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
  • 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
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
  • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
  • C21D8/0473—Final recrystallisation 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
  • C21D9/46—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
  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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
• • 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
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
  • C22C38/60—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
  • 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

Definitions

• the present invention relates to a high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after electrodeposition coating even when the content of Si or Mn is high.
• the present invention also relates to a method for manufacturing the high-strength steel sheet.
• steel sheets for automobiles are used after being coated.
• the steel sheets are subjected to a chemical conversion treatment, called a phosphate treatment, as a pretreatment for coating.
• the chemical conversion treatment of a steel sheet is one of important treatments for ensuring corrosion resistance after coating.
• Si and Mn are effective in increasing the strength and ductility of a steel sheet.
• Si and Mn are oxidized during continuous annealing to form surface oxides (such as SiO 2 and MnO, hereinafter referred to as selective surface oxides) selectively containing Si or Mn in the outermost surface layer of the steel sheet. Since the selective surface oxides inhibit the formation reaction of a chemical conversion coating during a chemical conversion treatment, a fine region (hereinafter referred to as a lack of hiding in some cases) where no chemical conversion coating is produced is formed, leading to a reduction in chemical conversion treatability.
• Patent Literature 1 discloses a method for forming a 20-1,500 mg/m 2 iron coating layer on a steel sheet by an electroplating process as a conventional technique for improving the chemical conversion treatability of a steel sheet containing Si and Mn.
• this method has a problem that an electroplating line is necessary and therefore the increase in number of steps causes an increase in cost.
• Phosphate treatability is enhanced by regulating the Mn/Si ratio as described in Patent Literature 2 or by adding Ni as described in Patent Literature 3.
• the effect depends on the content of Si or Mn in a steel sheet; hence, further improvements are probably necessary for steel sheets with a high Si or Mn content.
• Patent Literature 4 discloses a method in which an internal oxidation layer made of an Si-containing oxide is formed within a depth of 1 ⁇ m from the surface of an underlayer of a steel sheet by controlling the dew point at -25°C to 0°C during annealing such that the proportion of the Si-containing oxide in a length of 10 ⁇ m in a surface of the steel sheet is 80% or less.
• the control of the dew point is difficult and stable operation is also difficult because an area where the dew point is controlled is based on the whole inside of a furnace.
• Patent Literature 5 discloses a method in which an oxide film is formed on a surface of a steel sheet in an oxidizing atmosphere by increasing the temperature of the steel sheet to 350°C to 650°C and the steel sheet is heated to a recrystallization temperature in a reducing atmosphere and is then cooled.
• the thickness of the oxide film formed on the steel sheet surface is uneven depending on an oxidation process, oxidation does not occur sufficiently, or the oxide film is too thick so that the oxide film remains or peels off during the subsequent annealing in the reducing atmosphere and therefore surface properties are poor in some cases.
• a technique for performing oxidation in air is described.
• oxidation in air has a problem that, for example, thick oxides are produced and subsequent reduction is difficult or a reducing atmosphere with a high hydrogen concentration is necessary.
• Patent Literature 6 discloses a method in which an oxide film is formed on a surface of a cold-rolled steel sheet containing 0.1% or more Si and/or 1.0% or more Mn on a mass basis at a steel sheet temperature of 400°C or higher in an atmosphere oxidizing iron, followed by reducing the oxide film on the steel sheet surface in an atmosphere reducing iron.
• Fe in the steel sheet surface is oxidized at 400°C or higher using a direct fired burner with an air ratio of 0.93 to 1.10 and the steel sheet is then annealed in an N 2 + H 2 gas atmosphere reducing Fe oxides, whereby selective surface oxidation which deteriorates chemical conversion treatability is suppressed and an Fe oxidation layer is formed on the outermost surface.
• Patent Literature 6 does not particularly describe the heating temperature of the direct fired burner.
• a large amount (about 0.6% or more) of Si is contained, the amount of oxidized Si, which is more likely to be oxidized than Fe, is large; hence, the oxidation of Fe is suppressed or is slight.
• the formation of a surface Fe reduction layer after reduction is insufficient or a lack of hiding is caused in a chemical conversion coating in some cases because SiO 2 is present on the steel sheet surface after reduction.
