Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
  • B22D11/124—Accessories for subsequent treating or working cast stock in situ for cooling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/10—Spot welding; Stitch welding
  • B23K11/11—Spot welding
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
  • C22C38/14—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
  • C22C38/60—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
  • C23C2/02—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
  • C23C2/04—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
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
  • C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel

Definitions

• the present invention relates to a steel plate, particularly a steel plate that can be suitably used as a material for structural parts of automobiles and the like.
• the present invention also relates to a resistance spot welding method using the steel plate, a resistance spot welded component, and a method for manufacturing the steel plate.
• resistance welding resistance spot welding
• TSS Tensile shear strength
• CTS cross tension strength
• L-shape tension strength L-shape tension strength
• steel plates used in automobile parts and the like are required to have excellent delayed fracture resistance after resistance welding to prevent delayed fracture caused by hydrogen penetrating from the usage environment.
• a high tensile strength of 1600 MPa or more it is necessary to add a large amount of alloying elements, but the addition of alloying elements leads to a deterioration of delayed fracture resistance after resistance welding. For this reason, it has been difficult to achieve both high strength and excellent delayed fracture resistance after resistance welding.
• Patent Document 1 proposes a method of improving the peeling strength of the welded joint by inserting an insert plate having specific dimensions between multiple overlapping steel plates when they are resistance spot welded together.
• Patent Document 2 also proposes a method of improving the strength of a weld by performing resistance spot welding to form the weld and then performing a post-current application.
• Patent Documents 1 and 2 do not take the problem of delayed fracture into consideration.
• Patent Document 1 requires a process of preparing an insert that meets certain conditions and inserting it between the steel sheets when welding, which results in poor productivity.
• the method of Patent Document 2 also requires post-current application after welding, which results in poor productivity. In order to obtain excellent properties without requiring additional processes that reduce productivity, it is necessary to improve the resistance weldability of the steel sheet itself.
• the present invention was made in consideration of the above situation, and aims to provide a steel plate that has a tensile strength of 1600 MPa or more, high L-shaped tensile strength after resistance welding, and also has excellent delayed fracture resistance after resistance welding.
• the tensile strength in an L-shaped tensile test can be improved by changing the microstructure in the corona bond part around the nugget. Specifically, softening the corona bond part improves resistance to crack propagation, resulting in high tensile strength in an L-shaped tensile test.
• the above-mentioned changes in the microstructure in the corona bond area also have the effect of improving delayed fracture resistance after resistance welding. Therefore, in order to improve delayed fracture resistance after resistance welding, it is necessary to form a sufficient amount of ferrite in the region 7 to 12 ⁇ m from the surface in the plate thickness direction.
• the present invention is based on the above findings and has the following gist:
• the composition, in mass%, is C: 0.22-0.38%, Si: 0.05-1.35%, Mn: 2.4 to 3.5%, P: 0.02% or less, S: 0.002% or less, Al: 0.01-0.10%, N: 0.008% or less, B: 0.0002 to 0.0050%, and at least one selected from the group consisting of Ti: 0.005 to 0.07%, Nb: 0.005 to 0.07%, and V: 0.005 to 0.07%,
• the balance is Fe and unavoidable impurities,
• the microstructure at the 1/4 position of the plate thickness is In terms of volume fraction, Ferrite: 0-5%, Retained austenite: 0 to 5% Bainite: 0-7%; and Martensite: 93% or more; Average grain size of ferrite: 3 ⁇ m or less, Average grain size of retained austenite: 3 ⁇ m or less, The average grain size of bainite is 5 ⁇ m or less, and the average grain size of martensite is 7 ⁇ m or less.
• the microstructure in the region 7 to 12 ⁇ m from the surface in the plate thickness direction is The volume fraction of ferrite is 30% or more,
• the average grain size of ferrite is 10 ⁇ m or less,
• a steel plate in which the average number density of carbides having a grain size of 0.10 ⁇ m or more in a region 50 to 100 ⁇ m from the surface in the plate thickness direction is 5 particles/100 ⁇ m2 or more.
• composition of the components is, in mass%, Sb: 0.02% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Sn: 0.30% or less,
• a resistance spot welding method in which a plate assembly including at least one steel plate described in any one of 1 to 3 above is clamped between a pair of welding electrodes and joined by passing electricity through the plate assembly while applying pressure.
