WO2015125465A1 - High-strength steel sheet and method for producing same - Google Patents

High-strength steel sheet and method for producing same Download PDF

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Publication number
WO2015125465A1
WO2015125465A1 PCT/JP2015/000726 JP2015000726W WO2015125465A1 WO 2015125465 A1 WO2015125465 A1 WO 2015125465A1 JP 2015000726 W JP2015000726 W JP 2015000726W WO 2015125465 A1 WO2015125465 A1 WO 2015125465A1
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Prior art keywords
steel sheet
temperature
steel plate
continuous annealing
less
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PCT/JP2015/000726
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French (fr)
Japanese (ja)
Inventor
祐介 伏脇
由康 川崎
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Ｊｆｅスチール株式会社
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Publication of WO2015125465A1 publication Critical patent/WO2015125465A1/en

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire
    • C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F1/00—Electrolytic cleaning, degreasing, pickling or descaling
    • C25F1/02—Pickling; Descaling
    • C25F1/04—Pickling; Descaling in solution
    • C25F1/06—Iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere

Definitions

  • the present invention relates to a high-strength steel sheet having excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the content of Si or Mn is large, and a method for producing the same.
  • automotive steel plates are used after painting.
  • a chemical conversion treatment called a phosphate treatment is performed on the steel plate for automobiles.
  • Chemical conversion treatment of steel sheets for automobiles is one of the important treatments for ensuring corrosion resistance after painting.
  • Si In order to increase the strength and ductility of the steel plate, it is effective to contain Si in the steel plate.
  • Si is oxidized even in a reducing N 2 + H 2 gas atmosphere in which Fe is not oxidized (which reduces Fe oxide). Due to the oxidation of Si, Si oxide (SiO 2 ) is formed on the surface of the steel sheet. Since this SiO 2 inhibits the formation reaction of the chemical conversion film during the chemical conversion treatment, a minute region (hereinafter referred to as a scale) where the chemical conversion film is not formed is formed on the surface of the steel sheet, and the chemical conversion treatment performance decreases.
  • Patent Document 1 As a conventional technique for improving the chemical conversion processability of a high Si content steel sheet, in Patent Document 1, an iron coating layer of 20 to 1500 mg / m 2 is formed on the steel sheet using an electroplating method, a compound coating method, a vapor deposition method, or the like. A method is disclosed. However, in this method, there is a problem in that an additional equipment such as an electroplating equipment is required for forming the iron coating layer, and the cost is increased due to the increased number of processes.
  • Patent Document 2 the Mn / Si ratio of the steel sheet is defined, and in Patent Document 3, Ni is added to improve phosphate processability.
  • the effect depends on the Si content in the steel sheet, and further improvement is necessary for the steel sheet having a high Si content.
  • Patent Document 4 discloses a method in which an internal oxide layer made of Si-containing oxide is formed within a depth of 1 ⁇ m from the steel sheet surface, and the ratio of the Si-containing oxide in the steel sheet surface length of 10 ⁇ m is 80% or less. Has been.
  • the method described in Patent Document 4 as the means for forming the internal oxide layer that is, the method in which the dew point during annealing is controlled to -25 to 0 ° C. and the oxygen partial pressure during annealing is increased more than usual, the dew point is reduced. Since the area to be controlled is premised on the entire inside of the furnace, the controllability of the dew point is difficult and stable operation is difficult.
  • Patent Document 5 describes a method in which a steel sheet temperature reaches 350 to 650 ° C. in an oxidizing atmosphere to form an oxide film on the steel sheet surface, and then heated to a recrystallization temperature in a reducing atmosphere and cooled. .
  • this method there is a difference in the thickness of the oxide film formed on the surface of the steel sheet depending on the method of oxidation, and oxidation may not occur sufficiently.
  • the oxide film becomes too thick, and the oxide film may remain or peel off during annealing in a reducing atmosphere, which may deteriorate the surface properties.
  • atmosphere is described. Oxidation in the air has a problem that a thick oxide is formed and subsequent reduction is difficult, or a reduction atmosphere with a high hydrogen concentration is necessary.
  • Patent Document 6 a cold rolled steel sheet containing, by mass%, Si of 0.1% or more and / or Mn of 1.0% or more, a condition that the steel sheet temperature is 400 ° C. or more and under an iron oxidizing atmosphere. Describes a method of forming an oxide film on the steel sheet surface and then reducing the oxide film on the steel sheet surface in an iron reducing atmosphere. Specifically, for example, N 2 + H reduces Fe oxide after oxidizing Fe on the surface of the steel sheet under the condition of using a direct fire burner having a steel sheet temperature of 400 ° C. or more and an air ratio of 0.93 to 1.10. Annealing the steel sheet in a two gas atmosphere.
  • Patent Document 6 does not specifically describe the heating temperature of an open flame burner.
  • a large amount of Si is contained (approximately 0.6% or more)
  • the amount of oxidation of Si which is easier to oxidize than Fe, is increased, and the oxidation of Fe is suppressed. Too little.
  • formation of the surface Fe reduced layer after reduction may be insufficient, or SiO 2 may be present on the surface of the steel plate after reduction, resulting in the occurrence of scaling of the conversion coating.
  • JP-A-5-320952 Japanese Patent No. 4319559 Japanese Patent No. 2951480 Japanese Patent No. 3840392 JP 55-145122 A JP 2006-45615 A
  • the present invention has been made in view of such circumstances, and even when the content of Si and Mn is large, a high-strength steel sheet having excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating, and its production It aims to provide a method.
  • an object is to provide a high-strength and high-strength steel sheet having a TS ⁇ El of 20000 or more and a method for producing the same.
  • the dew point or oxygen concentration is increased by simply increasing the steam partial pressure or oxygen partial pressure in the entire annealing furnace, and the inside or outside of the steel sheet is excessively oxidized. Yes.
  • the conventional technique has a problem in dew point control or oxidation controllability in the whole furnace, unevenness in chemical conversion treatment occurs, or corrosion resistance after electrodeposition coating deteriorates.
  • the steel plate passage time in the temperature range where the steel plate temperature is 600 ° C. or higher and 750 ° C. or lower is set to 30 seconds or longer and 10 minutes or shorter.
  • the present invention is based on the above findings, and features are as follows.
  • the heating furnace temperature In the temperature range of 500 ° C. or more and A ° C.
  • the steel sheet is heated under the condition of the hydrogen concentration of the atmosphere: 20 vol% or more, and the furnace temperature: A In the temperature range of over ° C and below B ° C (B: any value selected from 550 ⁇ B ⁇ 750), the steel sheet is heated under the condition that the dew point of the atmosphere is -10 ° C or higher, and the steel sheet reaches the maximum in the continuous annealing
  • the temperature is 600 ° C. or more and 750 ° C. or less, and the steel plate temperature is 6 in the continuous annealing.
  • 0 method of producing a high strength steel sheet excellent in chemical conversion treatability characterized in that ° C. or higher 750 ° C.
  • the steel sheet passage time of the temperature range below the 10 minutes or less than 30 seconds.
  • the steel sheet has a component composition in mass%, further B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to Among 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W: 0.001 to 0.10% and V: 0.001 to 0.10% (1)
  • the manufacturing method of the high-strength steel plate as described in (1) characterized by including 1 or more types of elements chosen from.
  • a high-strength steel sheet having excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating can be obtained even when the content of Si or Mn is large.
  • the method for producing a high-strength steel sheet according to the present invention employs the following (Condition 1) to (Condition 3) when the steel sheet is continuously annealed.
  • (Condition 1) In the heating process of continuous annealing, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ⁇ A ⁇ 600), the hydrogen concentration: 20 vol% or more.
  • the steel sheet is heated under the conditions, and in the temperature range of the heating furnace temperature: A ° C. to B ° C. (B: any value selected from 550 ⁇ B ⁇ 750), the dew point of the atmosphere is ⁇ 10 ° C. or more. Heat the steel plate.
  • the manufacturing method of a steel plate used as the object of continuous annealing is demonstrated.
  • the manufacturing method of a steel plate is not specifically limited.
  • a method of producing a hot-rolled sheet by hot rolling steel a method of producing a cold-rolled sheet by cold rolling after hot rolling the steel, hot rolling the steel, pickling, A method of manufacturing a cold-rolled sheet by cold rolling can be employed.
  • the hot-rolled sheet and the cold-rolled sheet thus obtained can be used as an object for continuous annealing.
  • the conditions for hot rolling and pickling at the time of manufacturing the steel sheet are not particularly limited, and may be set as appropriate.
  • the cold rolling is preferably performed at a rolling reduction of 40% or more and 80% or less.
  • the rolling reduction is less than 40%, it is difficult to obtain a plate thickness as a steel plate for automobiles.
  • the rolling reduction exceeds 80%, the steel sheet is a high-strength steel sheet, so that not only the rolling cost is increased, but surface concentration during annealing is increased, and chemical conversion treatment properties may be deteriorated.
  • Continuous annealing can be performed using a general continuous annealing facility.
  • a general continuous annealing facility has a heating zone in the front stage, a soaking zone in the rear stage, and a cooling zone in the rear stage.
  • the steel sheet is heated to a predetermined temperature in the preceding heating zone, the steel sheet is held under conditions of a predetermined temperature and a predetermined time in the subsequent soaking zone, and then the soaked steel sheet is cooled in the cooling zone.
  • the present invention is characterized in that the above (Condition 1) to (Condition 3) are employed during continuous annealing.
  • the reason for adopting these conditions is as follows.
  • the hydrogen concentration in the atmosphere By controlling the hydrogen concentration in the atmosphere to 20 vol% or more in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ⁇ A ⁇ 600), The oxygen potential is lowered, and selective surface oxidation and surface concentration can be suppressed. Further, the upper limit of the hydrogen concentration that can be employed in this temperature range is not particularly limited. However, if the hydrogen concentration exceeds 80 vol%, the effect obtained by setting the hydrogen concentration to 20 vol% or more is saturated, and the cost for setting the hydrogen concentration high increases. For this reason, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ⁇ A ⁇ 600), the hydrogen concentration should be set in the range of 20 vol% or more and 80 vol% or less. Is desirable.
  • the reason for setting the temperature to 500 ° C. or higher and A ° C. or lower (A: any value selected from 520 ⁇ A ⁇ 600) is as follows.
  • the lower limit of the temperature range is 500 ° C.
  • A is an arbitrary value selected from 520 ⁇ A ⁇ 600.
  • the reason why the upper limit temperature is set to A ° C. is that, as described later, by setting the dew point of the atmosphere to ⁇ 10 ° C. or higher, internal oxidation is promoted and surface concentration hardly occurs. Therefore, the hydrogen concentration does not have to be 20 vol% or more in a temperature range higher than A ° C.
  • Components other than hydrogen gas contained in the atmosphere in the above temperature range are not particularly limited as long as the effects of the present invention are not impaired.
  • the atmosphere gas is composed of hydrogen gas, nitrogen gas, and inevitable impurity gas.
  • other gases may be included as long as the effects of the present invention are not impaired.
  • the hydrogen concentration outside the temperature range where the hydrogen concentration is controlled is not particularly limited.
  • the hydrogen concentration is 1 vol% or more and 50 vol% or less. If the hydrogen concentration is less than 1 vol%, the activation effect due to reduction cannot be obtained and the plating peel resistance may deteriorate, and if it exceeds 50 vol%, the cost increases and the effect is saturated.
  • the atmospheric gas in the above temperature range the atmospheric gas in other temperature ranges is composed of hydrogen gas, nitrogen gas, and inevitable impurity gas. In addition, other gases may be included as long as the effects of the present invention are not impaired. In addition, it is preferable from a viewpoint of manufacturing cost to control hydrogen concentration only in a specific temperature range.
  • the temperature in the heating furnace A ° C. and B ° C. or less (A: 520 ⁇ A ⁇ 600, B: 550 ⁇ B ⁇ 750)
  • the steel sheet is heated under the condition that the dew point of the atmosphere is ⁇ 10 ° C. or higher.
  • oxides of oxidizable elements Si, Mn, etc.
  • internal oxidation oxides of oxidizable elements
  • the reason why the lower limit of the temperature range between A ° C. and B ° C. is A ° C. (an arbitrary value selected from 520 ⁇ A ⁇ 600) is as follows. In the temperature range of 520 ° C. or lower, even if the dew point is controlled to be ⁇ 10 ° C. or higher, almost no internal oxidation is formed. Internal oxidation begins to occur at temperatures above 520 ° C. On the other hand, when the dew point in the temperature range of 600 ° C. or higher is less than ⁇ 10 ° C., the surface concentration increases, the inward diffusion of oxygen is hindered, and internal oxidation hardly occurs. Therefore, the dew point must be controlled to at least ⁇ 10 ° C. from a temperature range exceeding 600 ° C. From the above, the allowable range of A is 520 ⁇ A ⁇ 600, and for the reason described above, it is desirable that A be as low as possible within this range.
  • the reason why the upper limit of the temperature range between A ° C. and B ° C. is B ° C. (an arbitrary value selected from 550 ⁇ B ⁇ 750) is as follows.
  • surface thickening can be suppressed by the following mechanism.
  • an easily oxidizable element Si, Mn, etc.
  • a deficient layer the amount of solid solution of the easily oxidizable element is reduced.
  • the surface diffusion of the easily oxidizable element from the steel is suppressed.
  • the dew point of the atmosphere other than the temperature range where the dew point is controlled is not particularly limited. Preferably, it is in the range of ⁇ 50 ° C. to ⁇ 10 ° C. In addition, it is preferable from a viewpoint of manufacturing cost to control a dew point only in a specific temperature range.
  • the maximum temperature reached in the steel sheet is 600 ° C. or higher and 750 ° C. or lower in continuous annealing.
  • the steel plate maximum temperature may be the maximum temperature B ° C. in the heating in the heating process, but when B ° C. is less than 750 ° C., it is a temperature increased by further heating from B ° C. Also good.
  • the maximum reached temperature of the steel sheet refers to a value at which the value obtained by measurement by the following method is the highest during continuous annealing. The temperature is measured by installing a thermometer (a method of the thermometer such as a multiple reflection thermometer and a radiation thermometer is not particularly limited) at the roll position of each pass in the annealing furnace.
  • the maximum temperature reached by the steel sheet is set to an arbitrary value selected from 600 ° C. to 750 ° C. is as follows. If the temperature lower than 600 ° C. is set as the maximum steel plate temperature, a good material cannot be obtained. Therefore, the maximum steel plate temperature for achieving the effects of the present invention is 600 ° C. or higher. On the other hand, under conditions where the maximum temperature reached by the steel sheet exceeds 750 ° C., surface concentration becomes remarkable, and deterioration of the chemical conversion treatment property starts to be recognized. Furthermore, from the viewpoint of the material, when the maximum steel sheet temperature exceeds 750 ° C., the effect of balance between strength and ductility (effect in which high strength and ductility are compatible) is saturated. From the above, the maximum temperature reached by the steel sheet is set to 600 ° C. or higher and 750 ° C. or lower.
  • the steel plate passage time in the temperature range where the steel plate temperature is 600 ° C. or higher and 750 ° C. or lower in continuous annealing is set to 30 seconds or longer and 10 minutes or shorter. If the steel plate passage time is less than 30 seconds, the target material (TS, El) cannot be obtained. On the other hand, if the steel plate passage time exceeds 10 minutes, the effect of balance between strength and ductility is saturated. Further, the steel plate temperature can be measured by employing the same measurement method as that used when deriving the maximum steel plate temperature.