• the present invention has been made in view of the above circumstances. It is an object of the present invention to provide a high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after electrodeposition coating even when the content of Si or Mn is high. It is an object of the present invention to provide a method for manufacturing the high-strength steel sheet.
• the steel sheet is annealed in such a manner that the steel sheet is heated at a heating rate of 7 °C/s or more in a temperature range corresponding to an annealing furnace inside temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600), the maximum end-point temperature of the steel sheet in an annealing furnace is controlled to be 600°C to 700°C, the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600°C to 700°C is controlled to be 30 seconds to 10 minutes, and the concentration of hydrogen in an atmosphere is controlled to be 20% by volume or more in the heating step. Subsequently, a chemical conversion treatment is performed.
• the heating rate in the temperature range corresponding to an annealing furnace inside temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600) is 7 °C/s or more
• the maximum end-point temperature of the steel sheet in the annealing furnace is 600°C to 700°C
• the concentration of hydrogen in the atmosphere in the temperature range corresponding to a steel sheet temperature of 600°C to 700°C is 20% by volume or more in the heating step
• a high-strength steel sheet obtained by the above method includes a surface portion within 100 ⁇ m from a surface of the steel sheet.
• the formation of the following oxide is suppressed: an oxide of one or more selected from among Fe, Si, Mn, Al, P, and further B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V.
• the amount of the formed oxide per side is limited to less than 0.030 g/m 2 in total. This leads to excellence in chemical conversion treatability and a significant increase in corrosion resistance after electrodeposition coating.
• the present invention is based on the above finding and is featured as described below.
• a high-strength steel sheet according to the present invention has a tensile strength TS of 590 MPa or more.
• the term "high-strength steel sheet” as used herein includes both a cold-rolled steel sheet and a hot-rolled steel sheet.
• a high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after electrodeposition coating is obtained even when the content of Si or Mn is high.
• annealing conditions which are the most important requirements for the present invention and which determine the surface structure of a steel sheet are described.
• the internal oxidation of a steel sheet surface layer that may possibly be the origin of corrosion needs to be minimized.
• Chemical conversion treatability can be enhanced by promoting the internal oxidation of Si and Mn. However, this causes the deterioration of corrosion resistance as described above. Therefore, it is necessary that good chemical conversion treatability is maintained by a method other than promoting the internal oxidation of Si and Mn and corrosion resistance is enhanced by suppressing internal oxidation.
• the potential of oxygen is reduced in an annealing step for the purpose of ensuring chemical conversion treatability and the activity of Si, Mn, and the like, which are oxidizable elements, in a base metal surface portion is reduced.
• This suppresses the external oxidation of these elements, resulting in improving chemical conversion treatability.
• the formation of internal oxides in a steel sheet surface portion is suppressed and therefore corrosion resistance after electrodeposition coating is improved.
• Such an effect is obtained in such a manner that when annealing is performed in a continuous annealing line, the heating rate in a temperature range corresponding to an annealing furnace inside temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600) is controlled to be 7 °C/s or more, the maximum end-point temperature of a steel sheet in an annealing furnace is controlled to be 600°C to 700°C, the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600°C to 700°C is controlled to be 30 seconds to 10 minutes, and the concentration of hydrogen in an atmosphere is controlled to be 20% by volume or more in a heating step.
• the heating rate in a temperature range corresponding to an annealing furnace inside temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600) is controlled to be 7 °C/s or more
• the maximum end-point temperature of a steel sheet in an annealing furnace is controlled to be 600°C to 700°C
• the temperature range in which the heating rate is controlled is 450°C or higher is as described below.
• a temperature range lower than 450°C surface oxidation and internal oxidation do not occur to such an extent that the occurrence of a lack of hiding and unevenness, the deterioration of corrosion resistance, and the like are problematic.
• the temperature range, in which an effect of the present invention is exhibited is 450°C or higher.
• the reason why the temperature range is A°C (where 500 ⁇ A ⁇ 600) or lower is as described below.
• the time for which the heating rate is controlled to be 7 °C/s or more is short and therefore an effect of the present invention is small.
• the effect of suppressing surface oxidation is insufficient. Therefore, A is 500 or more.