• Resistance spot welded components that include at least one steel plate according to any one of 1 to 3 above in a plate assembly
• the cooled hot-rolled steel sheet is coiled at a coiling temperature of 460° C. or less.
• the hot-rolled steel sheet after coiling is subjected to pickling.
• the pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700°C to obtain a heat-treated hot-rolled steel sheet;
• the heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the cold-rolled steel sheet is annealed under conditions in which the dew point in the temperature range of 600 to 980 ° C.
• the cold-rolled steel sheet is Average heating rate: 12° C./s or more to 650° C. Heat to an annealing temperature of 830 to 980°C at an average heating rate of less than 12°C/s; Holding at the annealing temperature for a holding time of 20 to 360 seconds, The steel sheet is cooled from the annealing temperature to room temperature at an average cooling rate of 3° C./s or more.
• the cooled hot-rolled steel sheet is coiled at a coiling temperature of 460° C. or less.
• the hot-rolled steel sheet after coiling is subjected to pickling.
• the pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700°C to obtain a heat-treated hot-rolled steel sheet;
• the heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the cold-rolled steel sheet is annealed under conditions in which the dew point in the temperature range of 600 to 980 ° C.
• the annealed cold-rolled steel sheet is subjected to hot-dip galvanization to form a zinc-based plating layer on at least one surface of the steel sheet;
• a method for producing a steel sheet comprising cooling the steel sheet after hot-dip galvanization to room temperature at an average cooling rate of 3 ° C./s or more, In the annealing, the cold-rolled steel sheet is Average heating rate: 12° C./s or more to 650° C.
• a method for producing a steel sheet comprising cooling the steel sheet from the annealing temperature to immersion in a hot-dip galvanizing bath at an average cooling rate of 3 ° C./s or more.
• the present invention provides a steel plate that has a tensile strength of 1600 MPa or more, high L-shaped tensile strength after resistance welding, and also has excellent delayed fracture resistance after resistance welding.
• the L-shaped tensile strength and delayed fracture resistance are sometimes collectively referred to as resistance weldability.
• the steel sheet of the present invention has the above-mentioned composition. The reasons for limiting the composition will be explained below.
• C 0.22-0.38%
• C is an element effective in increasing the strength of steel sheets, and in the present invention, it also contributes to the formation of martensite. Furthermore, C is also a component that forms carbides, which are one of the important elements in the present invention. If the C content is less than 0.22%, the necessary strength and martensite volume fraction cannot be ensured. Therefore, the C content is 0.22% or more, preferably 0.23% or more, and more preferably 0.24% or more. On the other hand, if the C content is excessively high, the toughness of the nugget after resistance welding decreases, and as a result, the L-shaped tensile strength decreases. Therefore, the C content is 0.38% or less, preferably 0.34% or less, and more preferably 0.33% or less.
• Si 0.05-1.35% Si is an element that has the effect of improving resistance weldability. This is because the addition of Si alleviates the segregation of Mn, and as a result, the variation in hardness in the thickness direction of the steel plate is alleviated.
• the Si content is set to 0.05% or more, preferably 0.15% or more, and more preferably 0.25% or more.
• the Si content is set to 1.35% or less, preferably 1.25% or less, and more preferably 1.15% or less.
• Mn 2.4-3.5%
• Mn is an element that has the effect of improving the strength of the steel sheet by stabilizing martensite formation and solid solution strengthening.
• Mn has the effect of stabilizing austenite, and is an element necessary for ensuring the volume fraction of martensite.
• Mn is set to 2.4% or more.
• the toughness of the nugget after spot welding decreases, and as a result, the L-shaped tensile strength decreases.
• Mn content is set to 3.5% or less, preferably 3.2% or less.
• the P content is set to 0.02% or less, preferably 0.015% or less, and more preferably 0.012% or less.
• the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction increases the steelmaking cost. Therefore, the P content is preferably 0.002% or more.
• S 0.002% or less
• Excessive S reduces resistance weldability. This is because the amount of sulfides such as MnS produced increases with an increase in S content, and cracks are generated from the sulfides when hydrogen penetrates. Therefore, the S content is set to 0.002% or less, preferably 0.0015% or less, and more preferably 0.0012% or less.
• the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction increases the steelmaking cost. Therefore, the S content is preferably set to 0.0002% or more.
