Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • 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
  • C22C38/06—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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • 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
• • 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
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; Strips

Definitions

• the present invention relates to a high-strength galvannealed steel sheet with excellent release of diffusible hydrogen from steel, and a method for manufacturing the same.
• Hot-dip galvanized steel sheets are generally produced by using hot-rolled or cold-rolled steel sheets as the base material, recrystallizing the base steel sheet in the annealing furnace of a continuous galvanizing line (CGL), and then hot-dip galvanizing it.
• Galvannealed steel sheets are produced by further alloying after hot-dip galvanizing.
• CGL annealing requires a reducing atmosphere containing hydrogen to suppress oxidation of the steel sheet surface, which causes unplated areas, and to achieve a good plating appearance.
• hydrogen in this atmosphere penetrates the steel sheet and remains as diffusible hydrogen, reducing its delayed fracture resistance.
• Patent Document 1 proposes a method in which hydrogen-infiltrated steel is heated at a predetermined temperature through a baking process, causing the hydrogen to diffuse and be released from the steel surface.
• Patent Documents 2 and 3 propose methods for releasing hydrogen through the coating layer, in which a certain number of cracks are formed in the coating, and the hydrogen in the steel sheet is released through the cracks to the outside of the steel sheet.
• the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength galvannealed steel sheet having a tensile strength of 780 MPa or more, good release of diffusible hydrogen in steel, and excellent delayed fracture resistance.
• the delayed fracture resistance can be evaluated by the method described in the Examples, and when the evaluation criteria of the Examples are satisfied, the delayed fracture resistance is judged to be excellent.
• the present inventors have found the following regarding hydrogen release from high-strength galvannealed steel sheets. They found that hydrogen release is affected not only by the presence or absence of cracks in the coating, but also by their morphology, and that it is particularly important that there be a certain number of crack intersections near the interface between the steel sheet and the coating. Furthermore, they found that a coating layer with a large number of crack intersections can be formed by optimizing the annealing atmosphere before hot-dip galvanizing and the cooling rate after alloying. The present invention was made based on these findings, and the gist of the present invention is as follows.
• a high-strength galvannealed steel sheet having a plating layer on a base steel sheet The composition of the base steel sheet is, in mass%, C: 0.06% or more and 0.30% or less, Si: 0.01% or more but less than 1.50%, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (not including 0%), S: 0.03% or less (not including 0%), sol.
• Al 0.1% or less (not including 0%), N: 0.007% or less (not including 0%), O: 0.003% or less (not including 0%), the mass ratio of Si to Mn (Si/Mn) is 0.25 or less, and the balance is Fe and unavoidable impurities, a galvannealed layer having a coating weight of 20 g/m2 or more and 120 g/ m2 or less per side on the substrate steel sheet; the oxygen content of the steel sheet surface layer portion immediately below the zinc-coated layer, within 100 ⁇ m from the surface of the base steel sheet toward the center of the sheet thickness, is less than 0.030 g/ m2 per side; A high-strength galvannealed steel sheet, wherein the number of crack intersections in the plating layer is 500 or more per mm2 on a plane parallel to the sheet surface of the steel sheet at a depth M [ ⁇ m] (1 ⁇ M ⁇ 5 ) from the surface of the zinc plating layer, the amount of diffusible hydrogen in the steel is 0.
• the dew point of the atmosphere in the heating furnace in the temperature range where the steel sheet temperature in the annealing heating furnace is 700°C or higher is -40°C or lower
• the atmosphere in the heating furnace contains, in addition to 20.0 vol% or less of hydrogen, at least one of SO2 in an amount of 0.1 volppm or more and 3.0 volppm or less and HCl in an amount of 0.5 volppm or more and 10.0 volppm or less
• a method for producing a high-strength galvannealed steel sheet comprising: cooling a galvannealed layer from an alloying treatment to 250°C at a rate of 5°C/sec or more; and, after cooling to room temperature, performing either or both of the following steps
• high-strength galvannealed steel sheet with excellent delayed fracture resistance can be obtained due to improved release of diffusible hydrogen from steel.
• the high-strength galvannealed steel sheet obtained by the present invention is suitable for structural members such as automobile parts, and its application to this application can contribute to improved fuel efficiency by reducing the vehicle body weight.
• This invention targets high-strength galvannealed steel sheets that have a hot-dip galvanized layer on at least one side of the base steel sheet (base steel sheet) and are alloyed after hot-dip galvanizing.
• a "high-strength” steel sheet means that the tensile strength (TS) of the steel sheet measured in accordance with JIS Z2241 (2011) is 780 MPa or greater.
• base steel sheet The chemical composition of the base steel sheet (base steel sheet) and the reasons for its limitations are explained below.
• % representing the content of the component elements in the base steel sheet means “mass %” unless otherwise specified.
• Tensile strength is referred to as TS.
• Step sheet may also include hot-dip galvanized steel sheet and galvannealed hot-dip galvanized steel sheet.
• C 0.06% or more and 0.30% or less C has the effect of improving workability by forming martensite or the like as a steel structure, but in order to obtain good weldability, the C content must be 0.30% or less, and more preferably 0.25% or less. In order to obtain good workability, the C content must be 0.06% or more, and preferably 0.09% or more.