• the case of higher than 600°C is disadvantageous from the viewpoints of the deterioration of annealing furnace internals (rolls and the like) and an increase in cost, although there is no problem for an effect of the present invention.
• A is 600 or less.
• the reason why the heating rate is 7 °C/s or more is as described below.
• the effect of suppressing surface oxidation is recognized when the heating rate is 7 °C/s or more.
• the upper limit of the heating rate is not particularly limited.
• the heating rate is 500 °C/s or more, the above effect is saturated, which is disadvantageous in terms of cost. Therefore, the heating rate is preferably 500 °C/s or less.
• the heating rate can be adjusted to 7 °C/s or more in such a manner that, for example, an induction heater is placed in an annealing furnace in which the temperature of the steel sheet is 450°C to A°C.
• the maximum end-point temperature of the steel sheet in the annealing furnace is 600°C to 700°C.
• the maximum end-point temperature of the steel sheet is lower than 600°C, good material quality is not obtained. Therefore, a temperature range in which an effect of the present invention is exhibited is 600°C or higher.
• a temperature range higher than 700°C surface oxidation is significant and the deterioration of chemical conversion treatability is serious.
• the effect of balancing between strength and ductility is saturated from the viewpoint of material quality. From the above, the maximum end-point temperature of the steel sheet is 600°C to 700°C.
• the reason why the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600°C to 700°C is 30 seconds to 10 minutes is as described below.
• target material quality TS, El
• the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600°C to 700°C is more than 10 minutes, the effect of balancing between strength and ductility is saturated.
• the reason why the concentration of hydrogen in the atmosphere in the temperature range corresponding to a steel sheet temperature of 600°C to 700°C is 20% by volume or more is as described below.
• the effect of suppressing surface oxidation begins to be recognized when the concentration of hydrogen is 20% by volume.
• the upper limit of the concentration of hydrogen is not particularly limited. When the concentration of hydrogen is more than 80% by volume, the above effect is saturated, which is disadvantageous in terms of cost. Therefore, the concentration of hydrogen is preferably 80% by volume or less.
• composition of the steel sheet of the high-strength steel sheet of the present invention are described below.
• the content of C forms martensite and the like as a steel microstructure to enhance workability. Therefore, the content of C needs to be 0.03% or more. However, when the content of C is more than 0.35%, strength increases extremely and elongation decreases, resulting in the deterioration of workability. Thus, the content of C is 0.03% to 0.35%.
• Si is an element that is effective in strengthening steel to obtain good material quality
• Si is an oxidizable element and therefore is disadvantageous for chemical conversion treatability.
• Si is an element that should be avoided being added as much as possible.
• about 0.01% of Si is inevitably contained in steel. Reducing the content of Si to 0.01% or less leads to an increase in cost. From the above, the lower limit is 0.01%.
• the content of Si is more than 0.50%, the effect of increasing the strength and elongation of steel is saturated and chemical conversion treatability is deteriorated. Thus, the content of Si is 0.01% to 0.50%.
• Mn is an element effective in increasing the strength of steel. In order to ensure mechanical properties and strength, 3.6% or more Mn needs to be contained. However, when more than 8.0% Mn is contained, it is difficult to ensure chemical conversion treatability and the balance between strength and ductility. Furthermore, there is a cost disadvantage. Thus, the content of Mn is 3.6% to 8.0%.
• Al is added for the purpose of deoxidizing molten steel.
• the content of Al is less than 0.01%, the purpose of deoxidizing molten steel is not achieved.
• the effect of deoxidizing molten steel is achieved when the content of Al is 0.01% or more.
• the content of Al is more than 1.0%, an increase in cost is caused.
• the surface oxidation of Al is increased and it is difficult to improve chemical conversion treatability.
• the content of Al is 0.01% to 1.0%.
• the content of P is one of inevitably contained elements. In order to adjust the content of P to less than 0.005%, an increase in cost may possibly be caused. Therefore, the content of P is preferably 0.005% or more. However, when more than 0.10% of P is contained, weldability is deteriorated. Furthermore, the deterioration of chemical conversion treatability is significant and it is difficult to enhance chemical conversion treatability even by the present invention. Thus, the content of P is 0.10% or less and the lower limit is preferably 0.005%.