• Al 0.01 ⁇ 0.10%
• Al is an element necessary for deoxidation. If the Al content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Al content is set to 0.01% or more, preferably 0.02%. On the other hand, if the Al content is higher than 0.10%, the ferrite phase is excessively generated during annealing, making it difficult to ensure strength. Therefore, the Al content is set to 0.10% or less, preferably 0.08% or less, more preferably 0.05% or less.
• N 0.008% or less
• the N content is 0.008% or less, preferably 0.007% or less, and more preferably 0.005% or less.
• the lower limit of the N content is not particularly limited and may be 0%. However, excessive reduction increases the steelmaking cost. Therefore, the N content is preferably 0.0005% or more, and more preferably 0.001% or more.
• B 0.0002-0.0050%
• B is an element that improves hardenability and contributes to high strength by forming martensite.
• B is useful for forming carbides because it improves hardenability without lowering the martensite transformation starting point.
• the B content is set to 0.0002% or more, preferably 0.0004% or more.
• the B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0035% or less.
• composition of the steel plate of the present invention contains at least one element selected from the group consisting of Ti, Nb, and V, each in the amounts listed below.
• Ti, Nb, and V are all elements that have the common function of improving resistance weldability by forming fine carbides.
• Ti 0.005-0.07%
• Ti improves delayed fracture resistance after resistance welding by forming fine carbides. Furthermore, Ti improves delayed fracture resistance after resistance welding by increasing hydrogen overvoltage.
• the Ti content is set to 0.005% or more, preferably 0.008% or more.
• the Ti content is set to 0.07% or less, preferably 0.05% or less.
• Nb 0.005-0.07%
• Nb improves the delayed fracture resistance of resistance spot welds by forming fine carbides.
• the Nb content is set to 0.005% or more, preferably 0.01% or more, in order to obtain the above effect.
• the Nb content is set to 0.07% or less, preferably 0.05% or less.
• V 0.005-0.07%
• V improves the delayed fracture resistance of resistance spot welds by forming fine carbides.
• the V content is set to 0.005% or more in order to obtain the above effect.
• the strength increase effect is small for an amount exceeding 0.07%, and the alloy cost also increases. Therefore, the V content is set to 0.07% or less, preferably 0.06% or less, and more preferably 0.05% or less.
• the steel sheet in one embodiment of the present invention has a composition containing the above components with the remainder being Fe and unavoidable impurities.
• the inevitable impurities include, for example, Co, Zn, Ta, Mg, and Zr.
• Co is included as the inevitable impurity
• the Co content is preferably 0.10% or less.
• Zn is included as the inevitable impurity
• the Zn content is preferably 0.10% or less.
• Ta is included as the inevitable impurity
• the Ta content is preferably 0.10% or less.
• Mg is included as the inevitable impurity
• Zr is included as the inevitable impurity, the Zr content is preferably 0.10% or less.
• composition of the steel sheet in another embodiment of the present invention may optionally contain at least one of the following elements in addition to the above-mentioned elements. Note that all of the following elements can be added optionally, and therefore their inclusion is not essential. Therefore, the lower limit of the content may be 0%.
• Sb 0.02% or less
• Sb is an element that has the effect of strengthening grain boundaries by segregating at the grain boundaries.
• the cross tensile strength can be further improved.
• the Sb content is higher than 0.02%, the formation of ferrite phase in the surface layer of the steel sheet is suppressed, and the microstructure in the surface layer cannot be made as desired. Therefore, when Sb is added, the Sb content is 0.02% or less, preferably 0.015% or less.
• the lower limit of the Sb content is not particularly limited, but from the viewpoint of enhancing the effect of adding Sb, the Sb content is preferably 0.001% or more, more preferably 0.002% or more.
• Cu 0.50% or less
• Cu is an element that has the effect of increasing hydrogen overvoltage and, as a result, further improving delayed fracture resistance after resistance welding.
• the Cu content exceeds 0.50%, the effect is saturated and surface defects are likely to occur. Therefore, when Cu is added, the Cu content is set to 0.50% or less.
• the Cu content is preferably set to 0.005% or more.
• Ni 0.50% or less
• Ni is an element that has the effect of increasing hydrogen overvoltage and further improving delayed fracture resistance.
• Ni when Ni is added together with Cu, it has the effect of suppressing surface defects caused by Cu.
