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
  • 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
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • 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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • 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
  • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing

Definitions

• the present invention relates to a high-strength steel sheet having excellent chemical conversion property and excellent corrosion resistance after electrodeposition coating even when the content of Si or Mn is large, and a method for producing the same.
• Si and Mn are oxidized even when annealing is performed in a reducing N 2 + H 2 gas atmosphere in which Fe does not oxidize (reducing Fe oxide) and is selected as the outermost layer of the steel sheet.
• a surface oxide containing Si or Mn for example, SiO 2 , MnO, etc., hereinafter may be referred to as a selective surface oxide
• ske fine region
• Patent Document 1 discloses a method of forming an iron coating layer of 20 to 1500 mg / m 2 on a steel sheet using an electroplating method. Yes.
• this method there is a problem that the cost is increased due to the additional steps required for the electroplating equipment.
• Patent Document 2 the phosphate processability is improved by prescribing the Mn / Si ratio.
• Patent Document 3 the phosphate processability is improved by adding Ni.
• the effects of Patent Document 2 and Patent Document 3 depend on the contents of Si and Mn in the steel sheet, and it is considered that further improvement is necessary for the steel sheet having a high Si and Mn content.
• Patent Document 4 by setting the dew point during annealing to ⁇ 25 to 0 ° C., an internal oxide layer made of an oxide containing Si is formed within a depth of 1 ⁇ m from the surface of the steel sheet substrate, and the steel sheet surface length A method is disclosed in which the proportion of the Si-containing oxide in 10 ⁇ m is 80% or less.
• the area for controlling the dew point since the area for controlling the dew point is premised on the entire inside of the furnace, the controllability of the dew point is difficult and stable operation is difficult.
• annealing was performed under unstable dew point control, variations were observed in the distribution of internal oxides formed on the steel sheet.
• 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 steel sheet under an iron oxidizing atmosphere at a steel sheet temperature of 400 ° C.
• a method is described in which an oxide film is formed on the surface and then the oxide film on the steel sheet surface is reduced in an iron reducing atmosphere.
• annealing is performed in an N 2 + H 2 gas atmosphere that reduces Fe oxide.
• selective surface oxidation that degrades chemical conversion properties is suppressed, and an Fe oxide layer is formed on the outermost surface.
• Patent Document 6 does not specifically describe the heating temperature of an open flame burner, but when it contains a large amount of Si (approximately 0.6% or more), the amount of oxidation of Si that is easier to oxidize than Fe. As a result, the oxidation of Fe is suppressed, and the oxidation of Fe itself becomes too small. As a result, the formation of the surface Fe reduction layer after reduction may be insufficient, or SiO 2 may be present on the steel sheet surface after reduction, resulting in the occurrence of a conversion coating.
• Si approximately 0.6% or more
• JP-A-5-320952 JP 2004-323969 A Japanese Patent Laid-Open No. 6-1000096 JP 2003-113441 A 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 chemical conversion treatment properties and excellent corrosion resistance after electrodeposition coating, and a method for producing the same The purpose is to provide.
• the inside of the steel sheet was excessively oxidized by simply raising the steam partial pressure or oxygen partial pressure in the entire annealing furnace to raise the dew point or oxygen concentration. For this reason, there are various problems such as dew point or oxidation controllability, uneven chemical conversion treatment, and deterioration of corrosion resistance after electrodeposition coating. Therefore, the present inventors have studied a method for solving the problem by a new method not confined to the conventional idea. As a result, high strength with excellent chemical conversion treatment and corrosion resistance after electrodeposition coating by controlling the structure and structure of the steel sheet surface layer that may become the starting point of corrosion resistance degradation after electrodeposition coating It has been found that a steel plate can be obtained.
• the maximum reached temperature of the steel plate in the annealing furnace is 600 ° C. or more and 750 ° C. or less
• the steel plate passage time in the temperature range of 600 ° C. or more and 750 ° C. or less is 30 seconds or more and 10 minutes or less.
• Continuous annealing is performed by controlling the dew point to be -10 ° C or higher. By performing such treatment, selective surface oxidation can be suppressed, surface concentration can be suppressed, and a high-strength steel sheet excellent in chemical conversion treatment properties and corrosion resistance after electrodeposition coating can be obtained.
• having excellent chemical conversion property means having a non-scaling and uneven appearance after chemical conversion treatment.
• the high-strength steel plate obtained by the above method is selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, and Ni on the steel sheet surface layer portion within 100 ⁇ m from the steel plate surface.