• S is one of the inevitably contained elements.
• the lower limit is not particularly limited. When a large amount of S is contained, weldability and corrosion resistance are deteriorated. Therefore, the content of S is 0.010% or less.
• the following element may be added where appropriate: an element that is one or more selected from among 0.001% to 0.005% B, 0.005% to 0.05% Nb, 0.005% to 0.05% Ti, 0.001% to 1.0% Cr, 0.05% to 1.0% Mo, 0.05% to 1.0% Cu, 0.05% to 1.0% Ni, 0.001% to 0.20% Sn, 0.001% to 0.20% Sb, 0.001% to 0.10% Ta, 0.001% to 0.10% W, and 0.001% to 0.10% V.
• an element that is one or more selected from among 0.001% to 0.005% B, 0.005% to 0.05% Nb, 0.005% to 0.05% Ti, 0.001% to 1.0% Cr, 0.05% to 1.0% Mo, 0.05% to 1.0% Cu, 0.05% to 1.0% Ni, 0.001% to 0.20% Sn, 0.001% to 0.20% Sb, 0.001% to 0.10% Ta, 0.001% to 0.10% W, and 0.001% to 0.10% V.
• Nb When the content of Nb is less than 0.005%, the effect of adjusting strength is unlikely to be obtained. However, when the content of Nb is more than 0.05%, an increase in cost is caused. Thus, when Nb is contained, the content of Nb is 0.005% to 0.05%.
• the content of Ti When the content of Ti is less than 0.005%, the effect of adjusting strength is unlikely to be obtained. However, when the content of Ti is more than 0.05%, the deterioration of chemical conversion treatability is caused. Thus, when Ti is contained, the content of Ti is 0.005% to 0.05%.
• the content of Mo When the content of Mo is less than 0.05%, the effect of adjusting strength is unlikely to be obtained. However, when the content of Mo is more than 1.0%, an increase in cost is caused. Thus, when Mo is contained, the content of Mo is 0.05% to 1.0%.
• the content of Cu When the content of Cu is less than 0.05%, the effect of accelerating the formation of a retained ⁇ -phase is unlikely to be obtained. However, when the content of Cu is more than 1.0%, an increase in cost is caused. Thus, when Cu is contained, the content of Cu is 0.05% to 1.0%.
• the content of Ni When the content of Ni is less than 0.05%, the effect of accelerating the formation of the retained ⁇ -phase is unlikely to be obtained. However, when the content of Ni is more than 1.0%, an increase in cost is caused. Thus, when Ni is contained, the content of Ni is 0.05% to 1.0%.
• Sn and Sb may be contained from the viewpoint of suppressing the nitriding or oxidation of a surface of the steel sheet or the decarburization of a region of tens of micrometers in the steel sheet surface, the decarburization being due to oxidation. Suppressing the nitriding or oxidation thereof prevents the amount of martensite produced in the steel sheet surface from being reduced, thereby improving fatigue properties and surface quality.
• the content of each of Sn and Sb is 0.001% or more.
• the content of either of Sn and Sb is more than 0.20%, the deterioration of toughness is caused. Therefore, the content of each of Sn and Sb is preferably 0.20% or less.
• Ta 0.001% to 0.10%
• Ta forms a carbide and a carbonitride with C and N to contribute to increasing strength and also contributes to increasing yield ratio (YR).
• Ta has the property of refining the microstructure of a hot-rolled steel sheet. This property reduces the diameter of ferrite grains after cold rolling and annealing. Thus, the amount of C segregated along grain boundaries increases with the increase in area of the grain boundaries and a high bake hardening value (BH value) can be obtained. From this viewpoint, 0.001% or more Ta may be contained. However, containing more than 0.10% Ta causes an increase in material cost and may possibly interfere with the formation of martensite in the course of cooling after annealing. Furthermore, TaC precipitated in the hot-rolled steel sheet increases deformation resistance during cold rolling to make stable actual manufacture difficult in some cases. Thus, when Ta is contained, the content of Ta is 0.10% or less.