• the Ni content exceeds 0.50%, the effect is saturated. Therefore, when Ni is added, the Ni content is set to 0.50% or less.
• the Ni content is set to 0.005% or more.
• Cr 0.50% or less Cr is an element that contributes to further increasing strength by generating a hard phase. However, if the Cr content exceeds 0.50%, surface defects are likely to occur. Therefore, when Cr is added, the Cr content is set to 0.50% or less, preferably 0.45% or less. On the other hand, from the viewpoint of enhancing the effect of adding Cr, the Cr content is preferably set to 0.02% or more, more preferably 0.05% or more.
• Mo 0.50% or less
• Mo is an element that contributes to further strengthening by generating a hard phase.
• a part of Mo also contributes to further strengthening by generating carbides.
• the Mo content is set to 0.50% or less, preferably 0.45% or less.
• the Mo content is preferably set to 0.02% or more, more preferably 0.05% or more.
• Sn 0.30% or less
• Sn is an element that increases the hydrogen overvoltage of the steel sheet, thereby further improving the delayed fracture resistance.
• the Sn content is set to 0.30% or less, preferably 0.25% or less.
• the Sn content is preferably set to 0.005% or more, more preferably 0.01% or more.
• Ca 0.0050% or less
• Ca is an element that contributes to further improving delayed fracture resistance after resistance welding by making the shape of sulfides spheroidal.
• the Ca content exceeds 0.0050%, the effect is saturated. Therefore, when Ca is added, the Ca content is set to 0.0050% or less.
• the Ca content is set to 0.0005% or more.
• REM 0.0050% or less
• REM rare earth metal
• the REM content is set to 0.0050% or less.
• the REM content is preferably set to 0.0005% or more.
• the microstructure at the 1/4 position of the plate thickness and the microstructure at a position in the region 7 to 12 ⁇ m from the surface of the steel plate in the plate thickness direction must each satisfy specific conditions. The reasons for this are explained below.
• the "1/4 position of the plate thickness” refers to a position at a depth of 1/4 of the plate thickness t of the steel plate from the surface of the steel plate, and may also be expressed as the 1/4t position.
• tempered martensite is also defined as being included in “martensite”. This is because it is difficult to distinguish between martensite and tempered martensite in the microstructure of the steel plate of the present invention.
• tempered martensite includes not only tempered martensite formed by self-tempering during the cooling process in annealing, but also tempered martensite formed by cooling to room temperature and then performing a tempering treatment.
• the volume fraction of ferrite at the 1/4 position of the sheet thickness is set to 5% or less, preferably 3% or less, and more preferably 1% or less.
• the lower the volume fraction of ferrite the better, so the lower limit of the volume fraction of ferrite is set to 0%.
• Average grain size of ferrite 3 ⁇ m or less If the average grain size of ferrite at the 1/4 position of the plate thickness is greater than 3 ⁇ m, the delayed fracture resistance property is deteriorated. This is because cracks due to hydrogen embrittlement are likely to occur at the interface between ferrite and martensite from the HAZ softened part to the base material after resistance welding. Therefore, the average grain size of ferrite at the 1/4 position of the plate thickness is set to 3 ⁇ m or less, preferably 2.5 ⁇ m or less. On the other hand, from the viewpoint of delayed fracture resistance, the smaller the average grain size of ferrite, the more preferable it is, so the lower limit of the average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, the average grain size of ferrite is preferably set to 0.2 ⁇ m or more, more preferably 0.5 ⁇ m or more, and even more preferably 1.0 ⁇ m or more.
• the volume fraction of the retained austenite at the 1/4 position of the plate thickness is set to 5% or less, preferably 4% or less.
• Average grain size of retained austenite 3 ⁇ m or less If the average grain size of retained austenite at the 1/4 position of the sheet thickness is greater than 3 ⁇ m, the delayed fracture resistance property is deteriorated. This is because martensite is easily generated during cold press forming due to the influence of the C distribution in the retained austenite. Therefore, the average grain size of retained austenite at the 1/4 position of the sheet thickness is set to 3 ⁇ m or less. On the other hand, the lower limit of the average grain size is not particularly limited.
• the average grain size is 0.3 ⁇ m, it has a large contribution to elongation, so the average grain size is preferably 0.3 ⁇ m or more, more preferably 1 ⁇ m or more, and even more preferably 2 ⁇ m or more.