• Si 0.01% or more but less than 1.50%
• Si is an effective element for achieving high strength in steel sheets because it has a significant effect of increasing the strength of steel through solid solution (solid solution strengthening ability) without significantly impairing workability.
• Si is also an element that adversely affects the resistance weld cracking resistance characteristics of welded parts.
• a content of 0.01% or more is required.
• the Si content is 1.50% or more, the amount of Si concentrated on the surface of the substrate steel sheet during CGL annealing increases, and Si oxides that cause unplated defects are formed on the surface of the substrate steel sheet, making it difficult to achieve good galvanizability, so Si must be contained in a range of less than 1.50%. From this perspective, the Si content is preferably 0.80% or less, and more preferably 0.60% or less.
• Mn 1.5% or more and 3.5% or less
• Mn is an element that strengthens steel by solid solution strengthening, improves hardenability, and promotes the formation of retained ⁇ , bainite, and martensite. These effects are achieved by including 1.5% or more of Mn. Therefore, the Mn content must be 1.5% or more, and is preferably 1.8% or more. On the other hand, if the Mn content is 3.5% or less, the above effects can be obtained without increasing costs. Therefore, the Mn content must be 3.5% or less, and is preferably 3.3% or less.
• P 0.1% or less (excluding 0%) By suppressing the P content, it is possible to prevent a decrease in weldability, and further to prevent P from segregating at grain boundaries, thereby preventing deterioration of ductility, bendability, and toughness. Furthermore, if a large amount of P is contained, the ferrite transformation is promoted, which increases the grain size. Therefore, the P content must be 0.1% or less. There is no particular lower limit for the P content, but it is usually preferable to set it to 0.001% or more due to constraints on production technology.
• S 0.03% or less (excluding 0%) It is preferable to reduce the S content as much as possible. By suppressing the S content, deterioration in weldability can be prevented, and deterioration in ductility during hot rolling can be prevented, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, by suppressing the S content, deterioration in delayed fracture resistance, ductility, bendability, and stretch flangeability of the steel sheet due to the formation of coarse sulfides as an impurity element can be prevented. Since the problems caused by S become significant when the S content exceeds 0.03%, the S content must be 0.03% or less, and preferably 0.02% or less.
• the S content is preferably 0.01% or less, and more preferably 0.003% or less.
• the lower limit of the S content it is usually preferable to set it to 0.0001% or more due to constraints on production technology.
• N 0.007% or less (excluding 0%)
• N content 0.007% or less (excluding 0%)
• the N content must be set to 0.007% or less, preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less.
• the N content There is no particular lower limit for the N content, but it is usually preferable to set it to 0.0005% or more due to production technology constraints.
• Sol. Al 0.1% or less (excluding 0%) Since Al is thermodynamically most easily oxidized, it oxidizes before Si and Mn, and has the effect of suppressing the oxidation of Si and Mn in the outermost layer of the substrate steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is obtained when the sol. Al content is 0.01% or more. On the other hand, if the sol. Al content exceeds 0.1%, the cost increases. Therefore, when sol. Al is contained, the sol. Al content must be 0.1% or less. Although the lower limit of the sol. Al content is not particularly limited, it is preferably set to 0.001% or more because removing sol. Al at an impurity level also leads to increased costs. Also, as described above, the sol. Al content is preferably set to 0.01% or more.
• O 0.003% or less (excluding 0%)
• O is an element that forms oxide-based inclusions such as Al 2 O 3 , SiO 2 , CaO, MgO, (Al,Ca)—O, and (Si,Mn)—O in steel, and the generation of these inclusions deteriorates delayed fracture resistance.
• the O content must be 0.003% or less. There is no particular lower limit for the O content, but the lowest limit currently practical for industrial use is approximately 0.0005%.
• Mass ratio of Si to Mn is 0.25 or less
• Si-based oxides are more likely to form on the surface of the base steel sheet during annealing, making plating defects due to the oxides more likely to occur.
• the mass ratio of Si to Mn (Si/Mn) needs to be 0.25 or less.
• the mass ratio of Si to Mn (Si/Mn) is more preferably 0.20 or less.
• the steel sheet preferably further contains, by mass %, one or more components selected from the following groups A to D. The composition of the components of groups A to D and the effects of their inclusion will be explained below.
• Nb 0.05% or less (excluding 0%), Ti: 0.08% or less (excluding 0%), V: 0.2% or less (excluding 0%), W: 0.15% or less (excluding 0%), and Zr: 0.15% or less (excluding 0%)]
• Nb, Ti, V, W, and Zr are all elements effective in increasing the strength of the base steel sheet and can be added as needed.
• Nb is an element that can obtain a fine structure even when added in small amounts, and can achieve high strength without impairing toughness.
• Nb 0.05% or less (excluding 0%)
• the effect of improving strength can be obtained by including 0.005% or more of Nb, from the viewpoint of preventing an increase in cost, the Nb content is preferably 0.05% or less.