• W and V are elements which form a carbonitride and which have the property of increasing the strength of steel by a precipitation effect and may be added as required. In the case where W and/or V is added, this property is exhibited when the content of each of W and V is 0.001% or more. However, when more than 0.10% W and/or V is contained, strength is extremely increased and ductility is deteriorated. From the above, when W and/or V is contained, the content of each of W and V is 0.001% to 0.10%.
• the remainder other than the above elements are Fe and inevitable impurities. If an element other than the above elements is contained, the present invention is not adversely affected.
• the upper limit of the element other than the above elements is 0.10%.
• Steel containing the above chemical components is hot-rolled, followed by cold rolling, whereby a steel sheet is obtained.
• the steel sheet is annealed in a continuous annealing line.
• the steel sheet is preferably electrolytically pickled in an aqueous solution containing sulfuric acid.
• the steel sheet is then subjected to a chemical conversion treatment.
• the heating rate in a temperature range corresponding to an annealing furnace inside temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600) is 7 °C/s or more
• the maximum end-point temperature of the steel sheet in an annealing furnace is 600°C to 700°C
• the transit time of the steel sheet in a temperature range corresponding to a steel sheet temperature of 600°C to 700°C is 30 seconds to 10 minutes
• the concentration of hydrogen in an atmosphere is 20% by volume or more in a heating step. This is the most important requirement in the present invention.
• hot rolling is directly followed by annealing without performing cold rolling in some cases.
• Hot rolling can be performed under conditions usually used.
• Pickling is preferably performed after hot rolling. Mill scales formed on a surface are removed in a pickling step, followed by cold rolling. Pickling conditions are not particularly limited.
• Cold rolling is preferably performed with a rolling reduction of 40% to 80%.
• the rolling reduction is less than 40%, the recrystallization temperature is reduced and therefore mechanical properties are likely to be deteriorated.
• the rolling reduction is more than 80%, not only rolling costs increase because of the high-strength steel sheet but also chemical conversion treatability is deteriorated because surface oxidation increases during annealing in some cases.
• the cold-rolled or hot-rolled steel sheet is annealed and is then subjected to the chemical conversion treatment.
• the heating step is performed such that the steel sheet is heated to a predetermined temperature in an upstream heating zone and a soaking step is performed such that the steel sheet is held at a predetermined temperature for a predetermined time in a downstream soaking zone.
• the heating rate in the temperature range corresponding to an annealing furnace inside temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600) is 7 °C/s or more
• the maximum end-point temperature of the steel sheet in an annealing furnace is 600°C to 700°C
• the transit time of the steel sheet in the temperature range corresponding to a steel sheet temperature of 600°C to 700°C is 30 seconds to 10 minutes
• the concentration of hydrogen in the atmosphere is controlled to be 20% by volume or more as described above.
• Gaseous components in the annealing furnace are nitrogen, hydrogen, and inevitable impurities. If no effect of the present invention is impaired, another gaseous component may be contained.
• the concentration of hydrogen in a temperature range other than the temperature range corresponding to a steel sheet temperature of 600°C to 700°C, that is, a temperature range lower than 600°C or higher than 700°C, is not particularly limited.
• concentration of hydrogen therein is less than 1% by volume, no activation effect due to reduction is obtained and chemical conversion treatability is deteriorated in some cases.
• the upper limit is not particularly limited. When the upper limit is more than 50% by volume, an increase in cost is caused and an effect is saturated.
• the concentration of hydrogen therein is preferably 1% to 50% by volume and more preferably 5% to 30% by volume. The remainder are N 2 and inevitable impurity gases. If no effect of the present invention is impaired, a gaseous component such as H 2 O, CO 2 , or CO may be contained.
• Tempering is preferably performed at a temperature of 150°C to 400°C. This is because elongation tends to be deteriorated when the temperature is lower than 150°C and hardness tends to be reduced when the temperature is higher than 400°C.
• electrolytic pickling is preferably performed in an aqueous solution containing sulfuric acid for the purpose of removing slight amounts of surface oxides inevitably formed during annealing to ensure better chemical conversion treatability.
• a pickling solution used for electrolytic pickling is not particularly limited.
• Nitric acid and hydrofluoric acid are highly corrosive to equipment, requires caution in handling, and therefore are not preferable.
• Hydrochloric acid may possibly generate a chlorine gas from a cathode and therefore is not preferable.
