that consists of the balance Fe and inevitable impurities . Second, in mass%, C: 0.0014 to 0.0030%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015%, Al: 0.005 to 0.1%, N ≦ 0.0040%, and Ti: 0.002 to 0.015% and Nb: 0.002 to 0.015% One or two types are contained so that Ti + Nb = 0.002 to 0.015%, and the steel consists of the balance Fe and inevitable impurities. Thirdly, it further contains B: 0.0001 to 0.0040% by mass. By setting the C content within the first range, sufficient BH properties and room temperature non-aging properties are ensured. When Ti and Nb are contained, the C content can be expanded to the second range. By making Si content into the description range, press moldability becomes favorable. By making Mn content into the description range, an ear crack caused by S is prevented and a suitable strength is secured. By containing P in the notation range, strength is increased and workability is improved. The lower limit of the P content is more preferably 0.045% by mass. Al is used for deoxidation and N fixation. Ti and Nb have a role of securing a slow aging property at room temperature by fixing a part of N, C, and S. B is effective in preventing secondary work embrittlement and has the effect of lowering the recrystallization temperature.

  In the present invention, the low-carbon aluminum killed steel is, in mass%, C; 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, It means steel composed of S ≦ 0.01%, Al: 0.005 to 0.1%, balance Fe and inevitable impurities. In the low-carbon aluminum killed steel having such a component, the overaging treatment is performed with the overaging zone entry side temperature in the range of 380 to 460 ° C. and the overaging band exit side temperature in the range of 250 to 300 ° C. That is, the plate temperature enables the most efficient overaging treatment.

  On the other hand, the temperature pattern in the overaging zone is not specified for the P-added extra low-carbon BH steel having the specified components. Paragraph [0033] of Patent Document 1 describes “heat treatment conditions” as temperature conditions in the overaging zone, and is not particularly limited as described in the paragraph.

  In the continuous annealing, annealing is performed by sequentially supplying many types of steel plates such as the above-mentioned low-carbon aluminum killed steel, P-added extra-low-carbon BH steel, and extra-low-carbon aluminum-killed steel other than those listed on the left. Of these, the temperature pattern in the overaging zone is specified only for low-carbon aluminum killed steel, and other grades are not specifically specified. The temperature pattern is fixed to the specified temperature pattern, and the same temperature pattern is used for all varieties.

  Here, as described above, as a phenomenon peculiar to the P-added ultra-low carbon BH steel, a symptom in which a soot was generated on the surface of the steel strip after the continuous annealing was observed. On the surface in the vicinity of the width end of the steel strip, there is generated a soot that is inclined by several tens of degrees from the longitudinal direction of the steel strip and has a length of less than 1 mm. Such generation of soot was a phenomenon observed only in the P-added extra low-carbon BH steel. Further, it was found that the operation amount of the center position control (CPC) provided in the overaging zone of the continuous annealing furnace is particularly large in the P-added ultra low carbon BH steel. Furthermore, in the treatment of P-added ultra-low charcoal BH steel, there was a tendency that the greater the amount of CPC operation, the greater the degree of generation of soot.

  Therefore, the stress-strain behavior in the temperature region of the overaged zone was evaluated for the P-added extremely low carbon BH steel. The behavior and contrast of low-carbon aluminum killed steel and Nb-Ti added ultra-low carbon steel were compared. The components of the steel used for the evaluation are as shown in Table 1.

  The stress-strain behavior was evaluated by setting the temperature in steps of 50 ° C. in a temperature range of 150 ° C. to 400 ° C., and applying 0 to 15% strain at each temperature to evaluate the stress behavior. Strain was increased at a constant rate, and the relationship between stress and strain was evaluated. Usually, as the strain increases, the stress increases monotonously. However, in the stress-strain measurement at a specific temperature, when the strain is increased at a constant speed, the stress may be observed as a phenomenon in which the stress repeatedly increases and decreases over time. This is due to dynamic strain aging (serration). Therefore, in FIG. 3, each product was graphed as a stress-strain diagram at each temperature from room temperature to 400 ° C.

  As a result, as shown in FIG. 3, it was found that dynamic strain aging (serration) was observed in the temperature range near 300 ° C. as a phenomenon peculiar to the P-added ultra low carbon BH steel. When passing the over-aging zone of a continuous annealing furnace in a temperature range where dynamic strain aging is observed, non-uniform elongation occurs in the steel strip in the furnace, the steel strip tends to be biased, and CPC is used to correct the bias. It is presumed that the steel strip generates micro-slip between the rolls when it is operated, and eventually leads to the generation of a unique thread.

  From the above observation results, it has been found that it is effective to avoid a temperature range in which dynamic strain aging occurs as a temperature pattern in the overaging zone when P-added ultra low carbon BH steel is continuously annealed. Here, as the temperature pattern in the overaging zone, it is necessary to select whether to avoid the temperature range where dynamic strain aging occurs on the high temperature side or on the low temperature side.