• Ti 0.08% or less (excluding 0%)
• Ti is an element effective for precipitation strengthening of steel. Although there is no particular limitation on the lower limit of Ti, it is preferably 0.005% or more in order to obtain the effect of adjusting strength. However, if Ti is added in excess, the hard phase becomes excessively large and formability deteriorates. Therefore, when Ti is contained, the Ti content is preferably 0.08% or less, and more preferably 0.05% or less. V: 0.2% or less (excluding 0%) When V is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing an increase in costs, when V is contained, the V content is preferably 0.2% or less.
• W 0.15% or less (excluding 0%)
• the W content is preferably 0.15% or less.
• Zr 0.15% or less (excluding 0%)
• Zr content is preferably 0.15% or less.
• Group B [one or more elements selected from Cr: 1.0% or less (excluding 0%), Ni: 1.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%), Mo: 1.0% or less (excluding 0%), Co: 1.0% or less (excluding 0%), and B: 0.005% or less (excluding 0%)] Cr, Ni, Cu, Mo, Co and B are all elements that improve the hardenability of the steel sheet.
• Cr 1.0% or less (excluding 0%)
• the Cr content is preferably set to 1.0% or less.
• Ni 1.0% or less (excluding 0%) When Ni is contained in an amount of 0.005% or more, it is possible to promote the formation of a residual ⁇ phase. However, from the viewpoint of preventing an increase in costs, when Ni is contained, the Ni content is preferably 1.0% or less.
• Cu: 1.0% or less (excluding 0%) Cu can promote the formation of residual ⁇ phase when it is contained in an amount of 0.005% or more, but from the viewpoint of preventing an increase in costs, when Cu is contained, the Cu content is preferably 1.0% or less.
• Mo 1.0% or less (excluding 0%) When Mo is contained in an amount of 0.005% or more, the effect of adjusting strength can be obtained, and this effect is particularly enhanced when the Mo content is 0.05% or more.
• the Mo content is preferably 1.0% or less.
• Co 1.0% or less (excluding 0%) Co is an element that is effective in improving stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet.
• the Co content is preferably 0.005% or more, and more preferably 0.010% or more.
• the Co content is preferably 1.0% or less.
• B 0.005% or less (excluding 0%) B is an element effective in improving the hardenability of steel.
• the B content is preferably 0.0003% or more, and more preferably 0.0005% or more.
• the B content is preferably 0.005% or less.
• C group [one or more elements selected from Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), and REM: 0.005% or less (excluding 0%)]
• Ca Mg and REM (rare earth elements) are all elements used as deoxidizers.
• Ca 0.005% or less (excluding 0%)
• the Ca content is preferably 0.005% or less.
• Mg 0.005% or less (excluding 0%)
• the morphology of sulfides can be controlled and ductility and toughness can be improved.
• the Mg content is preferably 0.005% or less.
• REM 0.005% or less (excluding 0%)
• the REM content is preferably 0.005% or less.
• Sn Group D [one or more elements selected from Sn: 0.2% or less (excluding 0%) and Sb: 0.2% or less (excluding 0%)]
• Sb and Sn are elements that suppress decarburization, denitrification, deboronization, etc., and are effective in suppressing a decrease in the strength of the steel sheet. Therefore, when Sb and Sn are contained, their contents are each set to more than 0%.
• Sn: 0.2% or less (excluding 0%) Sn is an element that is effective in suppressing denitrification, deboronation, etc., and thus suppressing a decrease in the strength of steel. To obtain such an effect, the Sn content is preferably 0.002% or more.
• the Sn content is preferably 0.2% or less.
• Sb 0.2% or less (excluding 0%) Sb can be added from the viewpoint of suppressing nitriding and oxidation of the surface of the substrate steel sheet, or decarburization of the surface of the substrate steel sheet in a region of several tens of microns caused by oxidation.
• Sb prevents a decrease in the amount of martensite formed on the surface of the substrate steel sheet, thereby improving the fatigue properties and surface quality of the substrate steel sheet.
• the Sb content is preferably 0.001% or more.
• the Sb content is preferably 0.2% or less.
• the base steel sheet preferably further contains, by mass %, one or more elements selected from the following groups E to H.
• groups E to H The chemical compositions of groups E to H and the effects of their inclusion will be explained below.
• Group E [Ta: 0.10% or less (excluding 0%)] Ta is an element effective in increasing the strength of the steel sheet and may be contained as needed. Ta can be effective in improving strength by containing 0.005% or more of Ta, but from the viewpoint of preventing an increase in costs, if Ta is contained, the Ta content is set to 0.10% or less.
• Te 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), and Hf: 0.10% or less (excluding 0%)
• Te 0.10% or less (excluding 0%)
• Te is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved.
• the Te content is set to 0.10% or less.
• Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness.
• Bi and Pb are contained, their respective contents should be more than 0%.
• Bi is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved.
• Bi also has the effect of improving machinability and improving the smoothness of the cut end surface, and has the effect of improving the delayed fracture resistance of the cut surface.
• the Bi content is set to 0.20% or less from the viewpoint of preventing an increase in cost.