• sulfuric acid is preferably used.
• the concentration of sulfuric acid is preferably 5% to 20% by mass. When the concentration of sulfuric acid is less than 5% by mass, conductivity is low; hence, the voltage of an electrolytic bath rises during electrolysis to increase the load of a power supply in some cases. However, when the concentration of sulfuric acid is more than 20% by mass, the loss due to drag-out is large, which is problematic in terms of cost.
• Conditions for electrolytic pickling are not particularly limited.
• alternating electrolysis is preferably performed at a current density of 1 A/dm 2 or more.
• the reason for performing alternating electrolysis is as described below.
• a pickling effect is small.
• Fe dissolved during electrolysis is accumulated in the pickling solution and therefore the concentration of Fe in the pickling solution is increased; hence, a problem with dry stains or the like occurs if the pickling solution is attached to a surface of the steel sheet.
• the temperature of an electrolytic solution is preferably 40°C to 70°C.
• the temperature of a bath is increased by heat generated by continuous electrolysis and therefore it is difficult to maintain the electrolytic solution at lower than 40°C in some cases. From the viewpoint of the durability of a lining of an electrolytic cell, it is not preferable that the temperature of the electrolytic solution exceeds 70°C. When the temperature of the electrolytic solution is lower than 40°C, the pickling effect is small. Therefore, the temperature of the electrolytic solution is preferably 40°C or higher.
• the high-strength steel sheet according to the present invention is obtained as described above.
• the surface structure of the steel sheet is featured as described below.
• the amount of the following oxide per side is limited to less than 0.030 g/m 2 in total: an oxide of one or more selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V.
• the steel sheet which is made from steel containing large amounts of Si and Mn
• the internal oxidation of a surface layer of a base steel sheet is minimized, chemical conversion treatment unevenness and a lack of hiding are suppressed, and corrosion and cracking during heavy machining are also suppressed. Therefore, in the present invention, the potential of oxygen is reduced in the annealing step for the purpose of ensuring good chemical conversion treatability, whereby the activity of Si, Mn, and the like, which are oxidizable elements, in a base metal surface portion is reduced. Furthermore, the external oxidation of these elements is suppressed and the formation of internal oxides in the base metal surface portion is also suppressed.
• the amount of the following oxide is limited to less than 0.030 g/m 2 in total: an oxide of at least one selected from among Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, Cu, Ni, Sn, Sb, Ta, W, and V.
• the internal oxidation amount When the sum (hereinafter referred to as the internal oxidation amount) of the amounts of formed oxides is 0.030 g/m 2 or more, not only corrosion resistance and workability are deteriorated but also chemical conversion treatment unevenness and a lack of hiding are caused. Even if the internal oxidation amount is limited to less than 0.0001 g/m 2 , the effect of improving corrosion resistance and the effect of enhancing workability are saturated. Therefore, the lower limit of the internal oxidation amount is preferably 0.0001 g/m 2 .
• Hot-rolled steel sheets with a steel composition shown in Table 1 were pickled, whereby mill scales were removed. Thereafter, the hot-rolled steel sheets were cold-rolled under conditions shown in Tables 2 and 3, whereby cold-rolled steel sheets with a thickness of 1.0 mm were obtained. Incidentally, after the mill scales were removed, some of the hot-rolled steel sheets (a thickness of 2.0 mm) were used without being cold-rolled.
• each steel sheet was processed and annealed by controlling the heating rate in a temperature range corresponding to a steel sheet temperature of 450°C to A°C (where 500 ⁇ A ⁇ 600) in an annealing furnace, the concentration of hydrogen in a temperature range corresponding to a steel sheet temperature of 600°C to 700°C in the annealing furnace, the transit time of the steel sheet, and the maximum end-point temperature of the steel sheet, followed by water quenching and then tempering at 300°C for 140 seconds.
• the steel sheet was pickled in such a manner that the steel sheet was immersed in a 40°C aqueous solution containing 5% by mass sulfuric acid.
• Some of the steel sheets were electrolytically pickled under current density conditions shown in Tables 2 and 3 by alternating electrolysis in such a manner that a specimen was held as an anode and a cathode in that order for 3 seconds, whereby specimens were obtained.