• An oxide of seeds or more is formed in an amount of 0.010 to 0.100 g / m 2 per side, and in a region within 10 ⁇ m from the steel sheet surface, an oxide containing Mn in grains within 1 ⁇ m from the grain boundary of the steel sheet. It becomes an existing organization and structure. As a result, it is possible to prevent deterioration of the corrosion resistance after electrodeposition coating, and the chemical conversion processability is excellent.
• the present invention is based on the above findings, and features are as follows.
• a method for producing a high-strength steel sheet characterized in that the steel sheet passage time in the temperature range of 600 ° C. or higher and 750 ° C. or lower is 30 seconds to 10 minutes and the dew point in the atmosphere is ⁇ 10 ° C. or higher.
• 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. -0.05%, Cr: 0.001-1.0%, Mo: 0.05-1.0%, Cu: 0.05-1.0%, Ni: 0.05-1.0%
• a method for producing a high-strength steel sheet comprising one or more elements selected from the inside.
• One or more oxides selected from Mo, Cu, and Ni are formed at 0.010 to 0.100 g / m 2 per side, and from the grain boundary of the steel sheet in a region within 10 ⁇ m from the steel sheet surface.
• a high-strength steel sheet characterized in that an oxide containing Mn is present in grains within 1 ⁇ m.
• the high strength steel plate is a steel plate having a tensile strength (TS) of 590 MPa or more.
• the high-strength steel sheet of the present invention includes both cold-rolled steel sheets and hot-rolled steel sheets.
• the unit of the content of each element of the steel component composition is “mass%”, and hereinafter, simply indicated by “%” unless otherwise specified.
• annealing atmosphere conditions that determine the structure of the steel sheet surface, which is the most important requirement in the present invention, will be described.
• the steel plate maximum temperature in the annealing furnace is 600 ° C. or higher and 750 ° C. or lower
• the steel plate passing time in the temperature range of 600 ° C. or higher and 750 ° C. or lower is 30 seconds or more and 10 minutes or less
• the dew point in the atmosphere Is controlled to be ⁇ 10 ° C.
• an appropriate amount of an oxide of an easily oxidizable element (Si, Mn, etc.) (hereinafter referred to as internal oxidation) is present within the surface of the steel sheet within 100 ⁇ m. It is possible to suppress selective surface oxidation (hereinafter referred to as surface concentration) in the steel sheet surface layer such as Si and Mn in the steel, which deteriorates the chemical conversion property.
• the reason why the maximum steel sheet temperature in the annealing furnace is 600 ° C. or higher and 750 ° C. or lower is as follows. In the temperature range below 600 ° C., surface concentration and internal oxidation to such an extent that deterioration of chemical conversion properties becomes a problem do not occur. Moreover, if it is less than 600 degreeC, a favorable material cannot be obtained. Therefore, the temperature range in which the effects of the present invention are manifested is 600 ° C. or higher. On the other hand, in the temperature range exceeding 750 ° C., the surface concentration becomes remarkable and the chemical conversion treatment property deteriorates. Furthermore, from the viewpoint of the material, the effect of balance between strength and ductility is saturated in a temperature range exceeding 750 ° C. From the above, the maximum temperature reached by the steel sheet is set to 600 ° C. or higher and 750 ° C. or lower.
• the reason why the steel plate passage time in the temperature range of 600 ° C. or more and 750 ° C. or less is 30 seconds or more and 10 minutes or less is as follows. If it is less than 30 seconds, the target material (TS, El) cannot be obtained. On the other hand, if it exceeds 10 minutes, the effect of balance between strength and ductility is saturated.
• the reason why the dew point in the atmosphere in the temperature range of 600 ° C. to 750 ° C. is set to ⁇ 10 ° C. or higher is as follows. By increasing the dew point, it is possible to increase the O 2 potential resulting from the decomposition of H 2 O and promote internal oxidation.
• a region (hereinafter referred to as a deficient layer) in which the amount of solid oxidizable elements (Si, Mn, etc.) within the surface of the steel sheet within 10 ⁇ m is reduced is formed. Suppresses surface diffusion of easily oxidizable elements.
• the dew point in the temperature range of 600 ° C. or higher and 750 ° C. or lower needs to be ⁇ 10 ° C. or higher. When the temperature is lower than ⁇ 10 ° C., sufficient internal oxidation is not formed.