  • the conditions for continuous annealing other than the above essential conditions are as follows.
  • the conditions of the soaking temperature and soaking time in the soaking zone are not particularly limited, and may be set as appropriate.
  • the soaking temperature may be the above-mentioned maximum steel plate temperature, or may be a temperature lower than the above-mentioned maximum steel plate temperature.
  • the surface concentration of Si and Mn increases in proportion to the amount of Si and Mn in the steel.
  • the amount of surface enrichment decreases as the oxygen potential of the atmosphere increases.
  • selective oxidation of Si and Mn in the steel is difficult to occur, so that the amount of surface enrichment decreases as the oxygen potential of the atmosphere decreases. Therefore, when controlling the dew point in the heating process, it is necessary to increase or decrease the oxygen potential of the atmosphere by increasing the hydrogen concentration or raising the dew point depending on the amount of Si and Mn in the steel. .
  • High strength steel sheets may be manufactured by performing the following treatment after continuous annealing.
  • tempering is preferably performed at a temperature of 150 to 400 ° C. If it is less than 150 ° C., the elongation tends to deteriorate, and if it exceeds 400 ° C., the hardness tends to decrease.
  • electrolytic pickling may be performed after the continuous annealing in order to remove a small amount of surface concentrate that is inevitably generated during the continuous annealing and to ensure better chemical conversion treatment.
  • the conditions for the electrolytic pickling are not particularly limited, but in order to efficiently remove oxides of Si and Mn that are inevitably surface-enriched during continuous annealing, an alternating electrolysis with a current density of 1 A / dm 2 or more may be used. desirable.
  • the reason for alternating electrolysis is that the pickling effect is small when the steel plate is held at the cathode, and conversely, Fe that is eluted during electrolysis accumulates in the pickling solution while the steel plate is held at the anode. This is because if the Fe concentration increases and adheres to the surface of the steel sheet, problems such as dry dirt occur.
  • the pickling solution used for the electrolytic pickling is not particularly limited, but nitric acid and hydrofluoric acid are not preferable because they are highly corrosive to equipment and require careful handling. Hydrochloric acid is not preferred because it may generate chlorine gas from the cathode. For this reason, use of sulfuric acid is preferable in consideration of corrosivity and environment.
  • the sulfuric acid concentration is preferably 5% by mass or more and 20% by mass or less. If the sulfuric acid concentration is less than 5% by mass, the electrical conductivity will be low, so the bath voltage during electrolysis will rise and the load on the power source will increase. On the other hand, if it exceeds 20% by mass, a loss due to drag-out is large, which causes a problem in cost.
  • the temperature of the electrolytic solution is preferably 40 ° C. or higher and 70 ° C. or lower. Since the bath temperature rises due to heat generated by continuous electrolysis, it is difficult to maintain the temperature below 40 ° C. Moreover, it is a problem that temperature exceeds 70 degreeC from a durable viewpoint of the lining of an electrolytic cell.
  • the present invention is a manufacturing method characterized by continuous annealing conditions for steel sheets.
  • the steel plate that is subject to this continuous annealing will be described.
  • “%” means “mass%”.
  • C 0.03-0.35%
  • C improves workability by forming martensite or the like in the steel structure.
  • the C content needs to be 0.03% or more.
  • the C content is 0.03% or more and 0.35% or less.
  • Si 0.01 to 0.50% Si is an effective element for strengthening steel and obtaining a good material.
  • Si is an easily oxidizable element, it is disadvantageous for chemical conversion treatment. From this viewpoint, Si is an element that should be avoided as much as possible. Moreover, about 0.01% Si is inevitably contained in the steel, and the cost increases in order to reduce the Si content below this. From the above, the lower limit of the Si content is 0.01%.
  • the Si content exceeds 0.50%, the steel strengthening ability and elongation improvement effect are saturated. Moreover, the chemical conversion property of a high strength steel plate deteriorates. Therefore, the Si amount is set to 0.01% or more and 0.50% or less.
  • One feature of the present invention is that the chemical conversion treatment can be improved even when the Si content is high.
  • Mn 3.6 to 8.0%
  • Mn is an element effective for increasing the strength of steel.
  • the Mn content needs to be 3.6% or more.
  • the content of Mn exceeds 8.0%, it becomes difficult to secure chemical conversion property and secure a balance between strength and ductility. Further, it is disadvantageous in terms of cost. Therefore, the Mn content is 3.6% or more and 8.0% or less.
  • Al 0.01 to 1.0% Al is added for the purpose of deoxidizing molten steel. If the Al content is less than 0.01%, the object is not achieved. The effect of deoxidation of molten steel can be obtained by making the Al content 0.01% or more. On the other hand, if the Al content exceeds 1.0%, the cost increases. Furthermore, if the Al content exceeds 1.0%, the surface concentration of Al increases, and it becomes difficult to improve the chemical conversion property. Therefore, the Al content is set to 0.01% to 1.0%.
  • P 0.10% or less
  • P is one of elements inevitably contained, and may not be contained. In order to reduce the P content to less than 0.005%, there is a concern about an increase in cost. Therefore, the P content is preferably 0.005% or more. On the other hand, if the P content exceeds 0.10%, the weldability deteriorates. Furthermore, when the P content exceeds 0.10%, the chemical conversion treatment property deteriorates so severely that it is difficult to improve the chemical conversion treatment property even with the present invention. Accordingly, the P content is preferably 0.10% or less, and the lower limit is preferably 0.005%.
  • S 0.010% or less S is one of elements inevitably contained, and S may not be contained.
  • the lower limit of the S content is not specified.
  • content of S shall be 0.010% or less.
  • the steel sheet subjected to continuous annealing is B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W : One or more elements selected from 0.001 to 0.10% and V: 0.001 to 0.10% may be included as necessary. The reason for limiting the proper content in the case of containing these elements is as follows.
  • B 0.001 to 0.005%
  • the content of B is less than 0.001%, it is difficult to obtain the quenching promoting effect.
  • the content of B exceeds 0.005%, chemical conversion property may be deteriorated. Therefore, when it contains B, it is preferable to make B amount into 0.001% or more and 0.005% or less.
  • the steel plate does not need to contain B. It is the same for other optional elements to be contained as necessary.
  • Nb 0.005 to 0.05% If the Nb content is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if the Nb content exceeds 0.05%, the cost increases. Therefore, when Nb is contained, the Nb content is 0.005% or more and 0.05% or less.
  • Ti 0.005 to 0.05% If the Ti content is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, when the Ti content exceeds 0.05%, chemical conversion treatment may be deteriorated. Therefore, when Ti is contained, the Ti content is preferably 0.005% or more and 0.05% or less.
  • Cr 0.001 to 1.0%
  • Cr content is less than 0.001%, it is difficult to obtain a hardenability effect.
  • Cr content exceeds 1.0%, Cr is concentrated on the surface, so that weldability is deteriorated. Therefore, when it contains Cr, it is preferable that Cr amount shall be 0.001% or more and 1.0% or less.
  • Mo 0.05 to 1.0% If the Mo content is less than 0.05%, the effect of adjusting the strength is difficult to obtain. On the other hand, if the Mo content exceeds 1.0%, the cost increases. Therefore, when it contains Mo, it is preferable to make Mo amount into 0.05% or more and 1.0% or less.
  • the amount of Cu is preferably 0.05% or more and 1.0% or less.
  • Ni 0.05 to 1.0%
  • the amount of Ni is preferably 0.05% or more and 1.0% or less.
  • Sn and Sb can be contained from the viewpoint of suppressing decarburization in the region of several tens of microns from the steel sheet surface caused by nitriding, oxidation, or oxidation of the steel sheet surface.
  • the content is 0.001% or more.
  • these content shall be 0.20% or less, respectively.
  • Ta 0.001 to 0.10%
  • Ta contributes to higher strength by forming carbides and carbonitrides with C and N, and further contributes to higher yield ratio (high YR).
  • Ta has the effect of refining the hot-rolled sheet structure, and this effect refines the ferrite grain size after cold rolling and annealing.
  • the amount of C segregation to the grain boundary accompanying the increase in grain boundary area increases, and a high seizure hardening amount (BH amount) can be obtained.
  • BH amount high seizure hardening amount
  • 0.001% or more of Ta can be contained.
  • the content of Ta exceeds 0.10%, not only the raw material cost is increased, but also the formation of martensite in the cooling process after annealing may be hindered.
  • TaC precipitated in the hot-rolled sheet increases the deformation resistance during cold rolling, and may make it difficult to manufacture a stable actual machine. Therefore, when Ta is contained, the content is preferably 0.10% or less.
  • W and V are elements that form carbonitrides and have the effect of increasing the strength of steel by precipitation effects, and can be added as necessary. Such an effect is observed when both W and / or V are added, containing 0.001% or more. On the other hand, when the content of these elements exceeds 0.10%, the steel sheet becomes excessively strong and the ductility may be deteriorated. From the above, when W and / or V are contained, the content is preferably 0.001% or more and 0.10% or less.
  • the remainder other than the above is Fe and inevitable impurities. Even if elements other than the elements described above are contained, the present invention is not adversely affected, and the upper limit is made 0.10%.
  • the structure and structure of the surface layer of the steel sheet which may be the starting point of corrosion cracking, have been improved. Need to control. Therefore, in the present invention, first, by performing hydrogen concentration control and dew point control as described above in the annealing process in order to ensure chemical conversion treatment properties, the easily oxidizable elements such as Si and Mn are formed as described above. Immediately before the treatment, internal oxidation is performed in advance, and the activity of Si and Mn in the surface layer portion of the iron base is lowered.
  • the structure and structure of the steel sheet surface layer of the steel strength steel sheet manufactured by the manufacturing method of the present invention has the following characteristics.
  • (Characteristic 1) Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V are in a region within 100 ⁇ m from the steel plate surface of the high-strength steel plate. At least one oxide selected from the group consisting of 0.010 to 0.100 g / m 2 per side.
  • (Feature 2) It has an oxide containing Mn in grains within 1 ⁇ m from the grain boundary of the steel sheet in a region within 10 ⁇ m from the steel sheet surface.
  • a high-strength steel sheet is formed in a region within 100 ⁇ m from the steel sheet surface, Fe, Si, Mn, Al, P, B, Nb, Ti, Cr,
  • One or more oxides selected from Mo, Cu, Ni, Sn, Sb, Ta, W, and V must be 0.010 g / m 2 or more in total per side. Further, if the total oxide exceeds 0.100 g / m 2 per side, there is a concern that a starting point of corrosion cracking occurs, and the amount of the oxide formed exceeds 0.100 g / m 2 . However, since the chemical conversion processability improving effect is not increased and is saturated, the upper limit is set to 0.100 g / m 2 .
  • the grain boundary diffusion of the easily oxidizable element in the steel can be suppressed, but the intragranular diffusion may not be sufficiently suppressed. Therefore, in the present invention, internal oxidation is performed not only at the grain boundaries but also within the grains by adopting the above-described continuous annealing conditions.
  • the high-strength steel sheet has an oxide containing Mn in grains within 1 ⁇ m from the grain boundary in a region of 10 ⁇ m from the steel sheet surface.
  • the presence of oxide in the grains reduces the amount of solid solution Mn in the grains near the oxide. As a result, concentration of Mn on the surface due to intragranular diffusion can be suppressed.
  • the structure of the steel sheet surface of the high-strength steel sheet obtained by the production method of the present invention is as described above. For example, there is no problem even if the oxide grows in a region exceeding 100 ⁇ m from the steel sheet surface. . Moreover, there is no problem even if an oxide containing Mn is present in a grain of 1 ⁇ m or more from the grain boundary in a region exceeding 10 ⁇ m from the steel plate surface.
  • the base metal structure in the region where the oxide containing Mn grows is preferably a soft and rich workability ferrite phase.
  • the high-strength steel plate of the present invention may be formed by subjecting the high-strength steel plate to a chemical conversion treatment.
  • the type of chemical conversion treatment liquid is not particularly limited, and general chemicals such as a chromate treatment liquid and a non-chromate treatment liquid can be used.
  • the chemical conversion treatment method is not limited, and various methods such as immersion (dip) treatment, spray treatment, and electrolytic treatment can be applied.
  • the high-strength steel sheet of the present invention may be formed by electrodeposition coating on the chemical conversion film of the steel sheet subjected to the above chemical conversion treatment.
  • the conditions for electrodeposition coating are not particularly limited, and may be set as appropriate.
  • the hot-rolled steel sheet having the steel composition shown in Table 1 was pickled, removed the black scale, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. A part of the hot-rolled steel sheet (thickness: 2.0 mm) after removing the black scale was used without performing cold rolling.
  • the cold-rolled steel plate and hot-rolled steel plate obtained above were charged into a continuous annealing facility.
  • annealing equipment as shown in Tables 2 and 3 (Tables 2-1 and 2-2 are combined into Table 2 and Tables 3-1 and 3-2 are combined into Table 3), temperature, hydrogen
  • the steel sheet was continuously annealed by controlling the concentration and dew point.
  • the atmospheric gas components were nitrogen gas, hydrogen gas, and inevitable impurity gas.
  • the chemical conversion processability and corrosion resistance of the specimens of the high-strength steel sheets obtained as described above were investigated. Further, the amount of oxide (internal oxidation amount) existing in the region from the steel plate surface of the high-strength steel plate to 100 ⁇ m was measured. The measurement method and evaluation criteria are shown below.
  • the evaluation method of chemical conversion property is as follows.
  • a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nippon Parkerizing Co., Ltd. was used as the chemical conversion treatment liquid, and chemical conversion treatment was performed by the following method.
  • the test material is degreased with a degreasing liquid fine cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washed with water, and then subjected to a surface adjustment for 30 seconds with a surface conditioning solution preparen Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. It was. After the surface adjustment, the substrate was immersed in a chemical conversion solution (Palbond L3080) at 43 ° C. for 120 seconds, washed with water, and dried with warm air.
  • a degreasing liquid fine cleaner registered trademark
  • a surface conditioning solution preparen Z registered trademark
  • test sample was immersed in a 5% NaCl aqueous solution (55 ° C.) for 240 hours, then taken out, washed with water and dried, and then the cross-cut portion was peeled off to measure the peeling width.
  • the measurement results were evaluated according to the following evaluation criteria. " ⁇ " is an acceptable level ⁇ : Peeling width is less than 2.5 mm on one side ⁇ : Peeling width is 2.5 mm or more on one side ⁇ Workability> As for workability, a JIS No.
  • the amount of internal oxidation is measured by “impulse furnace melting-infrared absorption method”. However, it is necessary to subtract the amount of oxygen contained in the material (that is, the steel plate before annealing).
  • the oxygen concentration in the steel was measured at positions polished by 100 ⁇ m or more from both surfaces of the high-strength steel sheet after continuous annealing, and the measured value was defined as the amount of oxygen OH contained in the material.
  • board thickness direction of the high strength steel plate surface after continuous annealing was measured, and the measured value was made into the oxygen amount OI after internal oxidation.
  • the high-strength steel plate produced by the method of the present invention is a high-strength steel plate containing a large amount of oxidizable elements such as Si and Mn. It turns out that it is excellent in workability. On the other hand, in a comparative example, any one or more of chemical conversion treatment property, corrosion resistance, and workability is inferior.
  • the high-strength steel sheet of the present invention has excellent chemical conversion properties, corrosion resistance, and workability, and can be used as a surface-treated steel sheet for reducing the weight and strength of an automobile body.