• Bainite 0-7% If the volume fraction of bainite at the 1/4 position of the sheet thickness is higher than 7%, the desired tensile strength cannot be obtained. Therefore, the volume fraction of bainite at the 1/4 position of the sheet thickness is set to 7% or less, preferably 5% or less. On the other hand, from the viewpoint of strength, the lower the volume fraction of bainite, the better, so the lower limit of the volume fraction of bainite is set to 0%.
• Average grain size of bainite 5 ⁇ m or less If the average grain size of bainite at the 1/4 position of the plate thickness is greater than 5 ⁇ m, the delayed fracture resistance is deteriorated. This is because cracks due to hydrogen embrittlement are likely to occur at the interface between bainite and martensite from the HAZ softened part to the base material after resistance welding. Therefore, the average grain size of bainite at the 1/4 position of the plate thickness is set to 5 ⁇ m or less, preferably 4 ⁇ m or less. On the other hand, from the viewpoint of delayed fracture resistance, the smaller the average grain size of bainite, the more preferable, so the lower limit of the average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, the average grain size of bainite is preferably set to 0.1 ⁇ m or more, more preferably 0.5 ⁇ m or more, and even more preferably 1.0 ⁇ m or more.
• the volume fraction of martensite at the 1/4 position of the sheet thickness must be 93% or more. Therefore, the volume fraction of martensite at the 1/4 position of the sheet thickness is 93% or more, preferably 95% or more.
• the upper limit of the volume fraction of martensite at the 1/4 position of the sheet thickness is not particularly limited, but may be 100%.
• Average grain size of martensite 7 ⁇ m or less If the average grain size of martensite at the 1/4 position of the sheet thickness is greater than 7 ⁇ m, the grains after resistance welding will become coarse, and the L-shaped tensile strength will decrease. Therefore, the average grain size of martensite at the 1/4 position of the sheet thickness is set to 7 ⁇ m or less, preferably 6 ⁇ m or less. On the other hand, from the viewpoint of L-shaped tensile strength, the smaller the average grain size of martensite, the better, so the lower limit of the average grain size is not particularly limited. However, from the viewpoint of ease of manufacture, the average grain size of martensite is preferably 1 ⁇ m or more, more preferably 3 ⁇ m or more, and even more preferably 4 ⁇ m or more.
• the microstructure at the 1/4 plate thickness position is In terms of volume fraction, Ferrite: 0-5%, Retained austenite: 0 to 5% Bainite: 0-7%; and Martensite: 93% or more.
• the above microstructure may further contain other structures as desired.
• the other structures are structures other than ferrite, retained austenite, bainite, and martensite.
• the other structures may be, for example, pearlite.
• the volume fraction of the other structures may be 7% or less, and is preferably 3% or less.
• the microstructure at the 1/4 position of the plate thickness in one embodiment of the present invention is: In terms of volume fraction, Ferrite: 0-5%, Retained austenite: 0 to 5% Bainite: 0-7% Martensite: 93% or more, and other structures: 0-7% It may consist of:
• the volume fraction of ferrite in the region 7 to 12 ⁇ m from the surface of the steel plate in the plate thickness direction is 30% or more, preferably 45% or more, and more preferably 60% or more.
• the upper limit of the volume fraction of ferrite is not particularly limited and may be 100%.
• the volume fraction of ferrite may be, for example, 90% or less, or 85% or less.
• the average grain size of ferrite in the region 7 to 12 ⁇ m from the surface of the steel sheet in the sheet thickness direction is 10 ⁇ m or less, preferably 9 ⁇ m or less, and more preferably 8 ⁇ m or less.
• the average grain size of ferrite is preferably 1 ⁇ m or more, more preferably 2 ⁇ m or more, and even more preferably 3 ⁇ m or more.
• the microstructure may also contain other structures.
• the other structures are structures other than ferrite, such as bainite, pearlite, retained austenite, martensite, and cementite. From the viewpoint of further increasing the tensile strength, it is preferable that 50% or more of the remainder other than ferrite is at least one of bainite and martensite. In other words, it is preferable that the total volume fraction of bainite and martensite in the remaining structure other than ferrite is 50% or more.
• the upper limit of the total volume fraction of bainite and martensite is not particularly limited, and may be 100%.
• the microstructure in the region 7 to 12 ⁇ m from the surface in the plate thickness direction may be a structure consisting of 30% or more ferrite by volume fraction and one or both of the remaining bainite and martensite.