• the upper limit of the dew point is not particularly defined, but if it exceeds 60 ° C., the oxidation amount of Fe increases, and there is a concern about deterioration in the annealing furnace or roll, so 60 ° C. or less is desirable.
• C 0.03-0.35%
• C improves workability by forming martensite or the like as a steel structure.
• 0.03% or more is necessary.
• the C content is 0.03% or more and 0.35% or less.
• Si 0.01 to 0.50% Si is an element effective for strengthening steel to obtain a good material, but it is an easily oxidizable element, which is disadvantageous for chemical conversion treatment and should be avoided as much as possible.
• about 0.01% is inevitably contained in steel, and in order to reduce it below this, the cost increases. Therefore, 0.01% is set as the lower limit.
• the Si amount is set to 0.01% or more and 0.50% or less.
• 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, it is necessary to contain 3.6% or more. On the other hand, if it exceeds 8.0%, it will be difficult to ensure chemical conversion treatment and to ensure 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. However, when the content is less than 0.01%, the object is not achieved. The effect of deoxidation of molten steel is obtained at 0.01% or more. On the other hand, if it exceeds 1.0%, the cost increases. Furthermore, the surface concentration of Al increases and it becomes difficult to improve chemical conversion properties. Therefore, the Al content is set to 0.01% to 1.0%.
• P ⁇ 0.10% P is one of the elements inevitably contained, and in order to make it less than 0.005%, there is a concern about an increase in cost, so 0.005% or more is desirable.
• P exceeds 0.10% weldability deteriorates.
• the chemical conversion processability deteriorates, and even with the present invention, it is difficult to improve the chemical conversion processability. Therefore, the P content is 0.10% or less.
• the lower limit is preferably 0.005%.
• S ⁇ 0.010% S is one of the elements inevitably contained. No lower limit is specified. However, if contained in a large amount, the weldability and corrosion resistance deteriorate, so the content is made 0.010% or less.
• B 0.001 to 0.005%
• Nb 0.005 to 0.05%
• Ti 0.005 to 0.05%
• Cr 0.001
• B 0.001 to 0.005%
• B amount shall be 0.001% or more and 0.005% or less.
• Nb 0.005 to 0.05% If Nb is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if it exceeds 0.05%, the cost increases. Therefore, when it contains, Nb amount shall be 0.005% or more and 0.05% or less.
• Ti 0.005 to 0.05% If Ti is less than 0.005%, the effect of adjusting the strength is difficult to obtain. On the other hand, if it exceeds 0.05%, chemical conversion processability is deteriorated. Therefore, when it contains, Ti amount shall be 0.005% or more and 0.05% or less.
• Cr 0.001 to 1.0%
• Cr 0.001 to 1.0%
• Mo 0.05 to 1.0% If Mo is less than 0.05%, the effect of adjusting the strength is difficult to obtain. On the other hand, if it exceeds 1.0%, cost increases. Therefore, when contained, the Mo content is 0.05% or more and 1.0% or less.
• Cu 0.05 to 1.0% If Cu is less than 0.05%, the effect of promoting the formation of the residual ⁇ phase is difficult to obtain. On the other hand, if it exceeds 1.0%, cost increases. Therefore, when contained, the Cu content is 0.05% or more and 1.0% or less.
• Ni 0.05 to 1.0% If Ni is less than 0.05%, the effect of promoting the formation of residual ⁇ phase is difficult to obtain. On the other hand, if it exceeds 1.0%, cost increases. Therefore, when it contains, Ni amount shall be 0.05% or more and 1.0% or less.
• the remainder other than the above is Fe and inevitable impurities.
• the steel having the above chemical components is hot-rolled and then cold-rolled into a steel plate, and then annealed in a continuous annealing facility. Furthermore, it is preferable to perform electrolytic pickling in an aqueous solution containing sulfuric acid. Next, chemical conversion treatment is performed. In the above, after the hot rolling, annealing may be performed as it is without performing cold rolling.
• the maximum reached temperature of the steel plate in the annealing furnace is 600 ° C. or higher and 750 ° C. or lower, and the steel plate passage time in the temperature range of 600 ° C. or higher and 750 ° C. or lower is 30 seconds or longer and 10 ° C.
• the dew point in the atmosphere should be -10 ° C or higher. This is the most important requirement in the present invention.