  • the steel sheet can be applied in a wide range of fields such as home appliances and building materials as a surface-treated steel sheet provided with rust prevention properties.

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Abstract

Provided is a high-strength steel sheet that has, even when the content of Si or Mn is high, excellent workability, chemical convertibility, and corrosion resistance after electrodeposition coating, and a method for producing said steel sheet. When continuously annealing the steel sheet, the belowmentioned (Condition 1)-(Condition 3) are employed. (Condition 1) In a heating process during continuous annealing, when the internal temperature of an annealing furnace is in a temperature region of between 500°C and A°C (A: an arbitrary value selected from 520 ≤ A < 600), the steel sheet is heated under conditions in which hydrogen concentration is 20 vol% or more, and when the internal temperature of the annealing furnace is in a temperature region of more than A°C but at most B°C (B: an arbitrary value selected from 550 ≤ B ≤ 750), the steel plate is heated under conditions in which the dew point of the atmosphere is -10°C or more. (Condition 2) The steel sheet maximum attained temperature during continuous annealing is configured to be between 600°C and 750°C, inclusive. (Condition 3) The steel sheet transit time, in which the steel sheet temperature is in a temperature region of between 600°C and 750°C, inclusive, during continuous annealing is configured to be between 30 seconds and 10 minutes, inclusive.

Description

高強度鋼板およびその製造方法High strength steel plate and manufacturing method thereof
　本発明は、ＳｉやＭｎの含有量が多い場合でも、優れた化成処理性と電着塗装後の耐食性を有する高強度鋼板およびその製造方法に関する。 The present invention relates to a high-strength steel sheet having excellent chemical conversion property and corrosion resistance after electrodeposition coating even when the content of Si or Mn is large, and a method for producing the same.
　近年、自動車の燃費向上および自動車の衝突安全性向上の観点から、車体材料の高強度化によって車体材料の薄肉化を図り、車体そのものを軽量化し、かつ高強度化する要望が高まっている。そのために高強度鋼板の自動車への適用が促進されている。 In recent years, from the viewpoint of improving the fuel efficiency of automobiles and improving the collision safety of automobiles, there is an increasing demand for reducing the thickness of the body material by increasing the strength of the body material, reducing the weight of the body itself, and increasing the strength. Therefore, application of high-strength steel sheets to automobiles is being promoted.
　一般に、自動車用鋼板は塗装して使用される。この塗装の前処理として、リン酸塩処理と呼ばれる化成処理が自動車用鋼板に施される。自動車用鋼板の化成処理は塗装後の耐食性を確保するための重要な処理の一つである。 Generally, automotive steel plates are used after painting. As a pretreatment for the coating, a chemical conversion treatment called a phosphate treatment is performed on the steel plate for automobiles. Chemical conversion treatment of steel sheets for automobiles is one of the important treatments for ensuring corrosion resistance after painting.
　鋼板の強度、延性を高めるためには、鋼板にＳｉを含有させることが有効である。しかしながら、連続焼鈍の際にＳｉは、Ｆｅの酸化が起こらない（Ｆｅ酸化物を還元する）還元性のＮ２＋Ｈ２ガス雰囲気でも酸化する。このＳｉの酸化により、鋼板表面にＳｉ酸化物（ＳｉＯ２）が形成される。このＳｉＯ２が、化成処理中の化成皮膜の生成反応を阻害するため、化成皮膜が生成されない微小領域（以後、スケ）が鋼板表面に形成され、化成処理性が低下する。 In order to increase the strength and ductility of the steel plate, it is effective to contain Si in the steel plate. However, during continuous annealing, Si is oxidized even in a reducing N 2 + H 2 gas atmosphere in which Fe is not oxidized (which reduces Fe oxide). Due to the oxidation of Si, Si oxide (SiO 2 ) is formed on the surface of the steel sheet. Since this SiO 2 inhibits the formation reaction of the chemical conversion film during the chemical conversion treatment, a minute region (hereinafter referred to as a scale) where the chemical conversion film is not formed is formed on the surface of the steel sheet, and the chemical conversion treatment performance decreases.
　高Ｓｉ含有鋼板の化成処理性を改善する従来技術として、特許文献１では、２０～１５００ｍｇ／ｍ２の鉄被覆層を、電気めっき法、化合物塗布法、蒸着法等を用いて鋼板上に形成する方法が開示されている。しかしながら、この方法では、鉄皮膜層形成のために電気めっき設備等の設備が別途必要となり工程が増加する分コストも増大するという問題がある。 As a conventional technique for improving the chemical conversion processability of a high Si content steel sheet, in Patent Document 1, an iron coating layer of 20 to 1500 mg / m 2 is formed on the steel sheet using an electroplating method, a compound coating method, a vapor deposition method, or the like. A method is disclosed. However, in this method, there is a problem in that an additional equipment such as an electroplating equipment is required for forming the iron coating layer, and the cost is increased due to the increased number of processes.
　また、特許文献２では鋼板のＭｎ／Ｓｉ比率を規定することによって、特許文献３ではＮｉを添加することによって、リン酸塩処理性を向上させている。しかしながら、その効果は鋼板中のＳｉ含有量に依存するものであり、Ｓｉ含有量の高い鋼板についてはさらなる改善が必要である。 In Patent Document 2, the Mn / Si ratio of the steel sheet is defined, and in Patent Document 3, Ni is added to improve phosphate processability. However, the effect depends on the Si content in the steel sheet, and further improvement is necessary for the steel sheet having a high Si content.
　さらに、特許文献４では、鋼板表面から深さ１μｍ以内にＳｉ含有酸化物からなる内部酸化層を形成し、鋼板表面長さ１０μｍに占めるＳｉ含有酸化物の割合を８０％以下にする方法が開示されている。しかしながら、特許文献４に前記内部酸化層の形成手段として記載された方法、すなわち焼鈍時の露点を－２５～０℃に制御して焼鈍時の酸素分圧を通常より高める方法の場合、露点を制御するエリアが炉内全体を前提としたものであるため、露点の制御性が困難であり安定操業が困難である。また、不安定な露点制御のもとでの焼鈍を行った場合、鋼板に形成される内部酸化物の分布状態にバラツキが認められ、鋼板の長手方向や幅方向で化成処理性のムラ（全体または一部でスケ）が発生する懸念がある。 Furthermore, Patent Document 4 discloses a method in which an internal oxide layer made of Si-containing oxide is formed within a depth of 1 μm from the steel sheet surface, and the ratio of the Si-containing oxide in the steel sheet surface length of 10 μm is 80% or less. Has been. However, in the case of the method described in Patent Document 4 as the means for forming the internal oxide layer, that is, the method in which the dew point during annealing is controlled to -25 to 0 ° C. and the oxygen partial pressure during annealing is increased more than usual, the dew point is reduced. Since the area to be controlled is premised on the entire inside of the furnace, the controllability of the dew point is difficult and stable operation is difficult. In addition, when annealing was performed under unstable dew point control, variations were observed in the distribution of internal oxides formed on the steel sheet, and chemical conversion treatment unevenness in the longitudinal and width directions of the steel sheet (overall) Or there is a concern that a part of the scale may occur.
　また、特許文献５には、酸化性雰囲気で鋼板温度を３５０～６５０℃に到達させ鋼板表面に酸化膜を形成させ、その後還元性雰囲気で再結晶温度まで加熱し冷却する方法が記載されている。しかしながら、この方法では、酸化する方法により鋼板表面に形成される酸化皮膜の厚みに差があり、十分に酸化が起こらない場合がある。また、特許文献５に記載の方法では、酸化皮膜が厚くなりすぎて、後の還元性雰囲気での焼鈍において酸化膜の残留または剥離を生じ、表面性状が悪化する場合がある。また、特許文献５の実施例では、大気中で酸化する技術が記載されている。大気中での酸化は、酸化物が厚く生成してその後の還元が困難であるという問題、あるいは、高水素濃度の還元雰囲気が必要であるという問題がある。 Patent Document 5 describes a method in which a steel sheet temperature reaches 350 to 650 ° C. in an oxidizing atmosphere to form an oxide film on the steel sheet surface, and then heated to a recrystallization temperature in a reducing atmosphere and cooled. . However, in this method, there is a difference in the thickness of the oxide film formed on the surface of the steel sheet depending on the method of oxidation, and oxidation may not occur sufficiently. Further, in the method described in Patent Document 5, the oxide film becomes too thick, and the oxide film may remain or peel off during annealing in a reducing atmosphere, which may deteriorate the surface properties. Moreover, in the Example of patent document 5, the technique oxidized in air | atmosphere is described. Oxidation in the air has a problem that a thick oxide is formed and subsequent reduction is difficult, or a reduction atmosphere with a high hydrogen concentration is necessary.
　さらに、特許文献６では、質量％で、Ｓｉを０．１％以上、及び／または、Ｍｎを１．０％以上含有する冷延鋼板について、鋼板温度４００℃以上、鉄の酸化雰囲気下という条件で鋼板表面に酸化膜を形成させ、その後、鉄の還元雰囲気下で上記鋼板表面の酸化膜を還元する方法が記載されている。具体的には、例えば、鋼板温度４００℃以上、空気比０．９３以上１．１０以下の直火バーナーを用いる条件で、鋼板表面のＦｅを酸化した後、Ｆｅ酸化物を還元するＮ２＋Ｈ２ガス雰囲気で鋼板を焼鈍する。これにより、化成処理性を劣化させるＳｉＯ２が鋼板表面に形成されることを抑制し、鋼板表面にＦｅの酸化層を形成させる方法である。特許文献６には、直火バーナーの加熱温度が具体的に記載されていない。しかし、特許文献６においてＳｉを多く（概ね０．６％以上）含有する場合には、Ｆｅよりも酸化しやすいＳｉの酸化量が多くなってＦｅの酸化が抑制されたり、Ｆｅの酸化そのものが少なすぎたりする。その結果、特許文献６に記載の技術では、還元後の表面Ｆｅ還元層の形成が不十分であったり、還元後の鋼板表面にＳｉＯ２が存在して化成皮膜のスケが発生する場合があったりする。 Furthermore, in Patent Document 6, a cold rolled steel sheet containing, by mass%, Si of 0.1% or more and / or Mn of 1.0% or more, a condition that the steel sheet temperature is 400 ° C. or more and under an iron oxidizing atmosphere. Describes a method of forming an oxide film on the steel sheet surface and then reducing the oxide film on the steel sheet surface in an iron reducing atmosphere. Specifically, for example, N 2 + H reduces Fe oxide after oxidizing Fe on the surface of the steel sheet under the condition of using a direct fire burner having a steel sheet temperature of 400 ° C. or more and an air ratio of 0.93 to 1.10. Annealing the steel sheet in a two gas atmosphere. This is a method of suppressing the formation of SiO 2 that deteriorates the chemical conversion property on the steel sheet surface and forming an Fe oxide layer on the steel sheet surface. Patent Document 6 does not specifically describe the heating temperature of an open flame burner. However, in Patent Document 6, when a large amount of Si is contained (approximately 0.6% or more), the amount of oxidation of Si, which is easier to oxidize than Fe, is increased, and the oxidation of Fe is suppressed. Too little. As a result, in the technique described in Patent Document 6, formation of the surface Fe reduced layer after reduction may be insufficient, or SiO 2 may be present on the surface of the steel plate after reduction, resulting in the occurrence of scaling of the conversion coating. Or
特開平５－３２０９５２号公報JP-A-5-320952 特許第４３１９５５９号公報Japanese Patent No. 4319559 特許第２９５１４８０号公報Japanese Patent No. 2951480 特許第３８４０３９２号公報Japanese Patent No. 3840392 特開昭５５－１４５１２２号公報JP 55-145122 A 特開２００６－４５６１５号公報JP 2006-45615 A
　本発明は、かかる事情に鑑みてなされたものであって、ＳｉやＭｎの含有量が多い場合でも、優れた加工性、化成処理性、電着塗装後の耐食性を有する高強度鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of such circumstances, and even when the content of Si and Mn is large, a high-strength steel sheet having excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating, and its production It aims to provide a method.
　特に、ＴＳ×Ｅｌが２００００以上の高強度高伸びの鋼板およびその製造方法の提供を目的とする。 In particular, an object is to provide a high-strength and high-strength steel sheet having a TS × El of 20000 or more and a method for producing the same.
　上記特許文献に記載があるような従来技術では、単に焼鈍炉内全体の水蒸気分圧または酸素分圧を上昇させることで露点または酸素濃度を上げて、過剰に鋼板の内部または外部を酸化させている。このため、上述したように、従来技術では炉全体での露点制御または酸化制御性に問題があったり、化成処理性にムラが発生したり、電着塗装後の耐食性が劣化したりする。 In the prior art as described in the above patent document, the dew point or oxygen concentration is increased by simply increasing the steam partial pressure or oxygen partial pressure in the entire annealing furnace, and the inside or outside of the steel sheet is excessively oxidized. Yes. For this reason, as described above, the conventional technique has a problem in dew point control or oxidation controllability in the whole furnace, unevenness in chemical conversion treatment occurs, or corrosion resistance after electrodeposition coating deteriorates.
　そこで、本発明者らは、従来の考えにとらわれない新たな方法で課題を解決する方法を検討した。その結果、電着塗装後の耐食性劣化の起点になる可能性がある鋼板表層の組織、構造に対してより高度な制御を行うことで、高強度鋼板の化成処理性および電着塗装後の耐食性が改善されることを知見した。具体的には、鋼板を連続焼鈍する際に以下の（条件１）～（条件３）を採用する。 Therefore, the present inventors examined a method for solving the problem by a new method not confined to the conventional idea. As a result, chemical treatment of high-strength steel sheets and corrosion resistance after electrodeposition coating are achieved by performing more sophisticated control over the structure and structure of the steel sheet surface layer, which may become the starting point of corrosion resistance deterioration after electrodeposition coating. Was found to be improved. Specifically, the following (Condition 1) to (Condition 3) are employed when continuously annealing the steel sheet.
　（条件１）連続焼鈍の加熱過程において、加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域では、水素濃度：２０ｖｏｌ％以上の条件で鋼板を加熱し、加熱炉内温度：Ａ℃超えＢ℃以下（Ｂ：５５０≦Ｂ≦７５０から選択される任意の値）の温度域では、雰囲気の露点：－１０℃以上の条件で鋼板を加熱する。 (Condition 1) In the heating process of continuous annealing, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), the hydrogen concentration: 20 vol% or more. The steel sheet is heated under the conditions, and in the temperature range of the heating furnace temperature: A ° C. to B ° C. (B: any value selected from 550 ≦ B ≦ 750), the dew point of the atmosphere is −10 ° C. or more. Heat the steel plate.
　（条件２）連続焼鈍において鋼板最高到達温度を６００℃以上７５０℃以下とする。 (Condition 2) In continuous annealing, the maximum temperature reached in the steel sheet is 600 ° C. or higher and 750 ° C. or lower.
　（条件３）連続焼鈍において鋼板温度が６００℃以上７５０℃以下の温度域の鋼板通過時間を３０秒以上１０分以下とする。 (Condition 3) In continuous annealing, the steel plate passage time in the temperature range where the steel plate temperature is 600 ° C. or higher and 750 ° C. or lower is set to 30 seconds or longer and 10 minutes or shorter.