• Pb 0.20% or less (excluding 0%)
• Pb is contained in an amount of 0.001% or more, it is possible to suppress grain boundary segregation and improve ductility and toughness.
• Pb also has the effect of improving machinability and improving the smoothness of cut edges, and has the effect of improving the delayed fracture resistance of cut edges.
• the Pb content is set to 0.20% or less in order to prevent an increase in costs.
• Zn, Ge, Sr, and Cs are elements that increase strength without significantly affecting mechanical properties or surface quality, and when Zn, Ge, Sr, or Cs is contained, each of them is set to exceed 0%.
• Ge 0.10% or less (excluding 0%) Even if Ge is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in cost, if Ge is contained, the Ge content is set to 0.10% or less. Sr: 0.10% or less (excluding 0%) Even if Sr is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Sr is contained, the Sr content is set to 0.10% or less. Cs: 0.10% or less (excluding 0%) Even if the Cs content is 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.
• the chemical composition of the base steel sheet (base steel sheet) consists of the remainder, other than the above-mentioned components, consisting of Fe and unavoidable impurities.
• a base steel sheet (cold-rolled steel sheet or hot-rolled steel sheet) having the above-described components is introduced into continuous hot-dip galvanizing equipment, and after continuous annealing in the equipment, hot-dip galvanizing is performed, and further alloying treatment is performed to obtain a galvannealed steel sheet.
• continuous hot-dip galvanizing equipment is composed of an annealing furnace and a hot-dip galvanizing device located downstream of the annealing furnace, and the hot-dip galvanizing device includes a hot-dip galvanizing bath and a snout connected to the steel strip outlet side of the annealing furnace and having a tip portion immersed in the hot-dip galvanizing bath.
• a general continuous hot-dip galvanizing line (CGL) configured to continuously perform a series of processes including heating, cooling, hot-dip galvanizing, and hot-dip galvanizing alloying treatment can be applied.
• the base steel sheet introduced into the continuous hot-dip galvanizing line is annealed while passing through an annealing furnace which is provided with a heating zone, a soaking zone, and a cooling zone in this order.
• the specific annealing conditions are as follows:
• the atmospheric dew point in the heating furnace in a temperature range where the temperature of the base steel sheet is 700°C or higher is set to -40°C or lower.
• the atmosphere in the heating furnace contains 20.0 vol% or less of hydrogen, as well as at least one of 0.1 volppm to 3.0 volppm of SO2 and 0.5 volppm to 10.0 volppm of HCl.
• ⁇ 40°C or lower At steel sheet temperatures of 700°C or higher, surface segregation due to the diffusion of Si and Mn increases.
• the atmospheric dew point is set to ⁇ 40°C or lower, the formation of internal oxides in the surface layer of the substrate steel sheet, i.e., at a depth of up to 100 ⁇ m from the surface of the substrate steel sheet, can be suppressed. This makes it possible to suppress cracks originating from oxides during severe deformation, thereby improving formability.
• the formation of Si oxides and Mn oxides on the surface of the substrate steel sheet can also be suppressed, thereby suppressing the occurrence of coating defects due to oxides.
• a temperature of ⁇ 45°C or lower is more preferable.
• Hydrogen concentration in the heating furnace atmosphere 20.0 vol% or less
• the hydrogen concentration in the heating furnace atmosphere during annealing must be 20.0 vol% or less. If the hydrogen concentration in the heating furnace atmosphere in the temperature range of 700°C or higher is too high, the amount of diffusible hydrogen in the steel increases, making hydrogen release difficult. As a result, there is a problem of reduced delayed fracture resistance due to hydrogen embrittlement. For these reasons, the hydrogen concentration in the temperature range of 700°C or higher must be 20.0 vol% or less. On the other hand, if the hydrogen concentration in the heating furnace atmosphere is 3.0 vol% or more, the surface of the substrate steel sheet is sufficiently reduced and activated, resulting in good galvanizability. Therefore, it is preferable that the hydrogen concentration in the heating furnace atmosphere be 3.0 vol% or more.
• this alloy phase has low ductility, cracks will form when a certain level of stress is applied to the plating layer, which can serve as the initiation point for crack initiation during plating.
• the grain size of the steel sheet surface layer decreases due to the presence of corrosive gases, an outburst reaction occurs, increasing the density of grain boundaries on the steel sheet surface.
• the formation interval of low-ductility alloy phases also decreases, which is thought to make the coating layer more susceptible to cracking due to tensile stress applied to the coating layer during cooling of the steel sheet after alloying, increasing the number of crack intersections.
• the improvement effect of this corrosive gas becomes apparent when SO2 is 0.1 ppm or more and HCl is 0.5 ppm or more.
• the SO2 concentration must be 0.1 vol ppm or more and 3.0 vol ppm or less, and the HCl concentration must be 0.5 vol ppm or more and 10.0 ppm or less.
• the remainder of the atmosphere in the heating furnace during annealing may contain gases such as nitrogen, CO, and CO2 .