• the oxygen potential is increased and the easily oxidizable elements such as Si and Mn are internally oxidized in advance immediately before the chemical conversion treatment, and Si and Mn in the steel sheet surface layer portion. The activity of is reduced. As a result, external oxidation of these elements is suppressed, and as a result, chemical conversion treatment performance is improved.
• Hot rolling Usually, it can be performed on the conditions performed.
• the pickling treatment is preferable to perform a pickling treatment after hot pickling.
• the black scale formed on the surface in the pickling process is removed, and then cold-rolled.
• the pickling conditions are not particularly limited.
• Cold rolling is preferably performed at a rolling reduction of 40% to 80%. If the rolling reduction is less than 40%, the recrystallization temperature is lowered, and the mechanical characteristics are likely to deteriorate. On the other hand, if the rolling reduction exceeds 80%, the steel sheet is a high-strength steel plate, so that not only the rolling cost is increased, but also the surface concentration during annealing is increased, so that the chemical conversion property may be deteriorated.
• a cold-rolled steel plate or a hot-rolled steel plate is continuously annealed and then subjected to chemical conversion treatment.
• a heating process is performed in which the steel sheet is heated to a predetermined temperature in the preceding heating zone, and a soaking process is performed in which the temperature is maintained at a predetermined temperature for a predetermined time in the subsequent soaking zone.
• a cooling process is performed.
• the maximum steel sheet temperature in the annealing furnace is 600 ° C. or more and 750 ° C. or less
• the steel plate passage time in the temperature range of 600 ° C. or more and 750 ° C. or less is 30 seconds or more and 10 minutes or less, in the atmosphere
• the dew point is -10 ° C or higher.
• the dew point in the annealing furnace atmosphere other than the temperature range of 600 ° C. or higher and 750 ° C. or lower is not particularly limited. Preferably, it is ⁇ 50 ° C. to ⁇ 10 ° C.
• the gas component in the annealing furnace consists of nitrogen, hydrogen and unavoidable impurities.
• the upper limit is not particularly specified, but if it exceeds 50 vol%, the cost increases and the effect is saturated. Therefore, the hydrogen concentration is preferably 1 vol% or more and 50 vol% or less. Furthermore, 5 vol% or more and 30 vol% or less are desirable.
• the balance consists of nitrogen and unavoidable impurity gases. Other gas components such as H 2 O, CO 2 and CO may be contained as long as the effects of the present invention are not diminished.
• the surface enrichment amount of Si and Mn increases in proportion to the amount of Si and Mn in the steel.
• the amount of surface concentration decreases as the oxygen potential in the atmosphere increases. Therefore, when the amount of Si and Mn in steel is large, it is necessary to increase the oxygen potential in the atmosphere by increasing the dew point.
• tempering is preferably performed at a temperature of 150 to 400 ° C. If it is less than 150 ° C., the elongation tends to deteriorate. If it exceeds 400 ° C., the hardness tends to decrease.
• the pickling solution used for electrolytic pickling is not particularly limited. However, nitric acid and hydrofluoric acid are not preferred 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, it is preferable to use sulfuric acid for the pickling solution 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 that the bath voltage during electrolysis will rise and the power load may become large. 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 conditions of the electrolytic pickling are not particularly limited, but in order to efficiently remove the inevitably surface-enriched Si and Mn oxides formed after annealing, an alternating electrolysis with a current density of 1 A / dm 2 or more is used. It is desirable.
• the reason for the alternating electrolysis is as follows. The pickling effect is small when the steel plate is held on the cathode. On the contrary, if the steel plate is held on the anode, Fe eluting at the time of electrolysis accumulates in the pickling solution, and the Fe concentration in the pickling solution increases. Resulting in.
• 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 may be difficult to maintain the temperature below 40 ° C. Moreover, it is not preferable that temperature exceeds 70 degreeC from a durable viewpoint of the lining of an electrolytic cell. Moreover, since the pickling effect will become small when it is less than 40 degreeC, 40 degreeC or more is preferable.
• the high-strength steel sheet of the present invention is obtained, and the structure of the steel sheet surface is characterized as follows. On the steel sheet surface layer portion within 100 ⁇ m from the steel sheet surface, Fe, Si, Mn, Al, P, and further one or more oxides selected from B, Nb, Ti, Cr, Mo, Cu, and Ni are contained. A total of 0.010 to 0.100 g / m 2 is formed per side. Further, in the region from the steel sheet surface to 10 ⁇ m, an oxide containing Mn exists in the grains within 1 ⁇ m from the grain boundary.