　このような処理を行うことによって、選択的表面酸化を抑制し、表面濃化を抑制することができ、加工性、化成処理性および電着塗装後の耐食性に優れる高強度鋼板が得られる。 By performing such treatment, selective surface oxidation can be suppressed, surface concentration can be suppressed, and a high-strength steel sheet excellent in workability, chemical conversion property and corrosion resistance after electrodeposition coating can be obtained.
　そして、以上の方法により得られる高強度鋼板の鋼板表層の組織、構造は、以下の特徴１、２を有する。 And the structure and structure of the steel sheet surface layer of the high-strength steel sheet obtained by the above method have the following features 1 and 2.
　（特徴１）高強度鋼板の鋼板表面から１００μｍ以内の領域に、Ｆｅ、Ｓｉ、Ｍｎ、Ａｌ、Ｐ、Ｂ、Ｎｂ、Ｔｉ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、Ｔａ、ＷおよびＶの中から選ばれる少なくとも１種以上の酸化物を、合計で片面あたり０．０１０～０．１００ｇ／ｍ2有する。 (Characteristic 1) Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V are in a region within 100 μm from the steel plate surface of the high-strength steel plate. And at least one oxide selected from the group consisting of 0.010 to 0.100 g / m 2 per side in total.
　（特徴２）鋼板表面から１０μｍ以内の領域における、鋼板結晶粒界から１μｍ以内の粒内に、Ｍｎを含む酸化物を有する。 (Characteristic 2) It has an oxide containing Mn in grains within 1 μm from the steel grain boundaries in the region within 10 μm from the steel sheet surface.
　このような特徴を有する鋼板表層になることで、化成処理性および電着塗装後の耐食性が改善される。 </ RTI> By forming a steel sheet surface layer having such characteristics, chemical conversion treatment and corrosion resistance after electrodeposition coating are improved.
　本発明は上記知見に基づくものであり、特徴は以下の通りである。 The present invention is based on the above findings, and features are as follows.
　（１）質量％で、Ｃ：０．０３～０．３５％、Ｓｉ：０．０１～０．５０％、Ｍｎ：３．６～８．０％、Ａｌ：０．０１～１．０％、Ｐ：０．１０％以下、Ｓ：０．０１０％以下を含有し、残部がＦｅおよび不可避的不純物からなる鋼板を連続焼鈍する際に、前記連続焼鈍の加熱過程において、加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域では、雰囲気の水素濃度：２０ｖｏｌ％以上の条件で前記鋼板を加熱し、加熱炉内温度：Ａ℃超えＢ℃以下（Ｂ：５５０≦Ｂ≦７５０から選択される任意の値）の温度域では、雰囲気の露点：－１０℃以上の条件で前記鋼板を加熱し、前記連続焼鈍において鋼板最高到達温度が６００℃以上７５０℃以下とし、前記連続焼鈍において鋼板温度が６００℃以上７５０℃以下の温度域の鋼板通過時間を３０秒以上１０分以下とすることを特徴とする化成処理性に優れた高強度鋼板の製造方法。 (1) By mass, C: 0.03 to 0.35%, Si: 0.01 to 0.50%, Mn: 3.6 to 8.0%, Al: 0.01 to 1.0% , P: 0.10% or less, S: 0.010% or less, and when continuously annealing a steel plate consisting of Fe and unavoidable impurities, the heating furnace temperature: In the temperature range of 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), the steel sheet is heated under the condition of the hydrogen concentration of the atmosphere: 20 vol% or more, and the furnace temperature: A In the temperature range of over ° C and below B ° C (B: any value selected from 550 ≤ B ≤ 750), the steel sheet is heated under the condition that the dew point of the atmosphere is -10 ° C or higher, and the steel sheet reaches the maximum in the continuous annealing The temperature is 600 ° C. or more and 750 ° C. or less, and the steel plate temperature is 6 in the continuous annealing. 0 method of producing a high strength steel sheet excellent in chemical conversion treatability, characterized in that ° C. or higher 750 ° C. The steel sheet passage time of the temperature range below the 10 minutes or less than 30 seconds.
　（２）前記鋼板は、成分組成として、質量％で、さらに、Ｂ：０．００１～０．００５％、Ｎｂ：０．００５～０．０５％、Ｔｉ：０．００５～０．０５％、Ｃｒ：０．００１～１．０％、Ｍｏ：０．０５～１．０％、Ｃｕ：０．０５～１．０％、Ｎｉ：０．０５～１．０％、Ｓｎ：０．００１～０．２０％、Ｓｂ：０．００１～０．２０％、Ｔａ：０．００１～０．１０％、Ｗ：０．００１～０．１０％およびＶ：０．００１～０．１０％の中から選ばれる１種以上の元素を含有することを特徴とする（１）に記載の高強度鋼板の製造方法。 (2) The steel sheet has a component composition in mass%, further B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to Among 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W: 0.001 to 0.10% and V: 0.001 to 0.10% (1) The manufacturing method of the high-strength steel plate as described in (1) characterized by including 1 or more types of elements chosen from.
　（３）前記連続焼鈍を行った後、硫酸を含む水溶液で電解酸洗を行うことを特徴とする（１）または（２）に記載の高強度鋼板の製造方法。 (3) The method for producing a high-strength steel sheet according to (1) or (2), wherein after the continuous annealing, electrolytic pickling is performed with an aqueous solution containing sulfuric acid.
　（４）（１）～（３）のいずれかに記載の高強度鋼板の製造方法で製造された高強度鋼板であって、鋼板表面から１００μｍ以内の領域に、Ｆｅ、Ｓｉ、Ｍｎ、Ａｌ、Ｐ、Ｂ、Ｎｂ、Ｔｉ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、Ｔａ、ＷおよびＶの中から選ばれる少なくとも１種以上の酸化物を、合計で片面あたり０．０１０～０．１００ｇ／ｍ2有し、さらに、鋼板表面から１０μｍ以内の領域における、鋼板結晶粒界から１μｍ以内の粒内に、Ｍｎを含む酸化物を有することを特徴とする高強度鋼板。 (4) A high-strength steel plate produced by the method for producing a high-strength steel plate according to any one of (1) to (3), wherein Fe, Si, Mn, Al, At least one oxide selected from P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W and V, in total, 0.010 to 0.100 g per side / m 2 has, furthermore, high strength steel sheet characterized by in the region within 10μm from the surface of the steel sheet, in grains within 1μm from steel grain boundaries have an oxide containing Mn.
　本発明によれば、ＳｉやＭｎの含有量が多い場合でも、優れた加工性、化成処理性及び電着塗装後の耐食性を有する高強度鋼板が得られる。 According to the present invention, a high-strength steel sheet having excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating can be obtained even when the content of Si or Mn is large.
　また、本発明によれば、外観に優れるとともに、優れた加工性、化成処理性及び電着塗装後の耐食性を有する高強度鋼板を得ることもできる。 Further, according to the present invention, it is possible to obtain a high-strength steel sheet having excellent appearance and excellent workability, chemical conversion treatment, and corrosion resistance after electrodeposition coating.
　なお、外観に優れるとは、化成処理後にスケやムラがほとんど認められないことを指す。 Note that “excellent in appearance” means that scars and unevenness are hardly recognized after chemical conversion treatment.
　以下、本発明の実施形態について説明する。なお、本発明は以下の実施形態に限定されない。 Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.
　本発明の高強度鋼板の製造方法は、鋼板を連続焼鈍する際に以下の（条件１）～（条件３）を採用する。
（条件１）連続焼鈍の加熱過程において、加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域では、水素濃度：２０ｖｏｌ％以上の条件で鋼板を加熱し、加熱炉内温度：Ａ℃超えＢ℃以下（Ｂ：５５０≦Ｂ≦７５０から選択される任意の値）の温度域では、雰囲気の露点：－１０℃以上の条件で鋼板を加熱する。
（条件２）連続焼鈍において鋼板最高到達温度を６００℃以上７５０℃以下とする。
（条件３）連続焼鈍において鋼板温度が６００℃以上７５０℃以下の温度域の鋼板通過時間を３０秒以上１０分以下とする。 The method for producing a high-strength steel sheet according to the present invention employs the following (Condition 1) to (Condition 3) when the steel sheet is continuously annealed.
(Condition 1) In the heating process of continuous annealing, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), the hydrogen concentration: 20 vol% or more. The steel sheet is heated under the conditions, and in the temperature range of the heating furnace temperature: A ° C. to B ° C. (B: any value selected from 550 ≦ B ≦ 750), the dew point of the atmosphere is −10 ° C. or more. Heat the steel plate.
(Condition 2) In continuous annealing, the maximum temperature reached by the steel sheet is 600 ° C. or higher and 750 ° C. or lower.
(Condition 3) In continuous annealing, the steel plate passage time in the temperature range where the steel plate temperature is 600 ° C. or higher and 750 ° C. or lower is set to 30 seconds or longer and 10 minutes or shorter.
　先ず、連続焼鈍の対象となる鋼板の製造方法について説明する。鋼板の製造方法は特に限定されない。例えば、鋼を熱間圧延して熱延板を製造する方法、鋼を熱間圧延した後、冷間圧延して冷延板を製造する方法、鋼を熱間圧延した後、酸洗し、冷間圧延して冷延板を製造する方法等を採用することができる。このようにして得られた熱延板や冷延板を連続焼鈍する対象として用いることができる。 First, the manufacturing method of the steel plate used as the object of continuous annealing is demonstrated. The manufacturing method of a steel plate is not specifically limited. For example, a method of producing a hot-rolled sheet by hot rolling steel, a method of producing a cold-rolled sheet by cold rolling after hot rolling the steel, hot rolling the steel, pickling, A method of manufacturing a cold-rolled sheet by cold rolling can be employed. The hot-rolled sheet and the cold-rolled sheet thus obtained can be used as an object for continuous annealing.
　なお、上記鋼板を製造する際の、熱間圧延の条件、酸洗の条件は特に限定されず、適宜設定すればよい。また、冷間圧延については、４０％以上８０％以下の圧下率で行うことが好ましい。圧下率が４０％未満では自動車用の鋼板としての板厚が得られにくい。一方、圧下率が８０％超えでは、高強度鋼板であるため圧延コストがアップするだけでなく、焼鈍時の表面濃化が増加して、化成処理性が劣化する場合がある。 In addition, the conditions for hot rolling and pickling at the time of manufacturing the steel sheet are not particularly limited, and may be set as appropriate. Further, the cold rolling is preferably performed at a rolling reduction of 40% or more and 80% or less. When the rolling reduction is less than 40%, it is difficult to obtain a plate thickness as a steel plate for automobiles. On the other hand, when the rolling reduction exceeds 80%, the steel sheet is a high-strength steel sheet, so that not only the rolling cost is increased, but surface concentration during annealing is increased, and chemical conversion treatment properties may be deteriorated.
　次いで、鋼板を連続焼鈍する工程について説明する。連続焼鈍は一般的な連続焼鈍設備を用いて行うことができる。一般的な連続焼鈍設備は前段に加熱帯、後段に均熱帯、より後段に冷却帯を有する。通常、前段の加熱帯で鋼板を所定温度まで加熱し、後段の均熱帯で所定温度、所定時間の条件で鋼板を保持し、次いで、冷却帯で均熱後の鋼板を冷却する。 Next, the process of continuously annealing the steel sheet will be described. Continuous annealing can be performed using a general continuous annealing facility. A general continuous annealing facility has a heating zone in the front stage, a soaking zone in the rear stage, and a cooling zone in the rear stage. Usually, the steel sheet is heated to a predetermined temperature in the preceding heating zone, the steel sheet is held under conditions of a predetermined temperature and a predetermined time in the subsequent soaking zone, and then the soaked steel sheet is cooled in the cooling zone.
　本発明においては、連続焼鈍の際に上記（条件１）～（条件３）を採用することが特徴である。これらの条件を採用する理由は以下の通りである。 The present invention is characterized in that the above (Condition 1) to (Condition 3) are employed during continuous annealing. The reason for adopting these conditions is as follows.
　（条件１）に記載の通り、連続焼鈍の際の加熱過程において、加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域では、雰囲気の水素濃度を２０ｖｏｌ％以上に制御して鋼板の加熱を行う。なお、この加熱は通常、加熱帯で行われる。 As described in (Condition 1), in the heating process during continuous annealing, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), The steel sheet is heated by controlling the hydrogen concentration in the atmosphere to 20 vol% or more. In addition, this heating is normally performed in a heating zone.
　加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域での、雰囲気の水素濃度を２０ｖｏｌ％以上に制御することで、鋼板表面の酸素ポテンシャルが低下し、選択的表面酸化、表面濃化を抑制することが可能となる。また、この温度域で採用することができる水素濃度の上限は特に制限されない。しかし、水素濃度が８０ｖｏｌ％超えでは、水素濃度を２０ｖｏｌ％以上にすることで得られる効果が飽和し、また、水素濃度を高く設定するためのコストが増大する。このため、加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域では、水素濃度を２０ｖｏｌ％以上８０ｖｏｌ％以下の範囲に設定することが望ましい。 By controlling the hydrogen concentration in the atmosphere to 20 vol% or more in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), The oxygen potential is lowered, and selective surface oxidation and surface concentration can be suppressed. Further, the upper limit of the hydrogen concentration that can be employed in this temperature range is not particularly limited. However, if the hydrogen concentration exceeds 80 vol%, the effect obtained by setting the hydrogen concentration to 20 vol% or more is saturated, and the cost for setting the hydrogen concentration high increases. For this reason, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), the hydrogen concentration should be set in the range of 20 vol% or more and 80 vol% or less. Is desirable.
　また、５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）とする理由は以下の通りである。 Further, the reason for setting the temperature to 500 ° C. or higher and A ° C. or lower (A: any value selected from 520 ≦ A <600) is as follows.
　５００℃未満の温度域では、低温のため表面拡散する易酸化性元素の量が少ない。また、表面濃化がもともと少ない温度域であり、水素濃度の制御を行わなくても、化成処理性が阻害されることが少ない。よって、上記温度域の下限は５００℃とする。 In the temperature range below 500 ° C, the amount of easily oxidizable elements that diffuse on the surface is small due to low temperature. Further, the surface concentration is originally in a low temperature range, and the chemical conversion property is hardly hindered without controlling the hydrogen concentration. Therefore, the lower limit of the temperature range is 500 ° C.
　また、Ａは、５２０≦Ａ＜６００から選択される任意の値である。上限温度をＡ℃とした理由は、後述するように、雰囲気の露点を－１０℃以上とすることにより、内部酸化が促進され、表面濃化が殆ど起こらなくなるためである。したがって、Ａ℃より高い温度域では水素濃度を２０ｖｏｌ％以上にしなくてもよい。 A is an arbitrary value selected from 520 ≦ A <600. The reason why the upper limit temperature is set to A ° C. is that, as described later, by setting the dew point of the atmosphere to −10 ° C. or higher, internal oxidation is promoted and surface concentration hardly occurs. Therefore, the hydrogen concentration does not have to be 20 vol% or more in a temperature range higher than A ° C.
　上記温度域の雰囲気に含まれる水素ガス以外の成分は、本発明の効果を害さない範囲で特に限定されない。通常、雰囲気のガスは、水素ガス、窒素ガスおよび不可避的不純物ガスから構成される。また、本発明の効果を害さない範囲であれば、これら以外のガスを含んでもよい。 Components other than hydrogen gas contained in the atmosphere in the above temperature range are not particularly limited as long as the effects of the present invention are not impaired. Usually, the atmosphere gas is composed of hydrogen gas, nitrogen gas, and inevitable impurity gas. In addition, other gases may be included as long as the effects of the present invention are not impaired.