• the concentrations of these trace amounts of corrosive gases such as SO2 and HCl can be controlled by adjusting the amount of gas containing these corrosive gases that is introduced directly into the furnace. Alternatively, the concentrations can be controlled by applying a liquid containing H2SO4 or HCl to the substrate steel sheet before it enters the furnace, or by diluting this liquid with water. In short, it is important to control the concentrations of trace amounts of corrosive gases such as SO2 and HCl, and the method for controlling the concentrations of these trace amounts of corrosive gases such as SO2 and HCl is not limited to the above-mentioned method.
• the substrate steel sheet it is preferable to anneal the substrate steel sheet at a maximum temperature of 700°C or higher and 900°C or lower in order to recrystallize the strain imparted by rolling.
• a maximum temperature of 700°C or higher iron oxide on the surface of the substrate steel sheet is sufficiently reduced, resulting in a good plating appearance, so it is preferable to set the maximum temperature of the substrate steel sheet to 700°C or higher.
• the temperature by setting the temperature to 900°C or lower, it is possible to suppress surface segregation of Si, Mn, and Cr, resulting in a good plating appearance, so it is preferable to set the maximum temperature of the substrate steel sheet to 900°C or lower.
• the base steel sheet that has been continuously annealed under the above conditions is cooled and then immersed in a hot-dip galvanizing bath to undergo hot-dip galvanizing.
• the cooling temperature is preferably 200 to 520°C, and the sheet is preferably heated as needed before being immersed in the hot-dip galvanizing bath.
• the bath temperature of the hot-dip galvanizing bath is generally approximately 440 to 500°C.
• the hot-dip galvanizing bath is not particularly limited, but may, for example, contain an Al content of 0.10% by mass or more and 0.23% by mass or less, and further contain one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total content of 0% by mass or more and 3.5% by mass or less, with the remainder consisting of Zn and unavoidable impurities.
• the temperature of the base steel sheet before coating be equal to or higher than the coating bath temperature but not higher than the coating bath temperature + 50°C.
• a galvannealing treatment is further performed to form an alloyed hot-dip galvanized layer.
• the alloying treatment is preferably performed in a temperature range of 480°C or higher and 570°C or lower. If the alloying temperature is lower than 480°C, the Zn-Fe alloying rate becomes excessively slow, making alloying extremely difficult. On the other hand, if the alloying temperature exceeds 570°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and El.
• the alloying treatment is more preferably performed in a temperature range of 490°C or higher and 560°C or lower, and even more preferably in a temperature range of 490°C or higher and 530°C or lower.
• the coating weight of the galvannealed steel sheet (GA) is 20 to 120 g/ m2 per side. The coating weight can be adjusted by performing gas wiping or the like after hot-dip galvanizing.
• the steel sheet is cooled to 50°C or below. Furthermore, the average cooling rate from the alloying temperature to 250°C is set to 5°C/s or more. The reason for this will be explained below.
• Average cooling rate 5°C/s or more. This is one of the important requirements of the present invention. By setting the average cooling rate from the end of the alloying treatment to 250°C to 5°C/s or more, the number of crack intersections in the coating required for releasing diffusible hydrogen from the steel can be obtained. Because the thermal expansion coefficient of the galvannealed hot-dip coating layer is higher than that of the base steel sheet, tensile stress is generated in the coating layer during cooling, which is thought to cause cracks to form in the coating of the galvannealed hot-dip coating layer.
• the average cooling rate is 5°C/s or more
• the temperature difference between the surface coating layer and the base steel sheet increases, sufficiently increasing the tensile stress in the coating layer, accelerating crack formation and achieving the number of crack intersections required for hydrogen release.
• the release of diffusible hydrogen from the steel is improved, and the amount of diffusible hydrogen in the steel can be reduced to 0.30 mass ppm.
• the average cooling rate is less than 5°C/s
• the average cooling rate is preferably 7°C/s or more, and more preferably 10°C/s or more.
• the galvannealed steel sheet cooled to 50°C or less may be rolled at a predetermined elongation rate.
• the elongation rate in this rolling is preferably 0.05% or more and 1.00% or less.
• the elongation rate in this rolling is more preferably 0.70% or less and 0.10% or more.
• the rolling may be performed online in an apparatus connected to the continuous hot-dip galvanizing facility, or may be performed offline from the continuous hot-dip galvanizing facility.
• the target elongation (e.g., 0.05% to 1.00%) may be achieved in a single rolling run, or the target elongation may be achieved by performing multiple rolling runs.
• temper rolling is generally carried out, but rolling by a method such as processing with a leveler may also be used as long as it can impart an elongation rate equivalent to that of temper rolling.
• the galvannealed steel sheet is either held at room temperature for 48 hours or more to release the diffusible hydrogen in the steel sheet, or reheated and held as described below, or both. Holding at room temperature for a certain period of time or more allows the diffusible hydrogen in the steel sheet to be released through cracks in the plating, improving delayed fracture resistance.
• the holding time must be 48 hours or more.
• the holding time is more preferably 96 hours or more, and even more preferably 144 hours or more. There is no particular upper limit to the holding time, but from the perspective of productivity, it is preferable that it be 1,800 hours or less.
• room temperature refers to a temperature between 0°C and 40°C.
