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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
• • 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
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • 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
• • 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
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
  • C23C2/29—Cooling or quenching
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• This disclosure relates to a galvanized steel sheet which is suitably used, for example, in an automobile and a home electric appliance field and which has a low yield stress and superior anti-aging property and bake hardenability and to a method for producing the galvanized steel sheet.
• a steel sheet which has low strength in press forming to be easily pressed and which exhibits high bake hardenability in a paint baking step performed after press formation has been developed.
• This steel sheet is obtained by controlling a dissolved C amount by addition of Ti and Nb to ultralow-carbon steel used as a base material, has superior surface distortion resistance since a yield stress (hereinafter referred to as “YP” in some cases) is low, such as approximately 240 MPa at a strength level of 340 MPa, and ensures dent resistance by increasing a yield stress (YP′) after press forming and paint baking to approximately 300 MPa.
• a yield stress hereinafter referred to as “YP” in some cases
• a steel sheet having a thickness smaller than that of a current 340BH steel sheet having a thickness of 0.65 to 0.80 mm has been desired and, for example, to reduce the thickness by 0.05 mm, the yield stress (YP′) after press formation and paint baking must be increased to approximately 350 MPa or more.
• a steel sheet is required which has high paint bake hardenability (hereinafter referred to as “BH” in some cases) and work hardenability (hereinafter referred to as “WH” in some cases).
• the steel sheet is formed by appropriately controlling annealing and cooling conditions of steel containing 0.04% or less of C, 0.5% to 3.0% of Mn, and 0.01% to 1.0% of Mo so that after annealing, a composite microstructure is obtained which includes 0.5% to less than 10% of retained austenite on a volume fraction basis and the balance being ferrite and a hard phase composed of bainite and/or martensite.
• BH high strength and high bake hardenability
• annealing was performed at an extremely high temperature region of 860 to 980° C.
• troubles such as sheet breakage, may occur in some cases. Accordingly, development is required to form a steel sheet having superior anti-aging property without performing high temperature annealing.
• the volume fraction of martensite and the solute C in ferrite are controlled, and as cooling after annealing to obtain high bake hardenability, cooling is performed from a temperature of 550 to 750° C. to a temperature of 200° C. or less at a cooling rate of 100° C./s.
• a specific method must be performed in which, for example, quenching is performed in jet water, as disclosed in Japanese Unexamined Patent Application Publication No. 2006-233249, and it is difficult to perform manufacturing in a current continuous galvanizing line.
• cooling in the temperature range of 650 to 450° C. is performed at a cooling rate of 15 to 200° C./s, and cooling in the temperature range defined by the amounts of C, Mn, and Cr is performed at less than 10° C./s.
• cooling from the annealing temperature to 680° C. is performed at 3° C./s
• rapid cooling is performed at a rate of 80° C./s to a temperature represented by Ts
• slow cooling is performed at a rate of less than 10° C./s to a temperature represented by Tf, and subsequently, cooling to 180° C.
• the technique described above can be performed in a CAL which performs no galvanizing treatment and which is provided with an overaging zone.
• the technique is difficult to perform in a CGL which performs a galvanizing treatment during cooling and which is not generally provided with an overaging apparatus (when a galvanizing treatment is performed, a steel sheet must be dipped in a galvanizing bath at a temperature of approximately 460° C. for several seconds, and when alloying is further performed, a steel sheet must be heated to 500 to 600° C. and maintained for several tens of seconds).
• the high strength galvanized steel sheet is a galvanized steel sheet having a tensile strength of 340 MPa or more.
• a high strength galvanized steel sheet having a low yield stress and superior anti-aging property and bake hardenability can be obtained.
• weight reduction can also be achieved by thickness reduction.
• the high strength galvanized steel sheet has the superior properties described above, besides an automobile steel sheet, it can be widely used for home electric appliance application and the like. Hence, the steel sheet has industrial advantages.
• the composition is defined such that the Mn content is more than 1.0% to 1.8% and the Cr content is more than 0.5, and in addition, the Mn equivalent is controlled in an appropriate range that satisfies 1.9 ⁇ Mn (mass percent)+1.3Cr (mass percent) ⁇ 2.8.