　なお、上記で水素濃度を制御する温度域以外の水素濃度は特に限定されない。好ましくは水素濃度が１ｖｏｌ％以上５０ｖｏｌ％以下である。水素濃度が１ｖｏｌ％未満では還元による活性化効果が得られず耐めっき剥離性が劣化する可能性があり、５０ｖｏｌ％超えではコストアップし、かつ効果が飽和する。その他の温度域の雰囲気ガスも、上記温度域の雰囲気ガスと同様に、雰囲気のガスは、水素ガス、窒素ガスおよび不可避的不純物ガスから構成される。また、本発明の効果を害さない範囲であれば、これら以外のガスを含んでもよい。なお、特定の温度域のみ水素濃度を制御することが、製造コストの観点から好ましい。 In addition, the hydrogen concentration outside the temperature range where the hydrogen concentration is controlled is not particularly limited. Preferably, the hydrogen concentration is 1 vol% or more and 50 vol% or less. If the hydrogen concentration is less than 1 vol%, the activation effect due to reduction cannot be obtained and the plating peel resistance may deteriorate, and if it exceeds 50 vol%, the cost increases and the effect is saturated. Similarly to the atmospheric gas in the above temperature range, the atmospheric gas in other temperature ranges is composed of hydrogen gas, nitrogen gas, and inevitable impurity gas. In addition, other gases may be included as long as the effects of the present invention are not impaired. In addition, it is preferable from a viewpoint of manufacturing cost to control hydrogen concentration only in a specific temperature range.
　上記（条件１）に記載の通り、連続焼鈍の際の加熱過程において、加熱炉内温度：Ａ℃超えＢ℃以下（Ａ：５２０≦Ａ＜６００、Ｂ：５５０≦Ｂ≦７５０）の温度域では、雰囲気の露点が－１０℃以上の条件で鋼板を加熱する。５００℃以上Ａ℃以下の温度域での上記加熱に引き続き、この加熱を行うことで、鋼板表面から１０μｍ以内の領域に、易酸化性元素（Ｓｉ、Ｍｎなど）の酸化物（以下、内部酸化と称する）を適量に存在させることができる。これにより、焼鈍後の化成処理性を劣化させる鋼中Ｓｉ、Ｍｎ等の鋼板表面における表面濃化を抑制することが可能となる。Ａ℃超えＢ℃以下の温度域における露点を－１０℃以上とすることにより、Ｈ２Ｏの分解から生じるＯ２ポテンシャルを上昇させ、内部酸化を促進することが可能である。露点が－１０℃を下回ると、内部酸化の形成量が少ない。また、露点の上限については特に定めない。しかし、露点が９０℃を超えてくるとＦｅの酸化量が多くなり、焼鈍炉内やロールの劣化が懸念される。このため、加熱炉内温度：Ａ℃超えＢ℃以下の温度域での露点は－１０℃以上９０℃以下が望ましい。 As described in (Condition 1) above, in the heating process during the continuous annealing, the temperature in the heating furnace: A ° C. and B ° C. or less (A: 520 ≦ A <600, B: 550 ≦ B ≦ 750) Then, the steel sheet is heated under the condition that the dew point of the atmosphere is −10 ° C. or higher. Following this heating in the temperature range of 500 ° C. or more and A ° C. or less, by performing this heating, oxides of oxidizable elements (Si, Mn, etc.) (hereinafter referred to as internal oxidation) are formed in a region within 10 μm from the steel sheet surface. Can be present in an appropriate amount. Thereby, it becomes possible to suppress surface concentration on the steel sheet surface of Si, Mn, etc. in steel which deteriorates the chemical conversion property after annealing. By setting the dew point in the temperature range from A ° C. to B ° C. to −10 ° C. or higher, it is possible to increase the O 2 potential resulting from the decomposition of H 2 O and promote internal oxidation. When the dew point is below −10 ° C., the amount of internal oxidation formed is small. There is no particular upper limit on the dew point. However, when the dew point exceeds 90 ° C., the amount of oxidation of Fe increases, and there is a concern about deterioration in the annealing furnace or roll. For this reason, the dew point in the temperature range of the heating furnace temperature: A ° C. to B ° C. is preferably −10 ° C. or more and 90 ° C. or less.
　Ａ℃超えＢ℃以下の温度域の下限をＡ℃（５２０≦Ａ＜６００から選択される任意の値）とする理由は以下の通りである。５２０℃以下の温度域では、露点を－１０℃以上に制御しても、内部酸化が殆ど形成しない。５２０℃を超える温度で内部酸化が起こり始める。また、６００℃以上の温度域の露点を－１０℃未満にすると、表面濃化が多くなり、酸素の内方拡散が阻害され、内部酸化が起こりにくくなる。従って、少なくとも６００℃未満を超える温度域から－１０℃以上の露点に制御しなければならない。以上から、Ａの許容範囲は５２０≦Ａ＜６００であり、上述した理由により、この範囲内においてＡはなるべく低い値であることが望ましい。 The reason why the lower limit of the temperature range between A ° C. and B ° C. is A ° C. (an arbitrary value selected from 520 ≦ A <600) is as follows. In the temperature range of 520 ° C. or lower, even if the dew point is controlled to be −10 ° C. or higher, almost no internal oxidation is formed. Internal oxidation begins to occur at temperatures above 520 ° C. On the other hand, when the dew point in the temperature range of 600 ° C. or higher is less than −10 ° C., the surface concentration increases, the inward diffusion of oxygen is hindered, and internal oxidation hardly occurs. Therefore, the dew point must be controlled to at least −10 ° C. from a temperature range exceeding 600 ° C. From the above, the allowable range of A is 520 ≦ A <600, and for the reason described above, it is desirable that A be as low as possible within this range.
　Ａ℃超えＢ℃以下の温度域の上限をＢ℃（５５０≦Ｂ≦７５０から選択される任意の値）とする理由は以下の通りである。本発明では以下のメカニズムで表面濃化を抑制できる。内部酸化を形成することにより、鋼板表面から１０μｍ以内にある領域において、易酸化性元素（Ｓｉ、Ｍｎなど）が内部酸化し、易酸化性元素の固溶量が減少した領域（以下、欠乏層と称する）が形成され、鋼中からの易酸化性元素の表面拡散が抑制される。この内部酸化を形成し、表面濃化を抑制するために十分な欠乏層を形成させるためには、Ｂを５５０≦Ｂ≦７５０とする必要がある。Ｂ℃が５５０℃を下回った場合、十分に内部酸化が形成されない。また、Ｂ℃が７５０℃を超えると内部酸化の形成量が過剰となり、電着塗装後の耐食性劣化の起点が生じる。 The reason why the upper limit of the temperature range between A ° C. and B ° C. is B ° C. (an arbitrary value selected from 550 ≦ B ≦ 750) is as follows. In the present invention, surface thickening can be suppressed by the following mechanism. By forming internal oxidation, in a region within 10 μm from the surface of the steel sheet, an easily oxidizable element (Si, Mn, etc.) is internally oxidized, and the amount of solid solution of the easily oxidizable element is reduced (hereinafter referred to as a deficient layer). And the surface diffusion of the easily oxidizable element from the steel is suppressed. In order to form this internal oxidation and to form a deficient layer sufficient to suppress surface concentration, it is necessary to set B to 550 ≦ B ≦ 750. When B ° C falls below 550 ° C, internal oxidation is not sufficiently formed. On the other hand, when B ° C. exceeds 750 ° C., the amount of internal oxidation formed becomes excessive, and a starting point for deterioration of corrosion resistance after electrodeposition coating occurs.
　上記で露点を制御する温度域以外の雰囲気の露点は特に限定されない。好ましくは－５０℃～－１０℃の範囲である。なお、特定の温度域のみ露点を制御することが、製造コストの観点から好ましい。 The dew point of the atmosphere other than the temperature range where the dew point is controlled is not particularly limited. Preferably, it is in the range of −50 ° C. to −10 ° C. In addition, it is preferable from a viewpoint of manufacturing cost to control a dew point only in a specific temperature range.
　上記（条件２）に記載の通り、連続焼鈍において鋼板最高到達温度を６００℃以上７５０℃以下とする。上記鋼板最高到達温度は、加熱過程での上記加熱における最高到達温度Ｂ℃であってもよいが、Ｂ℃が７５０℃未満の場合にはＢ℃からさらに加熱して上昇させた温度であってもよい。ここで、鋼板最高到達温度とは、以下の方法で測定して得られた値が、連続焼鈍中で最高となる値を指す。上記温度は焼鈍炉内の各パスのロール位置において、温度計（多重反射温度計及び放射温度計など、温度計の方式は特に限定されない）を設置し、測温される。 As described in (Condition 2) above, the maximum temperature reached in the steel sheet is 600 ° C. or higher and 750 ° C. or lower in continuous annealing. The steel plate maximum temperature may be the maximum temperature B ° C. in the heating in the heating process, but when B ° C. is less than 750 ° C., it is a temperature increased by further heating from B ° C. Also good. Here, the maximum reached temperature of the steel sheet refers to a value at which the value obtained by measurement by the following method is the highest during continuous annealing. The temperature is measured by installing a thermometer (a method of the thermometer such as a multiple reflection thermometer and a radiation thermometer is not particularly limited) at the roll position of each pass in the annealing furnace.
　鋼板最高到達温度を６００℃以上７５０℃以下から選択される任意の値とした理由は以下の通りである。６００℃を下回る温度を鋼板最高到達温度とすると、良好な材質が得られない。よって、本発明の効果が発現するための鋼板最高到達温度は、６００℃以上とする。一方、鋼板最高到達温度が７５０℃を上回る条件では、表面濃化が顕著となり、化成処理性の劣化が認められ始める。さらに、材質の観点からは、鋼板最高到達温度が７５０℃を上回る場合、強度と延性のバランスの効果（高強度と延性とが両立する効果）が飽和する。以上のことから、鋼板最高到達温度は６００℃以上７５０℃以下とする。 The reason why the maximum temperature reached by the steel sheet is set to an arbitrary value selected from 600 ° C. to 750 ° C. is as follows. If the temperature lower than 600 ° C. is set as the maximum steel plate temperature, a good material cannot be obtained. Therefore, the maximum steel plate temperature for achieving the effects of the present invention is 600 ° C. or higher. On the other hand, under conditions where the maximum temperature reached by the steel sheet exceeds 750 ° C., surface concentration becomes remarkable, and deterioration of the chemical conversion treatment property starts to be recognized. Furthermore, from the viewpoint of the material, when the maximum steel sheet temperature exceeds 750 ° C., the effect of balance between strength and ductility (effect in which high strength and ductility are compatible) is saturated. From the above, the maximum temperature reached by the steel sheet is set to 600 ° C. or higher and 750 ° C. or lower.
　上記（条件３）に記載の通り、連続焼鈍において鋼板温度が６００℃以上７５０℃以下の温度域の鋼板通過時間を３０秒以上１０分以下とする。上記鋼板通過時間が３０秒を下回れば目標とする材質（ＴＳ、Ｅｌ）が得られない。一方、上記鋼板通過時間が１０分を上回れば、強度と延性のバランスの効果が飽和する。また、鋼板温度は、鋼板最高到達温度を導出する際の測定方法と同様の測定方法を採用して測定することができる。 As described in (Condition 3) above, the steel plate passage time in the temperature range where the steel plate temperature is 600 ° C. or higher and 750 ° C. or lower in continuous annealing is set to 30 seconds or longer and 10 minutes or shorter. If the steel plate passage time is less than 30 seconds, the target material (TS, El) cannot be obtained. On the other hand, if the steel plate passage time exceeds 10 minutes, the effect of balance between strength and ductility is saturated. Further, the steel plate temperature can be measured by employing the same measurement method as that used when deriving the maximum steel plate temperature.
　連続焼鈍において、上記（条件１）～（条件３）を採用することで、上記の通り、化成処理直前に、地鉄表層部における、易酸化性元素であるＳｉやＭｎ等の活量が低下する。そして、これらの元素の外部酸化が抑制され、結果的に化成処理性が改善することになる。 By adopting the above (Condition 1) to (Condition 3) in continuous annealing, as described above, the activity of easily oxidizable elements such as Si and Mn decreases in the surface layer of the iron core immediately before the chemical conversion treatment. To do. And external oxidation of these elements is suppressed, and as a result, chemical conversion property improves.
　上記必須条件以外の連続焼鈍における条件は以下の通りである。 The conditions for continuous annealing other than the above essential conditions are as follows.
　本発明において、均熱帯における均熱温度、均熱時間の条件は特に限定されず適宜設定すればよい。なお、均熱温度は上記鋼板最高到達温度であってもよいし、上記鋼板到達最高温度よりも低い温度であってもよい。 In the present invention, the conditions of the soaking temperature and soaking time in the soaking zone are not particularly limited, and may be set as appropriate. In addition, the soaking temperature may be the above-mentioned maximum steel plate temperature, or may be a temperature lower than the above-mentioned maximum steel plate temperature.
　また、同一の連続焼鈍条件で比較した場合、Ｓｉ、Ｍｎの表面濃化量は、鋼中Ｓｉ、Ｍｎ量に比例して多くなる。また、同一鋼種の場合、比較的高い酸素ポテンシャル雰囲気では、鋼中Ｓｉ、Ｍｎが内部酸化に移行するため、雰囲気の酸素ポテンシャルの増加に伴い、表面濃化量も少なくなる。さらに、比較的低い酸素ポテンシャル雰囲気では、鋼中Ｓｉ、Ｍｎの選択酸化が起こりにくくなるため、雰囲気の酸素ポテンシャルの減少に伴い、表面濃化量は少なくなる。そのため、加熱過程での露点制御の際には、鋼中Ｓｉ、Ｍｎ量に応じて、水素濃度を増加させたり、露点を上昇させたりして、雰囲気の酸素ポテンシャルを増加または低減させる必要がある。 Also, when compared under the same continuous annealing conditions, the surface concentration of Si and Mn increases in proportion to the amount of Si and Mn in the steel. In the case of the same steel type, in a relatively high oxygen potential atmosphere, since Si and Mn in the steel move to internal oxidation, the amount of surface enrichment decreases as the oxygen potential of the atmosphere increases. Further, in a relatively low oxygen potential atmosphere, selective oxidation of Si and Mn in the steel is difficult to occur, so that the amount of surface enrichment decreases as the oxygen potential of the atmosphere decreases. Therefore, when controlling the dew point in the heating process, it is necessary to increase or decrease the oxygen potential of the atmosphere by increasing the hydrogen concentration or raising the dew point depending on the amount of Si and Mn in the steel. .
　連続焼鈍後に以下の処理を施して高強度鋼板を製造してもよい。 High strength steel sheets may be manufactured by performing the following treatment after continuous annealing.
　冷却帯での冷却後、必要に応じて焼入れ、焼き戻しを行ってもよい。この条件は特に限定しないが、焼き戻しは１５０～４００℃の温度で行うのが望ましい。１５０℃未満では伸びが劣化傾向にあり、４００℃超えでは硬度が低下する傾向にある。 ¡After cooling in the cooling zone, quenching and tempering may be performed as necessary. This condition is not particularly limited, but tempering is preferably performed at a temperature of 150 to 400 ° C. If it is less than 150 ° C., the elongation tends to deteriorate, and if it exceeds 400 ° C., the hardness tends to decrease.