• the temperature must be 50°C or higher, preferably 80°C or higher.
• temperatures above 400°C may remelt the coating layer, resulting in deterioration of the appearance. Therefore, the temperature must be 400°C or lower, preferably 200°C or lower.
• the temperature is the maximum temperature reached during the heat retention process, and may be constant or variable as long as it is within the range of 50°C to 400°C.
• the heat retention time must be 0.1 hour or longer to fully achieve the effect of promoting the release of diffusible hydrogen from the steel sheet.
• the heat retention time is preferably 0.5 hour or longer, and more preferably 1.0 hour or longer. There is no particular upper limit to the heat retention time, but from the perspective of productivity, it is preferable that it be 48 hours or less.
• the high-strength galvannealed steel sheet obtained by the above-described manufacturing method will be described.
• the steel sheet of the present invention produced by the above-described method can have a total amount of hydrogen released at 25° C. or higher and 300° C. or lower of 0.30 mass ppm or less when measured by temperature ramp analysis. As a result, a high-strength galvannealed steel sheet having excellent delayed fracture resistance can be obtained.
• the high-strength galvannealed steel sheet produced in the present invention has a TS of 780 MPa or more. Furthermore, when further increasing the strength, the TS can be made 980 MPa or more.
• the TS is measured in accordance with JIS Z2241 as follows. JIS No. 5 test pieces are taken from the hot-dip galvanized steel sheet so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using this test piece, a tensile test is performed at a crosshead displacement rate Vc of 1.67 ⁇ 10 -1 mm/s, and the TS is measured.
• the structure of the steel sheet is not particularly limited, but in order to ensure a tensile strength of 780 MPa or more, it is preferable that the steel sheet have the following structure:
• the steel sheet structure described below is the steel structure in the range of 1/8 to 3/8 thickness depth from the surface of the steel sheet. That is, it is preferable that the steel sheet structure has a total area ratio of martensite, bainite and retained ⁇ (retained austenite) of 30% or more, thereby obtaining a steel sheet having a tensile strength of 780 MPa or more.
• a steel sheet having a tensile strength of 980 MPa or more can be obtained, and by setting the total area ratio of martensite, bainite, and retained ⁇ to 70% or more, a steel sheet having a tensile strength of 1180 MPa or more can be obtained.
• a steel sheet having a tensile strength of 1310 MPa or more can be obtained, and by setting the total area ratio of martensite, bainite, and retained ⁇ to 90% or more, a steel sheet having a tensile strength of 1470 MPa or more can be obtained.
• the steel structure can be identified by etching the polished cross section of the steel sheet with acid or the like, followed by observation with an optical microscope or a scanning electron microscope (SEM). Because the structure of high-strength steel sheets is complex and fine, it is preferable to use an SEM, which allows for more detailed observation of the microstructure.
• the steel sheet can be polished by cutting the cross section, embedding it in resin, mechanically polishing it with abrasive paper, or the like, and then finish-polishing it with diamond paste or oxide particles. Furthermore, finish-polishing may be performed by electrolytic polishing or ion polishing. Furthermore, etching with nital at a concentration of 1 vol% to 5 vol% can produce steps on the cross section suitable for observing the steel structure, so the nital concentration is preferably 1 vol% to 5 vol%. Because the structure of high-strength steel sheets is fine, it is preferable to perform SEM observation at a magnification of 1000 to 3000 times.
• the number of fields of view of the SEM is preferably 10 or less.
• the high-strength galvannealed steel sheet of the present invention has a zinc coating layer on the surface of a substrate steel sheet with a coating weight per side of 20 g/ m2 or more and 120 g/ m2 or less. If the coating weight is less than 20 g/ m2 , not only is corrosion resistance likely to decrease, but it is also difficult to control the coating weight. On the other hand, if the coating weight per side exceeds 120 g/ m2 , coating adhesion is likely to decrease.
• the high-strength galvannealed steel sheet of the present invention has a good surface appearance according to the evaluation criteria described in the examples.
• the oxygen content of the surface layer portion of the substrate steel sheet directly below the galvanized layer, within 100 ⁇ m from the surface of the substrate steel sheet in the sheet thickness center direction, as measured by the method described in the Examples, is less than 0.030 g/ m2 per side.
• the oxygen content in the surface layer portion of the substrate steel sheet is derived from oxides present in the surface layer portion of the substrate steel sheet.
• a large amount of oxide present in the surface layer portion of the substrate steel sheet means that a large amount of oxide is formed on the surface of the substrate steel sheet, and coating defects caused by oxides occur. Therefore, from the viewpoint of suppressing coating defects, the oxygen content needs to be less than 0.030 g/ m2 per side.
• the oxygen content is preferably less than 0.020 g/ m2 per side.
• the high-strength galvannealed steel sheet of the present invention has a crack intersection number of 500 or more per mm2 in a plane parallel to the surface of the galvanized steel sheet at a depth M [ ⁇ m] (1 ⁇ M ⁇ 5 ) from the surface of the galvanized layer, which is an important requirement of the present invention.