  As is clear from FIG. 3, the high temperature characteristics of the P-added ultra low carbon BH steel were characterized by the fact that the high temperature strength was very high in addition to the occurrence of dynamic strain aging. Compared with the Nb—Ti-added ultra-low carbon steel, the difference in the vicinity of 350 ° C. is the largest, and the P-added ultra-low carbon BH steel has about twice the strength. When considering the contact between the roll and the steel strip in the overaging zone of the continuous annealing furnace, the higher the strength of the steel strip in the temperature range of the overaging zone, the smaller the friction coefficient between the steel strip and the roll, As a result, the steel strip tends to be biased to one side of the roll, and when the CPC is operated to correct the bias, the steel strip generates a microslip with the roll, and finally a unique slip occurs. It is estimated that Combined with the high-temperature strength of P-added ultra-low-carbon BH steel, when trying to avoid the temperature range where dynamic strain aging occurs as a temperature pattern of the overaging zone on the low temperature side, the high-temperature strength increases and the generation of cracks. May be encouraged.

  As a measure for preventing the meandering of the steel strip in the overaging zone of the continuous annealing furnace, the thermal crown of the roll is actively used. When the roll is viewed in the axial direction, since the hot steel strip always passes through the center of the roll, the temperature rises and thermally expands, and since the steel strip does not pass through both ends of the roll, the temperature rise is small and thermal expansion also occurs. Few. As a result, the roll in use for continuous annealing has a shape in which the diameter of the central portion is larger than that at both ends as compared to that at the normal temperature, and a crown is formed. Such a phenomenon is called a thermal crown. Due to the occurrence of the thermal crown of the roll, the occurrence of meandering of the steel strip is reduced. On the other hand, if you try to avoid the temperature range where dynamic strain aging occurs as a temperature pattern of the overaging zone on the low temperature side, the roll temperature will drop and the thermal crown will decrease, reducing the meandering prevention effect of the strip steel there's a possibility that.

  From the above situation, when continuously annealing the P-added ultra low carbon BH steel, it is judged to be effective to avoid the temperature range where dynamic strain aging occurs as a temperature pattern in the overaging zone on the high temperature side. can do. And it becomes possible to prevent generation | occurrence | production of a soot by making the outgoing side temperature of an overaging zone into the temperature which exceeds 300 degreeC at the time of P addition ultra-low carbon BH steel annealing. In the overaging zone, the temperature of the steel strip gradually decreases from the upstream to the downstream, so if the overaging zone outlet side temperature exceeds 300 ° C, the steel strip temperature should exceed 300 ° C throughout the overaging zone. Can do. Here, the overaging zone outlet side temperature means the secondary cooling zone inlet side temperature. Specifically, the temperature measurement result of the overaging band-side thermometer 21 shown in FIG. 2 is used as the overaging band-side temperature.

  On the other hand, in low-carbon aluminum killed steel, it is necessary to set the overaging band temperature to 250 to 300 ° C. In addition, for varieties other than the low-carbon aluminum killed steel and the P-added extra-low-carbon BH steel, the temperature pattern of the overaging zone is not particularly specified.

  Therefore, in the present invention, as the outgoing temperature pattern of the overaging zone in the continuous annealing furnace, the range is 250 to 300 ° C. at the time of annealing the low carbon aluminum killed steel, and 300 ° C. at the time of annealing the P-added ultra low carbon BH steel. The temperature is controlled so as to exceed the temperature.

  In the temperature control of a normal continuous annealing furnace, the time for raising the overaging zone temperature from 250 ° C. to 300 ° C. is about 90 minutes, and the time required for lowering the temperature from 300 ° C. to 250 ° C. is about 90 minutes. Therefore, as the order of the coils supplied to the continuous annealing furnace, the low-carbon aluminum killed steel and the P-added extra-low-carbon BH steel are not placed next to each other, and other varieties (hereinafter referred to as “buffer material”) between them. It is preferable to insert. The temperature of the buffer material is increased or decreased while it passes through the overaging band. With respect to the buffer material, the temperature pattern of the overaging zone is not particularly limited, and therefore the temperature can be raised or lowered in any time zone during which the buffer material passes through the overaging zone.

  The present invention uses a coil made of a cold-rolled steel strip that has been cold-rolled, rewinds the cold-rolled steel strip from the coil, and performs annealing in a continuous annealing furnace, as well as cold rolling and continuous annealing. The present invention can also be applied when directly connected.

  The present invention was applied to continuous annealing of a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order. The varieties to be treated are low-carbon aluminum killed steel, P-added ultra-low-carbon BH steel, and other ultra-low-carbon steels (Ti-added ultra-low-carbon aluminum killed steel, Nb-Ti-added ultra-low-carbon aluminum killed steel, etc.).