　本発明においては、連続焼鈍後に電解酸洗を実施しなくとも良好な化成処理性は確保可能である。しかし、連続焼鈍時に不可避的に発生する微量な表面濃化物を除去し、より良好な化成処理性を確保するため、連続焼鈍後に電解酸洗を行ってもよい。 In the present invention, it is possible to ensure good chemical conversion treatment performance without performing electrolytic pickling after continuous annealing. However, electrolytic pickling may be performed after the continuous annealing in order to remove a small amount of surface concentrate that is inevitably generated during the continuous annealing and to ensure better chemical conversion treatment.
　電解酸洗の条件は特に限定しないが、連続焼鈍時に不可避的に表面濃化したＳｉやＭｎの酸化物を効率的に除去するため、電流密度が１Ａ／ｄｍ２以上の交番電解とすることが望ましい。交番電解とする理由は、鋼板を陰極に保持したままでは酸洗効果が小さく、逆に鋼板を陽極に保持したままでは電解時に溶出するＦｅが酸洗液中に蓄積し、酸洗液中のＦｅ濃度が増大してしまい、鋼板表面に付着すると乾き汚れ等の問題が発生してしまうためである。さらに、電解酸洗に用いる酸洗液は特に限定しないが、硝酸やフッ化水素酸は設備に対する腐食性が強く取り扱いに注意を要するため、好ましくない。また塩酸は陰極から塩素ガスを発生する可能性があり好ましくない。このため、腐食性や環境を考慮すると硫酸の使用が好ましい。硫酸濃度は５質量%以上２０質量％以下が好ましい。硫酸濃度が５質量％未満では導電率が低くなることから電解時の浴電圧が上昇し、電源負荷が大きくなってしまう。一方、２０質量％超えの場合は、ドラッグアウトによる損失が大きくコスト的に問題となる。電解液の温度は４０℃以上７０℃以下が好ましい。連続電解することによる発熱で浴温が上昇することから、４０℃未満に温度を維持することは困難である。また、電解槽のライニングの耐久性の観点から温度が７０℃を超えることは問題である。 The conditions for the electrolytic pickling are not particularly limited, but in order to efficiently remove oxides of Si and Mn that are inevitably surface-enriched during continuous annealing, an alternating electrolysis with a current density of 1 A / dm 2 or more may be used. desirable. The reason for alternating electrolysis is that the pickling effect is small when the steel plate is held at the cathode, and conversely, Fe that is eluted during electrolysis accumulates in the pickling solution while the steel plate is held at the anode. This is because if the Fe concentration increases and adheres to the surface of the steel sheet, problems such as dry dirt occur. Furthermore, the pickling solution used for the electrolytic pickling is not particularly limited, but nitric acid and hydrofluoric acid are not preferable because they are highly corrosive to equipment and require careful handling. Hydrochloric acid is not preferred because it may generate chlorine gas from the cathode. For this reason, use of sulfuric acid is preferable in consideration of corrosivity and environment. The sulfuric acid concentration is preferably 5% by mass or more and 20% by mass or less. If the sulfuric acid concentration is less than 5% by mass, the electrical conductivity will be low, so the bath voltage during electrolysis will rise and the load on the power source will increase. On the other hand, if it exceeds 20% by mass, a loss due to drag-out is large, which causes a problem in cost. The temperature of the electrolytic solution is preferably 40 ° C. or higher and 70 ° C. or lower. Since the bath temperature rises due to heat generated by continuous electrolysis, it is difficult to maintain the temperature below 40 ° C. Moreover, it is a problem that temperature exceeds 70 degreeC from a durable viewpoint of the lining of an electrolytic cell.
　上記の通り、本発明は鋼板の連続焼鈍条件に特徴がある製造方法である。この連続焼鈍の対象となる鋼板について説明する。以下の成分組成の説明における「％」は「質量％」を意味する。 As described above, the present invention is a manufacturing method characterized by continuous annealing conditions for steel sheets. The steel plate that is subject to this continuous annealing will be described. In the following description of the component composition, “%” means “mass%”.
　Ｃ：０．０３～０．３５％
　Ｃは、鋼組織中にマルテンサイトなどを形成させることで加工性を向上させる。そのためには、Ｃの含有量を０．０３％以上にする必要がある。一方、Ｃの含有量が０．３５％を超えると強度が上昇しすぎて、延びが低下し、結果として加工性が劣化する。したがって、Ｃ量は０．０３％以上０．３５％以下とする。 C: 0.03-0.35%
C improves workability by forming martensite or the like in the steel structure. For this purpose, the C content needs to be 0.03% or more. On the other hand, if the C content exceeds 0.35%, the strength increases excessively, the elongation decreases, and the workability deteriorates as a result. Therefore, the C content is 0.03% or more and 0.35% or less.
　Ｓｉ：０．０１～０．５０％
　Ｓｉは鋼を強化して良好な材質を得るのに有効な元素ではある。しかし、Ｓｉは易酸化性元素であるため、化成処理性には不利である。この観点からは、Ｓｉは極力添加することを避けるべき元素である。また、０．０１％程度のＳｉは不可避的に鋼中に含まれ、Ｓｉの含有量をこれ以下に低減するためにはコストが上昇してしまう。以上より、Ｓｉの含有量は０．０１％を下限とする。一方、Ｓｉの含有量が０．５０％を超えると鋼の強化能や伸び向上効果が飽和してくる。また、高強度鋼板の化成処理性が劣化する。したがって、Ｓｉ量は０．０１％以上０．５０％以下とする。本発明は、Ｓｉの含有量が多い場合であっても化成処理性を改善できる点が特徴の１つである。 Si: 0.01 to 0.50%
Si is an effective element for strengthening steel and obtaining a good material. However, since Si is an easily oxidizable element, it is disadvantageous for chemical conversion treatment. From this viewpoint, Si is an element that should be avoided as much as possible. Moreover, about 0.01% Si is inevitably contained in the steel, and the cost increases in order to reduce the Si content below this. From the above, the lower limit of the Si content is 0.01%. On the other hand, when the Si content exceeds 0.50%, the steel strengthening ability and elongation improvement effect are saturated. Moreover, the chemical conversion property of a high strength steel plate deteriorates. Therefore, the Si amount is set to 0.01% or more and 0.50% or less. One feature of the present invention is that the chemical conversion treatment can be improved even when the Si content is high.
　Ｍｎ：３．６～８．０％
　Ｍｎは鋼の高強度化に有効な元素である。機械特性や強度を確保するためは、Ｍｎの含有量を３．６％以上にする必要がある。一方、Ｍｎの含有量が８．０％を超えると化成処理性の確保、強度と延性のバランスの確保が困難になる。さらに、コスト的に不利となる。したがって、Ｍｎ量は３．６％以上８．０％以下とする。 Mn: 3.6 to 8.0%
Mn is an element effective for increasing the strength of steel. In order to ensure mechanical properties and strength, the Mn content needs to be 3.6% or more. On the other hand, when the content of Mn exceeds 8.0%, it becomes difficult to secure chemical conversion property and secure a balance between strength and ductility. Further, it is disadvantageous in terms of cost. Therefore, the Mn content is 3.6% or more and 8.0% or less.
　Ａｌ：０．０１～１．０％
　Ａｌは溶鋼の脱酸を目的に添加される。Ａｌの含有量が０．０１％未満の場合、その目的が達成されない。溶鋼の脱酸の効果はＡｌの含有量を０．０１％以上にすることで得られる。一方、Ａｌの含有量が１．０％を超えるとコストアップになる。さらに、Ａｌの含有量が１．０％を超えると、Ａｌの表面濃化が多くなり、化成処理性の改善が困難になってくる。したがって、Ａｌ量は０．０１％以上１．０％以下とする。 Al: 0.01 to 1.0%
Al is added for the purpose of deoxidizing molten steel. If the Al content is less than 0.01%, the object is not achieved. The effect of deoxidation of molten steel can be obtained by making the Al content 0.01% or more. On the other hand, if the Al content exceeds 1.0%, the cost increases. Furthermore, if the Al content exceeds 1.0%, the surface concentration of Al increases, and it becomes difficult to improve the chemical conversion property. Therefore, the Al content is set to 0.01% to 1.0%.
　Ｐ：０．１０％以下
　Ｐは不可避的に含有される元素のひとつであり、含有しなくてもよい。Ｐの含有量を０．００５％未満にするためには、コストの増大が懸念されるため、Ｐの含有量は０．００５％以上が望ましい。一方、Ｐの含有量が０．１０％を超えると溶接性が劣化する。さらに、Ｐの含有量が０．１０％を超えると、化成処理性の劣化が激しくなり、本発明をもってしても化成処理性を向上させることが困難になる。したがって、Ｐ量は０．１０％以下とし、下限としては０．００５％が望ましい。 P: 0.10% or less P is one of elements inevitably contained, and may not be contained. In order to reduce the P content to less than 0.005%, there is a concern about an increase in cost. Therefore, the P content is preferably 0.005% or more. On the other hand, if the P content exceeds 0.10%, the weldability deteriorates. Furthermore, when the P content exceeds 0.10%, the chemical conversion treatment property deteriorates so severely that it is difficult to improve the chemical conversion treatment property even with the present invention. Accordingly, the P content is preferably 0.10% or less, and the lower limit is preferably 0.005%.
　Ｓ：０．０１０％以下
　Ｓは不可避的に含有される元素のひとつであり、Ｓを含有しなくてもよい。Ｓの含有量の下限は規定しない。Ｓの含有量が多量になると溶接性および耐食性が劣化する。このため、Ｓの含有量は０．０１０％以下とする。 S: 0.010% or less S is one of elements inevitably contained, and S may not be contained. The lower limit of the S content is not specified. When the S content is large, weldability and corrosion resistance deteriorate. For this reason, content of S shall be 0.010% or less.
　また、本発明の製造方法で製造される高強度鋼板の表面品質や強度と延性のバランスの改善を図るために、連続焼鈍が施される鋼板は、Ｂ：０．００１～０．００５％、Ｎｂ：０．００５～０．０５％、Ｔｉ：０．００５～０．０５％、Ｃｒ：０．００１～１．０％、Ｍｏ：０．０５～１．０％、Ｃｕ：０．０５～１．０％、Ｎｉ：０．０５～１．０％、Ｓｎ：０．００１～０．２０％、Ｓｂ：０．００１～０．２０％、Ｔａ：０．００１～０．１０％、Ｗ：０．００１～０．１０％、Ｖ：０．００１～０．１０％の中から選ばれる１種以上の元素を必要に応じて含んでもよい。これらの元素を含有する場合における適正含有量の限定理由は以下の通りである。 In order to improve the balance between the surface quality and strength and ductility of the high-strength steel sheet produced by the production method of the present invention, the steel sheet subjected to continuous annealing is B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W : One or more elements selected from 0.001 to 0.10% and V: 0.001 to 0.10% may be included as necessary. The reason for limiting the proper content in the case of containing these elements is as follows.
　Ｂ：０．００１～０．００５％
　Ｂの含有量が０．００１％未満では焼き入れ促進効果が得られにくい。一方、Ｂの含有量が０．００５％超えでは化成処理性が劣化する場合がある。よって、Ｂを含有する場合、Ｂ量は０．００１％以上０．００５％以下とすることが好ましい。ただし、機械的特性改善上、Ｂを含有する必要がないと判断される場合、鋼板はＢを含有する必要はない。必要に応じて含有することは、他の任意元素についても同様である。 B: 0.001 to 0.005%
When the content of B is less than 0.001%, it is difficult to obtain the quenching promoting effect. On the other hand, if the content of B exceeds 0.005%, chemical conversion property may be deteriorated. Therefore, when it contains B, it is preferable to make B amount into 0.001% or more and 0.005% or less. However, when it is determined that it is not necessary to contain B for improving mechanical properties, the steel plate does not need to contain B. It is the same for other optional elements to be contained as necessary.
　Ｎｂ：０．００５～０．０５％
　Ｎｂの含有量が０．００５％未満では強度調整の効果が得られにくい。一方、Ｎｂの含有量が０．０５％超えではコストアップを招く。よって、Ｎｂを含有する場合、Ｎｂ量は０．００５％以上０．０５％以下とする。 Nb: 0.005 to 0.05%
If the Nb content is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if the Nb content exceeds 0.05%, the cost increases. Therefore, when Nb is contained, the Nb content is 0.005% or more and 0.05% or less.
　Ｔｉ：０．００５～０．０５％
　Ｔｉの含有量が０．００５％未満では強度調整の効果が得られにくい。一方、Ｔｉの含有量が０．０５％超えでは化成処理性の劣化を招く場合がある。よって、Ｔｉを含有する場合、Ｔｉ量は０．００５％以上０．０５％以下とすることが好ましい。 Ti: 0.005 to 0.05%
If the Ti content is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, when the Ti content exceeds 0.05%, chemical conversion treatment may be deteriorated. Therefore, when Ti is contained, the Ti content is preferably 0.005% or more and 0.05% or less.
　Ｃｒ：０．００１～１．０％
　Ｃｒの含有量が０．００１％未満では焼き入れ性効果が得られにくい。一方、Ｃｒの含有量が１．０％超えではＣｒが表面濃化するため、溶接性が劣化する。よって、Ｃｒを含有する場合、Ｃｒ量は０．００１％以上１．０％以下とすることが好ましい。 Cr: 0.001 to 1.0%
When the Cr content is less than 0.001%, it is difficult to obtain a hardenability effect. On the other hand, if the Cr content exceeds 1.0%, Cr is concentrated on the surface, so that weldability is deteriorated. Therefore, when it contains Cr, it is preferable that Cr amount shall be 0.001% or more and 1.0% or less.
　Ｍｏ：０．０５～１．０％
　Ｍｏの含有量が０．０５％未満では強度調整の効果が得られにくい。一方、Ｍｏの含有量が１．０％超えではコストアップを招く。よって、Ｍｏを含有する場合、Ｍｏ量は０．０５％以上１．０％以下とすることが好ましい。 Mo: 0.05 to 1.0%
If the Mo content is less than 0.05%, the effect of adjusting the strength is difficult to obtain. On the other hand, if the Mo content exceeds 1.0%, the cost increases. Therefore, when it contains Mo, it is preferable to make Mo amount into 0.05% or more and 1.0% or less.
　Ｃｕ：０．０５～１．０％
　Ｃｕの含有量が０．０５％未満では残留γ相形成促進効果が得られにくい。一方、Ｃｕの含有量が１．０％超えではコストアップを招く。よって、Ｃｕを含有する場合、Ｃｕ量は０．０５％以上１．０％以下とすることが好ましい。 Cu: 0.05 to 1.0%
If the Cu content is less than 0.05%, the effect of promoting the formation of the residual γ phase is difficult to obtain. On the other hand, if the Cu content exceeds 1.0%, the cost increases. Therefore, when Cu is contained, the amount of Cu is preferably 0.05% or more and 1.0% or less.
　Ｎｉ：０．０５～１．０％
　Ｎｉの含有量が０．０５％未満では残留γ相形成促進効果が得られにくい。一方、Ｎｉの含有量が１．０％超えではコストアップを招く。よって、Ｎｉを含有する場合、Ｎｉ量は０．０５％以上１．０％以下とすることが好ましい。 Ni: 0.05 to 1.0%
When the Ni content is less than 0.05%, it is difficult to obtain the effect of promoting the formation of the residual γ phase. On the other hand, if the Ni content exceeds 1.0%, the cost increases. Therefore, when Ni is contained, the amount of Ni is preferably 0.05% or more and 1.0% or less.