• the number of intersections is measured by reducing the thickness of the galvanized layer from the surface to 1 ⁇ m to 5 ⁇ m and then observing the surface, which makes it possible to observe the morphology of cracks that are likely to be connected to the surface of the substrate steel sheet. If the thickness is reduced to more than 5 ⁇ m, the substrate steel sheet may be exposed due to the influence of unevenness on the surface of the substrate steel sheet.
• the thickness reduction of the plating layer can be performed by measuring the thickness of the sample piece before, during, and after polishing, and mechanically polishing the sample piece so that the reduced thickness is 1 ⁇ m to 5 ⁇ m. The reduced thickness is calculated from the difference in the thickness of the sample piece before and after polishing.
• the cracks have a large number of intersections and are sufficiently connected, even if some of the cracks are crushed, hydrogen can be released from the remaining cracks, and good hydrogen release properties can be achieved.
• the cracks collapse, eliminating paths for hydrogen release, which is thought to lead to deterioration of hydrogen desorption properties.
• the number of crack intersections is preferably 700 points/ mm2 or more, and more preferably 1000 points/ mm2 or more.
• the high-strength galvannealed steel sheet according to the present invention has a diffusible hydrogen content of 0.30 ppm by mass or less. If the diffusible hydrogen content in the steel exceeds 0.30 ppm by mass, delayed fracture resistance becomes insufficient, so the diffusible hydrogen content in the steel needs to be 0.30 ppm by mass or less.
• the diffusible hydrogen content in the steel is preferably 0.15 ppm by mass or less, and more preferably 0.05 ppm by mass or less.
• the thickness of the hot-dip galvanized steel sheet produced in this invention is not particularly limited, but is typically approximately 0.3 mm or more and 2.8 mm or less.
• the steel sheets were cooled under the conditions shown in Table 2, and some samples were reheated and heat-retained.
• the heat-retaining temperature during this process was kept constant at the maximum temperature.
• the surface of the galvannealed layer of the thus obtained galvannealed steel sheet was polished to a depth of 1 ⁇ m to 5 ⁇ m from the surface of the galvannealed layer, and the number of intersections of cracks in the galvannealed layer observed in the observed image was counted.
• the thickness of the galvannealed layer, the amount of diffusible hydrogen in the steel sheet, and the delayed fracture resistance were measured using the measurement and evaluation methods described below. Furnace gas analysis, and evaluation of the tensile strength and structure of the steel sheet were also performed. The above results, along with the manufacturing conditions, are shown in Table 2.
• furnace gas was sampled from the annealing furnace, and the amounts of SO 2 and HCl were determined by ion chromatography. The analysis was carried out three times, and the average value was taken as the furnace gas concentration.
• a 10 mm x 10 mm sample was cut from the width center of the galvannealed steel sheet obtained by the above method. The sample was embedded in resin so that the plated surface was perpendicular, and polished with waterproof abrasive paper. It was then finish-polished with 1 ⁇ m diamond abrasive grains. After polishing, the sample was etched with 0.05% nital for 30 seconds, and the cross section was observed using an SEM at 400x magnification. For SEM observation, secondary electron images were acquired at an accelerating voltage of 10 kV. The observation field consisted of five consecutive fields from the center of the sample, and the plating thickness was measured at six equal positions (five locations) in each field. The average thickness value of all 25 locations obtained by the above procedure was taken as the thickness of the zinc plating layer.
• a 20 mm x 20 mm sample was cut from the width center of the galvannealed steel sheet and degreased with alcohol.
• the plating surface was then mechanically polished with diamond abrasive grains of 3 ⁇ m diameter, followed by finish polishing with diamond abrasive grains of 1 ⁇ m diameter and 0.25 ⁇ m diameter diamond abrasive grains to reduce the plating layer thickness.
• the thickness reduction was calculated from the difference in thickness of the sample before and after polishing, and adjusted so that the thickness reduction of the plating layer after polishing was in the range of 1 ⁇ m to 5 ⁇ m.
• etching was performed with 1% nital for 30 seconds, and SEM observation was performed at 500x magnification.
• SEM observation backscattered electron images were observed at an accelerating voltage of 15 kV.
• the observation field was 9 consecutive fields of view (3 fields x 3 fields) from the center of the sample, and the number of crack intersections in the plating was counted in each field.
• a crack intersection was defined as a point where three or more cracks originated from one point, and the number of intersections per area (mm 2 ) was calculated by dividing the sum of the number of intersections in the nine fields of view by the area of the observation field.
• Measurement of diffusible hydrogen content in steel sheet A rectangular test piece measuring 30 mm in major axis length and 5 mm in minor axis length was taken from the width center of a galvannealed steel sheet. The coating layer of the test piece was removed using a router. Hydrogen analysis was performed using a temperature-programmed hydrogen release analyzer under conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C/hour. The amount of released hydrogen (ppm by mass), which is the amount of hydrogen released from the surface of the test piece at each temperature, was measured. The sum of the amounts of hydrogen detected at 300°C or less was taken as the diffusible hydrogen content in the steel sheet.