• the concentration of hydrogen in the annealing furnace other than a region where the concentration of hydrogen was controlled as described above was basically 10% by volume.
• Gaseous components in an atmosphere were a nitrogen gas, a hydrogen gas, and inevitable impurity gases.
• the dew point of the atmosphere was controlled by absorbing or removing moisture in the atmosphere.
• the dew point of the atmosphere was -35°C.
• the specimens obtained as described above were measured for TS and El. Furthermore, the specimens were investigated for chemical conversion treatability and corrosion resistance after electrodeposition coating. The amount (internal oxidation amount) of oxides present in a surface portion of each steel sheet, the surface portion being located directly under a surface layer of the steel sheet and being within 100 ⁇ m from the surface layer, was measured. Measurement methods and evaluation standards are as described below.
• a chemical conversion solution used was a chemical conversion solution (PALBOND ® L3080) produced by Nihon Parkerizing Co., Ltd. Each specimen was subjected to a chemical conversion treatment by a method below. The specimen was degreased with a degreasing solution, FINECLEANER ® , produced by Nihon Parkerizing Co., Ltd.; was washed with water; was surface-modified with a surface modifier, PREPALENE ® Z, produced by Nihon Parkerizing Co., Ltd. for 30 seconds; was immersed in the chemical conversion solution (PALBOND ® L3080) at 43°C for 120 seconds; was washed with water; and was then dried with hot air.
• a degreasing solution FINECLEANER ®
• PREPALENE ® Z a surface modifier
• Randomly selected five fields of view of the specimen subjected to the chemical conversion treatment were observed with a scanning electron microscope (SEM) at 500x magnification.
• SEM scanning electron microscope
• the area fraction of a lack of hiding in a chemical conversion coating was measured by image processing.
• the specimen was evaluated depending on the area fraction of a lack of hiding as described below. "A" is an acceptable level.
• a test piece with a size of 70 mm ⁇ 150 mm was cut out of each specimen, obtained by the above method, subjected to the chemical conversion treatment, followed by cationic electrodeposition coating (baking conditions: 170°C for 20 minutes, a thickness of 25 ⁇ m) using PN-150G ® manufactured by Nippon Paint Co., Ltd. Thereafter, end portions and a surface not evaluated were sealed with an Al tape and cross cuts (a cross angle of 60°) were made with a cutter knife so as to reach a base metal, whereby a specimen was prepared.
• the specimen was immersed in a 5% aqueous solution of NaCl (55°C) for 240 hours, was taken out, was washed with water, and was then dried, followed by peeling the tape from a cross cut portion.
• the separation width was measured and was evaluated as described below. "A" is an acceptable level.
• a JIS #5 tensile test piece was taken from each sample in a 90° direction with respect to a rolling direction and was measured for workability in such a manner that tensile testing was performed at a constant cross head speed of 10 mm/min in accordance with JIS Z 2241 and the tensile strength (TS/MPa) and elongation (El/%) were determined.
• TS/MPa tensile strength
• El/% elongation
• the internal oxidation amount was measured by "impulse furnace fusion-infrared absorption spectrometry".
• the amount of oxygen contained in steel that is, an unannealed high-strength steel sheet
• the concentration of oxygen in steel was measured, and the measurement was defined as the oxygen amount OH in steel.
• the concentration of oxygen in steel was measured over the continuously annealed high-strength steel sheet in a thickness direction and the measurement was defined as the oxygen amount OI after internal oxidation.
• high-strength steel sheets manufactured by a method according to the present invention has excellent chemical conversion treatability, corrosion resistance after electrodeposition coating, and workability, although the high-strength steel sheets contain large amounts of oxidizable elements such as Si and Mn.
• one or more of chemical conversion treatability, corrosion resistance after electrodeposition coating, and workability are inferior.
• a high-strength steel sheet according to the present invention has excellent chemical conversion treatability, corrosion resistance, and workability and can be used as a surface-treated steel sheet for lightening and strengthening automobile bodies.
• the high-strength steel sheet can be used in various fields, such as home appliances and building materials, other than automobiles in the form of a surface-treated steel sheet manufactured by imparting rust resistance to a base steel sheet.

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• 2014
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