• the dew point control is performed as described above in order to increase the oxygen potential in the annealing process in order to ensure chemical conversion treatment.
• easily oxidizable elements such as Si and Mn are internally oxidized in advance immediately before the chemical conversion treatment, and the activities of Si and Mn in the steel sheet surface layer portion are lowered.
• this improvement effect is one type selected from Fe, Si, Mn, Al, P, and further B, Nb, Ti, Cr, Mo, Cu, Ni on the steel sheet surface layer portion within 100 ⁇ m from the steel sheet surface.
• the above oxide is present at 0.010 g / m 2 or more per side.
• this effect is saturated even if it exceeds 0.100 g / m 2 , the upper limit is made 0.100 g / m 2 .
• the maximum steel plate temperature in the annealing furnace is 600 ° C. or higher and 750 ° C. or lower
• the steel plate passage time in the temperature range of 600 ° C. or higher and 750 ° C. or lower is 30 seconds or longer and 10 minutes.
• an oxide containing Mn is present in grains within 1 ⁇ m from the steel plate crystal grain boundary.
• the presence of oxide in the grains reduces the amount of solid solution Si and Mn in the grains near the oxide. As a result, concentration on the surface due to intragranular diffusion of Si and Mn can be suppressed.
• the structure of the steel plate surface of the high-strength steel plate obtained by the production method of the present invention is as described above.
• the oxide grows in a region exceeding 100 ⁇ m from the steel plate surface.
• the region exceeding 10 ⁇ m from the steel sheet surface there is no problem even if an oxide containing Mn is present in the grain of 1 ⁇ m or more from the grain boundary.
• the steel plate structure on which the oxide containing Mn grows is preferably a soft and rich ferrite phase.
• the cold-rolled steel sheet obtained above was charged into a continuous annealing facility.
• the annealing equipment as shown in Tables 2 and 3, after the steel sheet in the annealing furnace is controlled by controlling the dew point, the steel plate passage time, and the maximum steel plate temperature in the temperature range of 600 °C to 750 °C, after annealing Then, water quenching was performed and tempering was performed at 300 ° C. ⁇ 140 s. Then, it pickled by being immersed in sulfuric acid aqueous solution of 40 mass% and 5 mass%.
• test material Part of the sample was subjected to electrolytic pickling by alternating electrolysis in which the test material was in the order of 3 seconds each in the order of anode and cathode under the current density conditions shown in Table 2 to obtain the test material.
• the dew point in the annealing furnace other than the region where the dew point was controlled was ⁇ 35 ° C.
• the atmospheric gas components were nitrogen gas, hydrogen gas, and inevitable impurity gas, and the dew point was controlled by absorbing and removing moisture in the atmosphere.
• the hydrogen concentration in the atmosphere was 10 vol%.
• TS and El were measured for the specimens obtained as described above.
• chemical conversion properties and corrosion resistance after electrodeposition coating were investigated.
• the amount of oxide (internal oxidation amount) present in the steel sheet surface layer up to 100 ⁇ m immediately below the steel sheet surface layer was measured. The measurement method and evaluation criteria are shown below.
• a degreasing liquid Fine Cleaner registered trademark
• surface conditioning solution preparen Z registered trademark
• ⁇ is 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 is obtained by taking a JIS No. 5 tensile test piece from the sample in a 90 ° direction with respect to the rolling direction.
• a tensile test is performed at a constant crosshead speed of 10 mm / min, and tensile strength (TS / MPa) and elongation (El /%) are measured. Good and TS ⁇ El ⁇ 24000 were regarded as bad.
• the amount of internal oxidation in the region up to 100 ⁇ m of the steel sheet surface layer is measured by the “impulse furnace melting-infrared absorption method”.
• the surface layer portions on both surfaces of the high-strength steel plate after continuous annealing are polished by 100 ⁇ m or more in the steel.
• Measure the oxygen concentration set the measured value as the amount of oxygen OH contained in the material, measure the oxygen concentration in the steel in the entire thickness direction of the high-strength steel sheet after continuous annealing, and measure the measured value internally.
• the subsequent oxygen amount OI was used.
• the high-strength steel sheet produced by the method of the present invention is a high-strength steel sheet containing a large amount of oxidizable elements such as Si and Mn, but it is chemically treated. It can be seen that it has excellent corrosion resistance and workability after electrodeposition coating. On the other hand, in the comparative example, any one or more of chemical conversion property, corrosion resistance after electrodeposition coating, 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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• 2013
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