  In the conventional example, a temperature pattern common to all varieties has been adopted as the temperature pattern in the overaging zone. It is a temperature pattern required in low-carbon aluminum killed steel. The primary cooling zone outlet temperature was 430 ° C., the overaging zone heater was turned off, and the overaging zone outlet side temperature was 270 ° C.

  On the other hand, in the example of the present invention, the same temperature pattern as in the conventional example is used when processing the low-carbon aluminum killed steel, and the primary cooling zone outlet temperature is increased to 460 ° C. when processing the P-added extremely low-carbon BH steel, The heater was turned on and the overaging zone temperature was 305 ° C. Other materials (Ti-added ultra-low-carbon aluminum killed steel, Nb-Ti-added ultra-low-carbon aluminum killed steel, etc.) were always inserted between the low-carbon aluminum killed steel and the P-added ultra-low-carbon BH steel as buffer materials. When the low-carbon aluminum killed steel and the P-added extra-low-carbon BH steel are replaced, the temperature of the overaging zone is changed during the treatment time of the buffer material inserted between the two, and the heating rate is 40 ° C / hour. The temperature was lowered at a rate of 40 ° C./hour.

  The amount of CPC deviation of the overaged zone when processing P-added ultra-low carbon BH steel and Nb-Ti ultra-low carbon steel as a comparative material was investigated. The CPC deviation amount means a plate deviation amount from the center on the exit side of the CPC roll arranged in the top hearth roll immediately before the last stage of the overaging zone. The results are shown in Table 2.

  As is apparent from Table 2, the CPC deviation amount of the P-added extra low-carbon BH steel, both the average value and the standard deviation, is significantly improved in the present invention example compared to the conventional example, and Nb-Ti which is a comparative material The value was comparable to ultra-low charcoal steel.

  Next, for the P-added ultra-low carbon BH steel, a comparison was made on the level of steel plate slag generation after annealing. The degree of the soot was evaluated by the depth of the soot, and the depth of 5 μm or less was “small”, 5 to 10 μm was “medium”, and 10 to 15 μm was “large”. Three passes “Small” or “None”. In the conventional example, “medium” and “large” occur 80% in total, but in the example of the present invention, the occurrence of “medium” and “large” is eliminated, “small” is 70%, and “none” is 30. %, And the pass rate improved significantly.

DESCRIPTION OF SYMBOLS 1 Continuous annealing furnace 2 Payoff reel 3 Welding machine 4 Entry side looper 5 Heating zone 6 Soaking zone 7 Primary cooling zone 8 Overaging zone 9 Secondary cooling zone 10 Outlet looper 11 Temper rolling mill 12 Winding reel 20 CPC
21 Overaging zone thermometer 22 Primary cooling zone thermometer

Claims (4)

A continuous annealing method for a steel sheet using a continuous annealing furnace having a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone in this order,
In mass%, C: 0.03-0.05%, Si ≦ 0.02%, Mn: 0.1-0.2%, P ≦ 0.02%, S ≦ 0.01%, Al: 0 0.005 to 0.1% steel, balance Fe and inevitable impurities (hereinafter referred to as “low-carbon aluminum killed steel”),
In mass%, C: 0.0014 to 0.0025%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0.15%, S ≦ 0.015% , Al: 0.005 to 0.1%, N ≦ 0.0040%, including two types of steel (hereinafter referred to as “P-added ultra-low carbon BH steel”) composed of Fe and inevitable impurities. Continuously anneal multiple types of steel plates,
The exit temperature of the overaging zone is in the range of 250 to 300 ° C. during the annealing of the low-carbon aluminum killed steel, and the temperature exceeds 300 ° C. during the annealing of the P-added extremely low-carbon BH steel. Continuous annealing method for steel sheet.   The P-added ultra low carbon BH steel is in mass%, C: 0.0014 to 0.0030%, Si ≦ 0.5%, Mn: 0.03 to 1.0%, P: 0.01 to 0. .15%, S ≦ 0.015%, Al: 0.005 to 0.1%, N ≦ 0.0040%, Ti: 0.002 to 0.015% and Nb: 0.002 The steel plate according to claim 1, wherein one or two of 0.015% are contained so that Ti + Nb = 0.002 to 0.015%, and the balance is Fe and inevitable impurities. Continuous annealing method.   The P-added ultra-low charcoal BH steel further contains B: 0.0001 to 0.0040% in mass%, and the method for continuously annealing a steel sheet according to claim 1 or 2. The method for continuously annealing a steel sheet according to any one of claims 1 to 3, wherein the low-carbon aluminum killed steel further contains B: 0.0020 to 0.0035% by mass.

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