• a diffusible hydrogen content in the steel of 0.05 ppm by mass or less was evaluated as excellent ( ⁇ )
• a diffusible hydrogen content of more than 0.05 ppm by mass and less than 0.15 ppm by mass was evaluated as good ( ⁇ )
• a diffusible hydrogen content of more than 0.15 ppm by mass and less than 0.30 ppm by mass was evaluated as fair ( ⁇ ).
• x the amount of diffusible hydrogen in steel exceeds 0.30 ppm by mass, the delayed fracture resistance property is often reduced, and therefore, steels having a diffusible hydrogen content of more than 0.30 ppm by mass were rated as poor ("x").
• the time from taking the strip-shaped test specimens from the galvannealed steel sheet to starting the delayed fracture tensile test was limited to within 10 minutes.
• the loading time in the tensile test was a maximum of 100 hours.
• the maximum stress at which no cracks here, "cracks” refers to fractures under tensile stress loading) occurred after 100 hours of loading was defined as the critical stress, and the delayed fracture resistance was evaluated by the ratio of the critical stress to the yield stress.
• the evaluation criteria for delayed fracture resistance were as follows: when critical stress/yield stress was 1.10 or more, it was given an excellent " ⁇ ", when it was less than 1.10 and 1.05 or more, it was given a good " ⁇ ", when it was less than 1.05 and 1.00 or more, it was given a fair " ⁇ ", and when it was less than 1.00, it was given a poor " ⁇ ". Note that the delayed fracture resistance evaluated in the delayed fracture test is generally lower (disadvantageous) for steel sheets with higher strength.
• the oxygen concentration in the steel throughout the entire thickness direction of the high-strength steel sheet after continuous annealing was measured, and this measured value was used as the amount of oxygen after oxidation, OI.
• OI amount of oxygen after oxidation
• OH amount of oxygen originally contained in the raw material
• the rating was excellent ( ⁇ ); when scale patterns were observed but no uncoated defects were observed, the rating was good ( ⁇ ); when scale patterns and fewer than five uncoated defects larger than 0.5 mm were observed, the rating was fair ( ⁇ ); and when five or more uncoated defects larger than 0.5 mm were observed, the rating was poor ( ⁇ ).
• the total area ratio of martensite, bainite, and retained ⁇ in the base steel sheet structure was measured as follows. A sample was cut out so that the observation surface was a thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet. This observation surface was polished with diamond paste and then finish-polished with alumina. The observation surface of the sample was then etched with 3 vol% nital to reveal the structure. The steel structure was observed at a depth of 1/8 to 3/8 of the sheet thickness on this sample observation surface, and five fields of view were observed at a magnification of 3000x using an SEM.
• the total area of martensite, bainite, and retained ⁇ was determined from the obtained structure image, and the area ratio was calculated by dividing this total area by the measured area for the five fields of view. The average of these values was used as the total area ratio of martensite, bainite, and retained ⁇ .
• the martensite, bainite, retained ⁇ , and other microstructures were identified as follows. Martensite There are two types of martensite: tempered martensite and fresh martensite. Tempered martensite is a gray or dark gray area that is close to black in an SEM photograph. Tempered martensite has a blocky morphology with boundaries at the interfaces with other structures such as prior ⁇ grain boundaries and ferrite.
• tempered martensite may contain other structures such as bainite inside, resulting in a concave shape. Tempered martensite contains many carbides inside, but depending on the plane orientation, there may be only a small amount of carbides.
• Fresh martensite Fresh martensite is the gray or white area in the SEM photograph. It is in the form of blocks, granules, plates, or films and does not contain carbides.
• Bainite Bainite is the dark gray region in the SEM photograph. Bainite appears as films, plates, or blocks formed by connecting some or all of these adjacent regions, and contains a small amount of carbides. Bainite that has been tempered after formation may contain coarse carbides.
• Bainitic ferrite contains almost no carbides inside and has mechanical properties similar to ferrite, so it belongs to the ferrite group. Ferrite may contain granular or massive fresh martensite, granular or massive retained ⁇ , or both. Fresh martensite and retained ⁇ contained in ferrite are not included in the ferrite area fraction, but are treated as the area fraction of fresh martensite or retained ⁇ . Carbides Carbides are the white areas in SEM photographs. Carbides are in the form of granules or films. Carbides are mainly formed as fine particles inside ferrite, martensite, and bainite, but their area ratios are small and can be ignored.
• the area ratio of carbides is not excluded from the area ratio of each structure containing carbides, but is included in the area ratio of each structure.
• Structures other than those mentioned above Each of the above structures may contain nitrides such as TiN, carbonitrides such as ( Nb, Ti)(C, N), sulfides such as MnS and CaS, and oxides such as Al2O3 and SiO2 , in a total area ratio of a few percent. Since the area ratios of these are small and can be ignored, the area ratios of these nitrides, carbonitrides, sulfides, or oxides are included in the area ratio of each structure containing them. Pearlite may also be included. In the case of pearlite, the area ratio of pearlite is calculated separately.
• the galvannealed steel sheets of the present invention have a beautiful surface appearance with no uncoated areas, and also have excellent release properties of diffusible hydrogen in the steel sheet and delayed fracture resistance.

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