• the microstructure is designed such that a ferrite phase and 2% to 15% of martensite on an area ratio basis are included, and that the total area ratio of pearlite and/or bainite is 1.0% or less.
• annealing/galvanizing conditions must be controlled, and annealing is performed at an annealing temperature of more than 750° C. to less than 820° C., cooling is performed at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed, and after galvanizing is performed, cooling is performed at an average cooling rate of 5° C./s or more.
• the content is set to 0.01% or more to ensure a predetermined amount or more of martensite.
• the C content is set to 0.08% or more, since the amount of martensite is excessively large, YP is increased, the BH amount is decreased, and in addition, the weldability is degraded.
• the C content is set to less than 0.08% and, to obtain a lower YP and a higher BH, the C content is preferably set to less than 0.06% and more preferably set to 0.05% or less.
• Si has a high solid-solution strengthening ability, and a lower Si content is preferable in terms of decrease in yield strength (decrease in YP). However, since a Si content of up to 0.2% is permissible, the Si content is set to 0.2% or less.
• Mn More than 1.0% to 1.8%
• Mn is the most important element.
• the Mn content is more than 1.8%, the amount of solute C in ferrite is decreased, and the BH property is degraded.
• the Mn content is 1.0% or less, a high BH property is obtained since the amount of solute C in ferrite is large.
• the anti-aging property may be degraded in some cases.
• the Mn content is set in the range of more than 1.0% to 1.8% and is preferably set in the range of more than 1.0% to 1.6%.
• P is an effective element to increase strength.
• the yield strength (YP) is increased, and surface-distortion resistance is degraded.
• an alloying speed of a galvanizing layer is decreased, surface defect occur, and in addition, resistance against secondary work-embrittlement is degraded due to segregation in grain boundaries of a steel sheet. Accordingly, the P content is set to 0.10% or less.
• the content is preferably decreased. Further, when the S content is more than 0.03%, the ductility of the steel sheet is degraded due to precipitation of fine MnS, and the press formability is degraded. Hence, the S content is set to 0.03% or less. In addition, in view of the press formability, the S content is preferably set to 0.015% or less.
• Al decreases inclusions in steel as a deoxidizing element and, in addition, it also functions to fix unnecessary solute N in steel in the form of a nitride.
• the Al content is set to 0.1% or less.
• 0.02% or more of Al is preferably contained.
• the content is preferably decreased.
• the N content is more than 0.008%, the amount of a nitride forming element necessary to fix N is increased. Hence, manufacturing cost is increased.
• the ductility and toughness are degraded.
• the N content is set to 0.008% or less.
• the N content is preferably set to less than 0.005% to ensure ductility and toughness.
• Cr is a hardenability improving element and is a very important element for formation of martensite.
• Cr since having a high hardenability and a low solid-solution hardenability as compared to those of Mn, Cr is effective to decrease YP, and Cr is positively added.
• the content is set to more than 0.5% and is preferably more than 0.65%.
• the content of Mn is limited.
• the Mn equivalent must be controlled to be a predetermined level by adjusting the Cr content. Accordingly, the Cr content is set to more than 0.5% and is preferably set to more than 0.65%.
• the value of Mn+1.3Cr is one index indicating the hardenability and it is important to control the value to form martensite.
• the value of Mn+1.3Cr is less than 1.9%, the hardenability becomes insufficient, and pearlite and bainite are liable to be generated during cooling performed after annealing, so that YP is increased.
• the value of Mn+1.3Cr is more than 2.8, the hardenability effect is saturated, and by excessive addition of alloying elements, manufacturing cost is increased.
• the value of Mn+1.3Cr is set in the range of 1.9% to 2.8% and is preferably set in the range of more than 2.3% to 2.8%.
• Targeted properties of the steel can be obtained by those essential addition elements described above. However, besides those elements, whenever necessary, the following elements may also be added.
• B is a hardenability improving element and can be added in an amount of 0.0005% or more to stably form martensite. Furthermore, when 0.0015% to 0.004% of B is added, besides improvement in grain growth properties of ferrite, BH can be improved, and balance between decrease in YP and increase in BH can be further improved. However, when more than 0.01% of B is added, adverse influence on the mechanical properties and the productivity in casting are enhanced. Hence, when B is added, the content thereof is set to 0.01% or less. At least one of Mo: 0.15% or less, V: 0.5% or less, Ti: 0.1% or less, and Nb: 0.1% or less
• Mo is an expensive element and is an element to increase YP.