　Ｓｎ：０．００１～０．２０％、Ｓｂ：０．００１～０．２０％
　ＳｎやＳｂは鋼板表面の窒化、酸化、あるいは酸化により生じる鋼板表面から数十ミクロン領域の脱炭を抑制する観点から含有することができる。窒化や酸化を抑制することで鋼板表面においてマルテンサイトの生成量が減少するのを防止し、得られる高強度鋼板の疲労特性や表面品質が改善する。以上の観点から、Ｓｎおよび／またはＳｂを含有する場合は、いずれも０．００１％以上の含有量とすることが好ましい。また、いずれかの含有量が０．２０％を超えると靭性の劣化を招くので、これらの含有量はそれぞれ０．２０％以下とすることが好ましい。 Sn: 0.001 to 0.20%, Sb: 0.001 to 0.20%
Sn and Sb can be contained from the viewpoint of suppressing decarburization in the region of several tens of microns from the steel sheet surface caused by nitriding, oxidation, or oxidation of the steel sheet surface. By suppressing nitriding and oxidation, the amount of martensite produced on the steel sheet surface is prevented from decreasing, and the fatigue properties and surface quality of the resulting high strength steel sheet are improved. From the above viewpoint, when Sn and / or Sb is contained, it is preferable that the content is 0.001% or more. Moreover, since the deterioration of toughness will be caused when either content exceeds 0.20%, it is preferable that these content shall be 0.20% or less, respectively.
　Ｔａ：０．００１～０．１０％
　ＴａはＣやＮと炭化物や炭窒化物を形成することで高強度化に寄与し、さらに高降伏比（高ＹＲ）化に寄与する。さらに、Ｔａは熱延板組織を微細化する作用を有し、この作用により、冷延、焼鈍後のフェライト粒径が微細化される。そして、粒界面積の増大に伴う粒界へのＣ偏析量が増大し、高焼付き硬化量（ＢＨ量）を得ることができる。このような観点から、Ｔａを０．００１％以上含有することができる。一方、Ｔａの含有量が０．１０％を超える場合、原料コストの増加を招くだけでなく、焼鈍後の冷却過程におけるマルテンサイトの形成を妨げる可能性がある。さらには、熱延板中に析出したＴａＣは、冷間圧延時の変形抵抗を高くし、安定した実機製造を困難にする場合がある。よって、Ｔａを含有する場合、その含有量は０．１０％以下とすることが好ましい。 Ta: 0.001 to 0.10%
Ta contributes to higher strength by forming carbides and carbonitrides with C and N, and further contributes to higher yield ratio (high YR). Furthermore, Ta has the effect of refining the hot-rolled sheet structure, and this effect refines the ferrite grain size after cold rolling and annealing. And the amount of C segregation to the grain boundary accompanying the increase in grain boundary area increases, and a high seizure hardening amount (BH amount) can be obtained. From such a viewpoint, 0.001% or more of Ta can be contained. On the other hand, when the content of Ta exceeds 0.10%, not only the raw material cost is increased, but also the formation of martensite in the cooling process after annealing may be hindered. Furthermore, TaC precipitated in the hot-rolled sheet increases the deformation resistance during cold rolling, and may make it difficult to manufacture a stable actual machine. Therefore, when Ta is contained, the content is preferably 0.10% or less.
　Ｗ：０．００１～０．１０％、Ｖ：０．００１～０．１０％
　ＷおよびＶは炭窒化物を形成し、鋼を析出効果により高強度化する作用を有する元素であり、必要に応じて添加できる。このような作用は、Ｗおよび／またはＶを添加する場合、いずれも０．００１％以上含有して認められる。一方、これらの元素の含有量が０．１０％を超える場合、鋼板が過度に高強度化し、延性が劣化してしまう場合がある。以上より、Ｗおよび／またはＶを含有する場合、いずれも含有量は０．００１％以上０．１０％以下が好ましい。 W: 0.001 to 0.10%, V: 0.001 to 0.10%
W and V are elements that form carbonitrides and have the effect of increasing the strength of steel by precipitation effects, and can be added as necessary. Such an effect is observed when both W and / or V are added, containing 0.001% or more. On the other hand, when the content of these elements exceeds 0.10%, the steel sheet becomes excessively strong and the ductility may be deteriorated. From the above, when W and / or V are contained, the content is preferably 0.001% or more and 0.10% or less.
　上記以外の残部はＦｅおよび不可避的不純物である。上記記載の元素以外の元素を含有しても、本発明には何ら悪影響を及ぼすものではなく、その上限は０．１０％とする。 The remainder other than the above is Fe and inevitable impurities. Even if elements other than the elements described above are contained, the present invention is not adversely affected, and the upper limit is made 0.10%.
　以上のような成分組成を有する鋼板の連続焼鈍の条件を調整することで、加工性、化成処理性および電着塗装後の耐食性に優れた高強度鋼板が得られる。以下、この高強度鋼板について説明する。 By adjusting the conditions for continuous annealing of a steel sheet having the above component composition, a high-strength steel sheet excellent in workability, chemical conversion treatment and corrosion resistance after electrodeposition coating can be obtained. Hereinafter, this high-strength steel sheet will be described.
　鋼中にＳｉおよび多量のＭｎを含有する高強度鋼板において、電着塗装後の耐食性を満足させるためには腐食の割れなどの起点になる可能性がある鋼板表層の組織、構造をより高度に制御する必要がある。そこで、本発明では、まず、化成処理性を確保するために焼鈍工程において水素濃度制御や露点制御を上述のように行うことによって、上述のように易酸化性元素であるＳｉやＭｎ等を化成処理直前に予め内部酸化させ、地鉄表層部におけるＳｉ、Ｍｎの活量を低下させる。そして、これらの元素の外部酸化が抑制され、結果的に化成処理性及び電着塗装後の耐食性が改善する。具体的には、本発明の製造方法で製造された鋼強度鋼板の鋼板表層の組織、構造は以下の特徴を有する。
（特徴１）高強度鋼板の鋼板表面から１００μｍ以内の領域に、Ｆｅ、Ｓｉ、Ｍｎ、Ａｌ、Ｐ、Ｂ、Ｎｂ、Ｔｉ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、Ｔａ、ＷおよびＶの中から選ばれる少なくとも１種以上の酸化物を、片面あたり合計で０．０１０～０．１００ｇ／ｍ2有する。
（特徴２）鋼板表面から１０μｍ以内の領域における、鋼板結晶粒界から１μｍ以内の粒内に、Ｍｎを含む酸化物を有する。 In a high-strength steel sheet containing Si and a large amount of Mn in the steel, in order to satisfy the corrosion resistance after electrodeposition coating, the structure and structure of the surface layer of the steel sheet, which may be the starting point of corrosion cracking, have been improved. Need to control. Therefore, in the present invention, first, by performing hydrogen concentration control and dew point control as described above in the annealing process in order to ensure chemical conversion treatment properties, the easily oxidizable elements such as Si and Mn are formed as described above. Immediately before the treatment, internal oxidation is performed in advance, and the activity of Si and Mn in the surface layer portion of the iron base is lowered. And external oxidation of these elements is suppressed, and as a result, chemical conversion property and corrosion resistance after electrodeposition coating are improved. Specifically, the structure and structure of the steel sheet surface layer of the steel strength steel sheet manufactured by the manufacturing method of the present invention has the following characteristics.
(Characteristic 1) Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V are in a region within 100 μm from the steel plate surface of the high-strength steel plate. At least one oxide selected from the group consisting of 0.010 to 0.100 g / m 2 per side.
(Feature 2) It has an oxide containing Mn in grains within 1 μm from the grain boundary of the steel sheet in a region within 10 μm from the steel sheet surface.
　上記化成処理性、電着塗装後の耐食性を改善するためには、高強度鋼板は、鋼板表面から１００μｍ以内の領域に、Ｆｅ、Ｓｉ、Ｍｎ、Ａｌ、Ｐ、Ｂ、Ｎｂ、Ｔｉ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、Ｔａ、Ｗ、Ｖの中から選ばれる１種以上の酸化物を片面あたり合計で０．０１０ｇ／ｍ２以上有する必要がある。また、上記酸化物を片面あたり合計で０．１００ｇ／ｍ２を超えて有すると、腐食の割れの起点が生じる懸念があることと、上記酸化物の形成量が０．１００ｇ／ｍ２を超えても化成処理性向上効果は高まらず飽和するため、上限は０．１００ｇ／ｍ２とする。 In order to improve the chemical conversion property and the corrosion resistance after electrodeposition coating, a high-strength steel sheet is formed in a region within 100 μm from the steel sheet surface, Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, One or more oxides selected from Mo, Cu, Ni, Sn, Sb, Ta, W, and V must be 0.010 g / m 2 or more in total per side. Further, if the total oxide exceeds 0.100 g / m 2 per side, there is a concern that a starting point of corrosion cracking occurs, and the amount of the oxide formed exceeds 0.100 g / m 2 . However, since the chemical conversion processability improving effect is not increased and is saturated, the upper limit is set to 0.100 g / m 2 .
　また、内部酸化物が粒界にのみ存在し、粒内に存在しない場合、鋼中易酸化性元素の粒界拡散は抑制できるが、粒内拡散は十分に抑制できない場合がある。したがって、本発明では、上記のような連続焼鈍の条件を採用することで、粒界のみならず粒内でも内部酸化させる。 Further, when the internal oxide exists only at the grain boundary and does not exist within the grain, the grain boundary diffusion of the easily oxidizable element in the steel can be suppressed, but the intragranular diffusion may not be sufficiently suppressed. Therefore, in the present invention, internal oxidation is performed not only at the grain boundaries but also within the grains by adopting the above-described continuous annealing conditions.
　具体的には、高強度鋼板は、鋼板表面から１０μｍの領域における、粒界から１μｍ以内の粒内に、Ｍｎを含む酸化物を有する。この粒内に酸化物が存在することで、酸化物近傍の粒内の固溶Ｍｎの量が減少する。その結果、Ｍｎの粒内拡散による表面への濃化を抑制することができる。 Specifically, the high-strength steel sheet has an oxide containing Mn in grains within 1 μm from the grain boundary in a region of 10 μm from the steel sheet surface. The presence of oxide in the grains reduces the amount of solid solution Mn in the grains near the oxide. As a result, concentration of Mn on the surface due to intragranular diffusion can be suppressed.
　なお、本発明の製造方法で得られる高強度鋼板の鋼板表面の構造は、上記の通りであるが、例えば、鋼板表面から１００μｍを超えた領域で上記酸化物が成長していても問題はない。また、鋼板表面から１０μｍを超えた領域おける、粒界から１μｍ以上の粒内にＭｎを含む酸化物が存在しても問題はない。 The structure of the steel sheet surface of the high-strength steel sheet obtained by the production method of the present invention is as described above. For example, there is no problem even if the oxide grows in a region exceeding 100 μm from the steel sheet surface. . Moreover, there is no problem even if an oxide containing Mn is present in a grain of 1 μm or more from the grain boundary in a region exceeding 10 μm from the steel plate surface.
　さらに、上記に加え、本発明では、塗装後の成形性確保の観点から、Ｍｎを含む酸化物が成長する領域における地鉄組織は、軟質で加工性に富むフェライト相が好ましい。 Furthermore, in addition to the above, in the present invention, from the viewpoint of ensuring formability after coating, the base metal structure in the region where the oxide containing Mn grows is preferably a soft and rich workability ferrite phase.
　本発明の高強度鋼板は、上記高強度鋼板に化成処理を施してなるものでもよい。化成処理液の種類は特に限定されず、クロメート処理液、ノンクロメート処理液等の一般的なものを使用できる。また、化成処理方法に関しても限定されず、浸漬（ディップ）処理、スプレー処理、電解処理等の種々の方法を適用することができる。 The high-strength steel plate of the present invention may be formed by subjecting the high-strength steel plate to a chemical conversion treatment. The type of chemical conversion treatment liquid is not particularly limited, and general chemicals such as a chromate treatment liquid and a non-chromate treatment liquid can be used. Further, the chemical conversion treatment method is not limited, and various methods such as immersion (dip) treatment, spray treatment, and electrolytic treatment can be applied.
　本発明の高強度鋼板は、上記化成処理を施した鋼板の化成皮膜上に電着塗装により塗膜を形成してなるものでもよい。電着塗装の条件は特に限定されず、適宜設定すればよい。 The high-strength steel sheet of the present invention may be formed by electrodeposition coating on the chemical conversion film of the steel sheet subjected to the above chemical conversion treatment. The conditions for electrodeposition coating are not particularly limited, and may be set as appropriate.
　以下、本発明を、実施例に基づいて具体的に説明する。 Hereinafter, the present invention will be specifically described based on examples.
　表１に示す鋼組成からなる熱延鋼板を酸洗し、黒皮スケール除去した後、冷間圧延し、厚さ１．０ｍｍの冷延鋼板を得た。なお、一部は冷間圧延を実施せず、黒皮スケール除去後の熱延鋼板（厚さ２．０ｍｍ）のまま用いた。 The hot-rolled steel sheet having the steel composition shown in Table 1 was pickled, removed the black scale, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.0 mm. A part of the hot-rolled steel sheet (thickness: 2.0 mm) after removing the black scale was used without performing cold rolling.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
　次いで、上記で得た冷延鋼板及び熱延鋼板を、連続焼鈍設備に装入した。焼鈍設備では、表２、３（表２－１と表２－２を合わせて表２とし、表３－１と表３－２を合わせて表３とする。）に示す通り、温度、水素濃度、露点を制御して、鋼板を連続焼鈍した。連続焼鈍において、雰囲気の気体成分は、窒素ガスと水素ガスおよび不可避不純物気体とした。－１０℃以上となる露点の制御については、窒素ガスが充満した空間に設置した水タンクを加熱して加湿した窒素ガスを流す配管を連続焼鈍設備に接続し、加湿した窒素ガス中に水素ガスを導入して混合し、これを上記配管から連続焼鈍設備に導入することで雰囲気の露点を制御した。また、連続焼鈍において露点を制御した領域以外の露点は－３５℃を基本とした。また、雰囲気中の水素濃度上昇は雰囲気中の窒素を減少させ、水素を増加させることで行った。また、水素濃度を制御する温度域以外の雰囲気の水素濃度は１０ｖｏｌ%を基本とした。 Next, the cold-rolled steel plate and hot-rolled steel plate obtained above were charged into a continuous annealing facility. For annealing equipment, as shown in Tables 2 and 3 (Tables 2-1 and 2-2 are combined into Table 2 and Tables 3-1 and 3-2 are combined into Table 3), temperature, hydrogen The steel sheet was continuously annealed by controlling the concentration and dew point. In the continuous annealing, the atmospheric gas components were nitrogen gas, hydrogen gas, and inevitable impurity gas. To control the dew point at -10 ° C or higher, connect a pipe that flows a humidified nitrogen gas by heating a water tank installed in a space filled with nitrogen gas to a continuous annealing facility, and hydrogen gas in the humidified nitrogen gas Were introduced and mixed, and the dew point of the atmosphere was controlled by introducing this into the continuous annealing facility from the above piping. In addition, the dew point outside the region where the dew point was controlled in the continuous annealing was basically -35 ° C. The increase in the hydrogen concentration in the atmosphere was performed by decreasing the nitrogen in the atmosphere and increasing the hydrogen. Further, the hydrogen concentration in the atmosphere other than the temperature range for controlling the hydrogen concentration was basically 10 vol%.