• Mo is also an effective element which improves zinc coating surface quality, or improves hardenability and stably obtains martensite, and 0.01% or more of Mo may be added.
• the Mo content is more than 0.15%, the effects thereof is saturated, and cost is seriously increased.
• Mo when Mo is added, 0.15% or less of Mo may be added so that an adverse influence thereof, increase in YP, is not so significant.
• the content of Mo is preferably decreased as small as possible, and Mo is preferably not to be added (0.02% or less of Mo being present as an inevitable impurity).
• V is a hardenability improving element and may be added in an amount of 0.01% or more to stably form martensite. However, even when V is excessively added, an effect corresponding to the cost cannot be obtained. Hence, when V is added, the content thereof is set to 0.5% or less.
• Ti and Nb each form carbide, nitride and carbonitride and decrease the amounts of solute C and N, and to prevent degradation of mechanical properties during aging, each element in an amount of 0.01% or more may be added. However, even when the element in an amount of more than 0.1% is excessively added, the effect is saturated, and an effect corresponding to the cost cannot be obtained. Hence, when Ti and/or Nb is added, the content of each element is set to 0.1% or less.
• the balance other than those elements described above includes Fe and inevitable impurities.
• the inevitable impurities for example, since O forms non-metal inclusions and has an adverse influence on the quality, the content of O is preferably decreased to 0.003% or less.
• the galvanized steel sheet has a dual phase microstructure containing a ferrite phase and 2% to 15% of martensite on an area ratio basis.
• the martensite is controlled in the range described above, the surface-distortion resistance and work-hardenability are improved, so that a steel sheet usable for automobile outer panel application can be obtained.
• the area ratio of the martensite is more than 15%, the strength is significantly increased, and for example, as a steel sheet for an automobile inner/outer plate panel, that is typically intended, sufficient surface-distortion resistance and press formability cannot be obtained.
• the area ratio of martensite is set to 15% or less.
• the area ratio of martensite is set in the range of 2% to 15% and is preferably set in the range of 2% to 10%.
• the area ratio can be obtained by the steps of polishing an L cross-section (vertical cross-section parallel to a rolling direction) of a steel sheet, etching the cross-section using nital, observing 12 visual fields at a magnification of 4,000 times power using a SEM, and performing image analysis of an obtained microstructure photograph.
• a blackish contrast region indicates ferrite, a region in which carbides are generated in the form of lamellas or points is regarded as pearlite and bainite, and particles having a white contrast are regarded as martensite.
• the microstructure can be controlled in the above area ratio range.
• the high strength galvanized steel sheet is manufactured by the steps of forming a slab by melting steel adjusted in the above chemical composition range; then performing hot rolling, followed by (pickling) cold rolling; then, after annealing, performing cooling at an average cooling rate of 3 to 15° C./s in a temperature range from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed; and after galvanizing, performing cooling at an average cooling rate of 5° C./s or more.
• the method for melting and refining steel is not particularly limited, and an electric furnace may be used, or a converter may be used.
• a method for casting steel after the melting and refining a cast slab may be formed by a continuous casting method, or an ingot may be formed by an ingot-making method.
• rolling may be performed after the slab is re-heated in a heating furnace, or direct rolling may be performed without heating the slab.
• hot rolling may be performed after blooming is performed for the ingot thus formed.
• Hot rolling may be performed in accordance with an ordinary method, for example, such that the temperature for heating the slab is set to 1,100 to 1,300° C., the finish rolling temperature is set to the Ar3 point or more, the cooling rate after the finish rolling is set to 10 to 200° C./s, and the coiling temperature is set to 400 to 750° C.
• the reduction ratio of cold rolling may be set to 50 to 85% which is the range performed in a general operation.
• Annealing Temperature More than 750° C. to Less than 820° C.
• the annealing temperature must be increased to an appropriate temperature to obtain a microstructure containing a ferrite phase and martensite.
• the annealing temperature is 750° C. or less, since austenite is not sufficiently formed, a predetermined amount of martensite cannot be obtained. Hence, for example, due to remaining YPEl, increase in YP, the surface-distortion resistance is degraded.