　連続焼鈍の後、水焼入れ後に３００℃×１４０ｓ間の焼き戻しを行った。Ｎｏ．３１～３３以外はこの焼き戻しで得られた高強度鋼板を供試材とした。Ｎｏ．３１～３３については、上記焼き戻し後、引き続き、４０℃、５％の硫酸水溶液中、表３に示す電流密度条件にて電解酸洗を行い、供試材を得た。電解酸洗では、供試材を陽極、陰極の順に３秒ずつの交番電解を行った。得られたＮｏ．１～６５の供試材について、ＪＩＳ　Ｚ　２２４１に準拠する金属材料引張試験方法に従い、ＴＳ、Ｅｌを測定した。測定結果を表２、３に示した。 After continuous annealing, tempering at 300 ° C. × 140 s was performed after water quenching. No. Except for 31 to 33, high strength steel plates obtained by this tempering were used as test materials. No. 31 to 33 were subjected to electrolytic pickling under the current density conditions shown in Table 3 in a sulfuric acid aqueous solution at 40 ° C. and 5% after the above tempering to obtain test materials. In the electrolytic pickling, the test material was subjected to alternating electrolysis for 3 seconds in order of the anode and the cathode. No. obtained With respect to the test materials 1 to 65, TS and El were measured according to a metal material tensile test method based on JIS Z 2241. The measurement results are shown in Tables 2 and 3.
　以上により得られた高強度鋼板の供試材に対して、化成処理性および耐食性を調査した。また、高強度鋼板の鋼板表面から１００μｍまので領域に存在する酸化物の量（内部酸化量）を測定した。測定方法および評価基準を下記に示す。 The chemical conversion processability and corrosion resistance of the specimens of the high-strength steel sheets obtained as described above were investigated. Further, the amount of oxide (internal oxidation amount) existing in the region from the steel plate surface of the high-strength steel plate to 100 μm was measured. The measurement method and evaluation criteria are shown below.
　＜化成処理性＞
　化成処理性の評価方法は次の通りである。化成処理液は日本パーカライジング社製の化成処理液（パルボンドＬ３０８０（登録商標））を用い、下記方法で化成処理を施した。 <Chemical conversion processability>
The evaluation method of chemical conversion property is as follows. A chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nippon Parkerizing Co., Ltd. was used as the chemical conversion treatment liquid, and chemical conversion treatment was performed by the following method.
　供試材を、日本パーカライジング社製の脱脂液ファインクリーナー（登録商標）で脱脂した後、水洗し、次に日本パーカライジング社製の表面調整液プレパレンＺ（登録商標）で３０秒間の表面調整を行った。表面調整後、４３℃の化成処理液（パルボンドＬ３０８０）に１２０秒間浸漬した後、水洗し、温風乾燥した。 The test material is degreased with a degreasing liquid fine cleaner (registered trademark) manufactured by Nihon Parkerizing Co., Ltd., washed with water, and then subjected to a surface adjustment for 30 seconds with a surface conditioning solution preparen Z (registered trademark) manufactured by Nihon Parkerizing Co., Ltd. It was. After the surface adjustment, the substrate was immersed in a chemical conversion solution (Palbond L3080) at 43 ° C. for 120 seconds, washed with water, and dried with warm air.
　化成処理後の供試材を走査型電子顕微鏡（ＳＥＭ）で、倍率５００倍の条件で無作為に５視野を観察した。化成処理皮膜のスケ面積率を画像処理により測定し、スケ面積率によって以下の評価を行った。「○」が合格レベルである。
○：１０％以下
×：１０％超
　＜電着塗装後の耐食性＞
　上記の方法で得られた化成処理鋼板より寸法７０ｍｍ×１５０ｍｍの試験片を切り出し、日本ペイント社製のＰＮ－１５０Ｇ（登録商標）でカチオン電着塗装（焼付け条件：１７０℃×２０分、膜厚２５μｍ）を行った。その後、端部と評価しない側の面をＡｌテープでシールし、カッターナイフにて鋼板表面に達するクロスカット（クロス角度６０°）を入れ、試験サンプルとした。 The sample material after the chemical conversion treatment was randomly observed with a scanning electron microscope (SEM) under the condition of 500 times magnification. The scale area ratio of the chemical conversion coating was measured by image processing, and the following evaluation was performed based on the scale area ratio. "○" is a pass level.
○: 10% or less ×: Over 10% <Corrosion resistance after electrodeposition coating>
A test piece having a size of 70 mm × 150 mm was cut out from the chemical conversion treated steel plate obtained by the above method, and was subjected to cationic electrodeposition coating with PN-150G (registered trademark) manufactured by Nippon Paint Co., Ltd. (baking conditions: 170 ° C. × 20 minutes, film thickness) 25 μm). Thereafter, the end surface and the side not evaluated were sealed with Al tape, and a cross cut (cross angle 60 °) reaching the steel plate surface with a cutter knife was put into a test sample.
　次に、試験サンプルを５％ＮａＣｌ水溶液（５５℃）中に、２４０時間浸漬後、取り出し、水洗、乾燥後にクロスカット部をテープ剥離し、剥離幅を測定した。この測定結果を以下の評価基準で評価した。「○」が合格レベルである
○：　剥離幅が片側２．５ｍｍ未満
×：　剥離幅が片側２．５ｍｍ以上
　＜加工性＞
　加工性は、供試材から圧延方向に対して９０°方向にＪＩＳ５号引張試験片を採取し、ＪＩＳ　Ｚ　２２４１の規定に準拠してクロスヘッド速度１０ｍｍ／ｍｉｎの条件で引張試験を行い、引張り強度ＴＳ（ＭＰａ）と伸びＥｌ（％）を測定し、ＴＳ×Ｅｌ≧２００００のものを良好、ＴＳ×Ｅｌ＜２００００のものを不良とした。 Next, the test sample was immersed in a 5% NaCl aqueous solution (55 ° C.) for 240 hours, then taken out, washed with water and dried, and then the cross-cut portion was peeled off to measure the peeling width. The measurement results were evaluated according to the following evaluation criteria. "○" is an acceptable level ○: Peeling width is less than 2.5 mm on one side ×: Peeling width is 2.5 mm or more on one side <Workability>
As for workability, a JIS No. 5 tensile test piece was sampled from the test material in the direction of 90 ° with respect to the rolling direction, and a tensile test was conducted under the condition of a crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241. The strength TS (MPa) and the elongation El (%) were measured, and those with TS × El ≧ 20000 were good and those with TS × El <20000 were bad.
　＜鋼板表層１００μｍまでの領域における内部酸化量＞
　内部酸化量は、「インパルス炉溶融－赤外線吸収法」により測定する。ただし、素材（すなわち焼鈍を施す前の鋼板）に含まれる酸素量を差し引く必要がある。本発明では、連続焼鈍後の高強度鋼板の両面からそれぞれ１００μｍ以上研磨した位置での鋼中酸素濃度を測定し、その測定値を素材に含まれる酸素量ＯＨとした。また、連続焼鈍後の高強度鋼板表面の板厚方向全体での鋼中酸素濃度を測定して、その測定値を内部酸化後の酸素量ＯＩとした。このようにして得られた高強度鋼板の内部酸化後の酸素量ＯＩと、素材に含まれる酸素量ＯＨとを用いて、ＯＩとＯＨの差（＝ＯＩ－ＯＨ）を算出し、さらに片面単位面積（すなわち１m２）当たりの量に換算した値（ｇ／ｍ２）を内部酸化量とした。 <Internal oxidation amount in the region of steel sheet surface layer up to 100 μm>
The amount of internal oxidation is measured by “impulse furnace melting-infrared absorption method”. However, it is necessary to subtract the amount of oxygen contained in the material (that is, the steel plate before annealing). In the present invention, the oxygen concentration in the steel was measured at positions polished by 100 μm or more from both surfaces of the high-strength steel sheet after continuous annealing, and the measured value was defined as the amount of oxygen OH contained in the material. Moreover, the oxygen concentration in the steel in the whole plate | board thickness direction of the high strength steel plate surface after continuous annealing was measured, and the measured value was made into the oxygen amount OI after internal oxidation. The difference between OI and OH (= OI−OH) is calculated using the oxygen amount OI after internal oxidation of the high-strength steel plate thus obtained and the oxygen amount OH contained in the material, and further, single-sided unit area (i.e. 1 m 2) value converted into the amount per (g / m 2) as an internal oxide amount.
　＜表面から１０μｍまでの領域における内部酸化物の有無の評価、粒界から１μｍ以内の粒内におけるＭｎ酸化物の有無の評価＞
　ＳＥＭまたはＴＥＭ観察により、倍率２００００倍の条件で無作為に５視野を観察し、必要に応じてＥＤＸ分析することで、内部酸化物の有無を確認した。なお、内部酸化物「無」は、上記５視野内にて観測数ゼロを意味する。 <Evaluation of presence / absence of internal oxide in region from surface to 10 μm, evaluation of presence / absence of Mn oxide in grains within 1 μm from grain boundary>
By SEM or TEM observation, five visual fields were randomly observed under the condition of a magnification of 20000, and the presence or absence of an internal oxide was confirmed by EDX analysis as necessary. The internal oxide “none” means that the number of observations is zero within the above five fields of view.
　以上により得られた結果を製造条件と併せて表２、３に示す。 The results obtained above are shown in Tables 2 and 3 together with the manufacturing conditions.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
　表２、３から明らかなように、本発明法で製造された高強度鋼板は、Ｓｉ、Ｍｎ等の易酸化性元素を多量に含有する高強度鋼板であっても、化成処理性、耐食性、加工性に優れることがわかる。一方、比較例では、化成処理性、耐食性、加工性、のいずれか一つ以上が劣る。 As is apparent from Tables 2 and 3, the high-strength steel plate produced by the method of the present invention is a high-strength steel plate containing a large amount of oxidizable elements such as Si and Mn. It turns out that it is excellent in workability. On the other hand, in a comparative example, any one or more of chemical conversion treatment property, corrosion resistance, and workability is inferior.
　本発明の高強度鋼板は、化成処理性、耐食性、加工性に優れ、自動車の車体そのものを軽量化かつ高強度化するための表面処理鋼板として利用することができる。また、自動車以外にも、素材鋼板に防錆性を付与した表面処理鋼板として、家電、建材の分野等、広範な分野で適用できる。 The high-strength steel sheet of the present invention has excellent chemical conversion properties, corrosion resistance, and workability, and can be used as a surface-treated steel sheet for reducing the weight and strength of an automobile body. In addition to automobiles, the steel sheet can be applied in a wide range of fields such as home appliances and building materials as a surface-treated steel sheet provided with rust prevention properties.

Claims (4)

  1. 　質量％で、Ｃ：０．０３～０．３５％、Ｓｉ：０．０１～０．５０％、Ｍｎ：３．６～８．０％、Ａｌ：０．０１～１．０％、Ｐ：０．１０％以下、Ｓ：０．０１０％以下を含有し、残部がＦｅおよび不可避的不純物からなる鋼板を連続焼鈍する際に、
    　前記連続焼鈍の加熱過程において、加熱炉内温度：５００℃以上Ａ℃以下（Ａ：５２０≦Ａ＜６００から選択される任意の値）の温度域では、雰囲気の水素濃度：２０ｖｏｌ％以上の条件で前記鋼板を加熱し、加熱炉内温度：Ａ℃超えＢ℃以下（Ｂ：５５０≦Ｂ≦７５０から選択される任意の値）の温度域では、雰囲気の露点：－１０℃以上の条件で前記鋼板を加熱し、
    　前記連続焼鈍において鋼板最高到達温度が６００℃以上７５０℃以下とし、
    　前記連続焼鈍において鋼板温度が６００℃以上７５０℃以下の温度域の鋼板通過時間を３０秒以上１０分以下とすることを特徴とする化成処理性に優れた高強度鋼板の製造方法。     In mass%, C: 0.03-0.35%, Si: 0.01-0.50%, Mn: 3.6-8.0%, Al: 0.01-1.0%, P: When continuously annealing a steel sheet containing 0.10% or less, S: 0.010% or less, the balance being Fe and inevitable impurities,
    In the heating process of the continuous annealing, in the temperature range of the heating furnace temperature: 500 ° C. or more and A ° C. or less (A: any value selected from 520 ≦ A <600), the hydrogen concentration in the atmosphere: 20 vol% or more In the temperature range of heating furnace temperature: A ° C. to B ° C. (B: any value selected from 550 ≦ B ≦ 750), the dew point of the atmosphere is −10 ° C. or higher. Heating the steel sheet;
    In the continuous annealing, the maximum steel sheet temperature is 600 ° C. or more and 750 ° C. or less,
    A method for producing a high-strength steel sheet excellent in chemical conversion property, characterized in that the steel sheet passage time in the temperature range of 600 ° C. to 750 ° C. in the continuous annealing is 30 seconds to 10 minutes.
  2. 　前記鋼板は、成分組成として、質量％で、さらに、Ｂ：０．００１～０．００５％、Ｎｂ：０．００５～０．０５％、Ｔｉ：０．００５～０．０５％、Ｃｒ：０．００１～１．０％、Ｍｏ：０．０５～１．０％、Ｃｕ：０．０５～１．０％、Ｎｉ：０．０５～１．０％、Ｓｎ：０．００１～０．２０％、Ｓｂ：０．００１～０．２０％、Ｔａ：０．００１～０．１０％、Ｗ：０．００１～０．１０％およびＶ：０．００１～０．１０％の中から選ばれる１種以上の元素を含有することを特徴とする請求項１に記載の高強度鋼板の製造方法。 The steel sheet has a component composition in mass%, B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0.05%, Cr: 0 0.001 to 1.0%, Mo: 0.05 to 1.0%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Sn: 0.001 to 0.20 %, Sb: 0.001 to 0.20%, Ta: 0.001 to 0.10%, W: 0.001 to 0.10% and V: 0.001 to 0.10% The method for producing a high-strength steel sheet according to claim 1, comprising at least one element.
  3. 　前記連続焼鈍を行った後、硫酸を含む水溶液で電解酸洗を行うことを特徴とする請求項１または２に記載の高強度鋼板の製造方法。 The method for producing a high-strength steel sheet according to claim 1 or 2, wherein after the continuous annealing, electrolytic pickling is performed with an aqueous solution containing sulfuric acid.
  4. 　請求項１～３のいずれかに記載の高強度鋼板の製造方法で製造された高強度鋼板であって、
    　鋼板表面から１００μｍ以内の領域に、Ｆｅ、Ｓｉ、Ｍｎ、Ａｌ、Ｐ、Ｂ、Ｎｂ、Ｔｉ、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂ、Ｔａ、ＷおよびＶの中から選ばれる少なくとも１種以上の酸化物を、合計で片面あたり０．０１０～０．１００ｇ／ｍ2有し、
    　さらに、鋼板表面から１０μｍ以内の領域における、鋼板結晶粒界から１μｍ以内の粒内に、Ｍｎを含む酸化物を有することを特徴とする高強度鋼板。
          A high-strength steel plate produced by the method for producing a high-strength steel plate according to any one of claims 1 to 3,
    At least one selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V in a region within 100 μm from the steel plate surface Having a total of 0.010 to 0.100 g / m 2 per side of the above oxides,
    Furthermore, in the area | region within 10 micrometers from a steel plate surface, it has the oxide containing Mn in the grain within 1 micrometer from a steel plate crystal grain boundary, The high strength steel plate characterized by the above-mentioned.
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