• the annealing temperature is 820° C. or more, the amount of solute C in ferrite is decreased, and a high BH amount may not be obtained in some cases.
• the annealing temperature is set to more than 750° C. to less than 820° C.
• the primary average cooling rate from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed is set to 3 to 15° C./s.
• the cooling rate is less than 3° C./s, since the pearlite and bainite significantly generate during cooling, YP is increased.
• YP is increased.
• the primary average cooling rate is set to 3 to 15° C./s from the annealing temperature to a temperature at which dipping into a galvanizing bath is performed.
• a preferable average cooling rate is 5 to 15° C./s.
• a galvanizing bath temperature in a galvanizing treatment may be a common temperature, such as approximately 400 to 480° C.
• the alloying treatment may be performed after the galvanizing treatment is performed.
• the alloying treatment after the galvanizing is performed, for example, such that after the dipping in a galvanizing bath is performed, whenever necessary, heating is performed to a temperature range of 500 to 700° C., and the temperature is maintained for several seconds to several tens of seconds.
• the mechanical properties are seriously degraded by the alloying treatment as described above.
• the increase in YP is small even if the alloying treatment as described above is performed.
• a coating amount per one surface is preferably 20 to 70 g/m 2 , and when the alloying treatment is performed, the Fe content in the coating layer is preferably set to 6% to 15%.
• the secondary cooling to be performed after the galvanizing treatment or the alloying treatment, to obtain a predetermined amount of martensite, cooling is performed at an average cooling rate of 5° C./s or more to a temperature of the Ms point or less.
• the secondary cooling rate is less than 5° C./s
• pearlite or bainite is generated at approximately 400 to 500° C., so that YP is increased.
• the second cooling rate is preferably 100° C./s or less. Accordingly, the secondary cooling rate is set to 5° C./s or more and is preferably set to 10 to 100° C./s.
• temper rolling may also be performed on the steel sheet after the heat treatment for shape flattening.
• a steel material is supposed to be manufactured by the steps including general steel making, casting, and hot rolling. However, by omitting part of the hot rolling step or all thereof, a steel material may be manufactured, for example, by thin-slab casting.
• the surface of the galvanized steel sheet may be further processed by an organic film treatment.
• samples obtained by cutting off from the cold-rolled steel sheets, which were obtained as described above, were sequentially processed by the steps of performing annealing at annealing temperatures shown in Table 2 for 60 seconds in an infrared image furnace; performing primary cooling under conditions shown in Table 2; performing galvanizing (galvanizing bath temperature: 460° C.); performing an alloying treatment (520° C. ⁇ 15 s); performing secondary cooling to a temperature of 150° C. or less; and performing temper rolling at an extension rate of 0.4%.
• the galvanizing treatment was adjusted to have a coating weight of 50 g/m 2 per one surface, and the alloying treatment was adjusted so that the Fe content in the coating layer was 9% to 12%.
• the compositions and the manufacturing conditions of Nos. 1 to 17 and 40 to 42 are within our range, and the microstructures thereof are our examples in which the area ratio of martensite is in the range of 2% to 15%, and the total area ratio of pearlite and/or bainite is 1.0% or less.
• our examples have a low YR and a high BH, and YPEl after aging is also low, such as 0.2% or less.
• Samples were obtained from the galvanized steel sheets thus obtained, and by methods similar to those in Example 1, the area ratio of martensite and the total area ratio of pearlite and/or bainite were measured. In addition, the tensile properties, work hardenability amount (WH), bake hardenability amount (BH), and YPEl after an acceleration aging test were measured.
• WH work hardenability amount
• BH bake hardenability amount
• YPEl after an acceleration aging test
• the compositions and the manufacturing conditions of Nos. 23, 25, 26, 28 to 31, and 35 to 39 are within our range, and the microstructures thereof are our examples in which the area ratio of martensite is in the range of 2% to 15%, and the total area ratio of pearlite and/or bainite is 1.0% or less.
• our examples have a lower YR and a higher BH, and YPEl after aging is also smaller, such as 0.2% or less.
• our high strength galvanized steel sheet has a low yield stress and also has superior anti-aging property and bake hardenability, the steel sheet can be applied to parts which require high formability, such as automobile inner and outer plate application.

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