• the average number density of carbides having a grain size of 0.10 ⁇ m or more in the region 50 to 100 ⁇ m from the surface of the steel sheet in the sheet thickness direction is set to 5/100 ⁇ m2 or more , preferably 7/100 ⁇ m2 or more , and more preferably 10/100 ⁇ m2 or more .
• the higher the average number density, the better, so the upper limit of the average number density is not particularly limited.
• the average number density may be, for example, 40 particles/100 ⁇ m 2 or less, or 30 particles/100 ⁇ m 2 or less.
• the type of the carbide is not particularly limited, but the steel plate of the present invention contains at least one selected from the group consisting of Ti, Nb, and V, as described above. Since these elements are likely to form carbides, the steel plate of the present invention may contain, as the carbides, at least one of Ti-based carbides, Nb-based carbides, and V-based carbides in addition to Fe-based carbides (cementite).
• the average number density of the carbides can be measured by TEM (transmission electron microscope) and EDS (energy dispersive X-ray spectroscopy), and more specifically, can be measured by the method described in the examples.
• the steel sheet of the present invention may be a cold-rolled steel sheet having no plating layer on its surface, but it is preferable that the steel sheet has a zinc-based plating layer on at least one surface.
• the steel sheet of the present invention may be a zinc-plated steel sheet or a zinc alloy-plated steel sheet.
• the zinc alloy plating layer is not particularly limited, and a plating layer made of any zinc alloy can be used.
• the zinc alloy plating layer it is preferable to use a zinc alloy plating layer having a composition selected from the group consisting of Zn-Al, Zn-Al-Mg, Zn-Al-Si, Zn-Al-Mg-Si, and Zn-Al-Mg-Ni.
• the zinc-based plating layer can be formed by any method.
• the zinc-based plating layer may be any of a hot-dip zinc-based plating layer, an alloyed hot-dip zinc-based plating layer, and an electrolytic zinc-based plating layer.
• the steel sheet of the present invention may be any of a hot-dip zinc-based plated steel sheet, an alloyed hot-dip zinc-based plated steel sheet, and an electrolytic zinc-based plated steel sheet.
• the coating weight of the zinc-based coating layer is not particularly limited, but from the viewpoints of corrosion resistance and ease of coating weight control, the coating weight is preferably 25 g/ m2 or more per one side of the steel sheet. On the other hand, from the viewpoint of adhesion of the coating layer, the coating weight is preferably 80 g/ m2 or less per one side of the steel sheet.
• a pre-plating layer may be further provided between the steel sheet (base steel sheet) and the zinc-based plating layer.
• the pre-plating layer is not particularly limited and may be a plating layer of any composition, but is preferably an Fe-based plating layer, and more preferably an Fe-based electroplating layer.
• the Fe-based plating layer may be, for example, an Fe plating layer or an Fe alloy plating layer.
• the Fe plating layer is a plating layer made of Fe and unavoidable impurities, and is also called a "pure Fe plating layer.”
• the Fe alloy plating layer is not particularly limited and may be a plating layer made of any Fe alloy.
• the Fe alloy plating layer may be, for example, a plating layer made of at least one alloy selected from the group consisting of an Fe-B alloy, an Fe-C alloy, an Fe-P alloy, an Fe-N alloy, an Fe-O alloy, an Fe-Ni alloy, an Fe-Mn alloy, an Fe-Mo alloy, and an Fe-W alloy.
• the Fe-based electroplating layer preferably has a composition containing a total of 10% or less of at least one selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder being Fe and unavoidable impurities.
• the Fe-based plating layer functions as a soft layer, the provision of the Fe-based plating layer can alleviate the stress applied to the steel sheet surface during welding. Furthermore, the presence of an Fe-based plating layer not only reduces the residual stress in the resistance weld, but also allows diffusible hydrogen to escape efficiently from the steel sheet surface, improving delayed fracture resistance.
• the amount of the Fe-based plating layer is not particularly limited, but from the viewpoint of enhancing the above-mentioned effects, it is preferably 0.5 g/ m2 or more, and more preferably 1.0 g/ m2 or more per side of the steel sheet. On the other hand, from the viewpoint of cost, the amount of the Fe-based plating layer is preferably 60 g/ m2 or less, more preferably 50 g/ m2 or less, even more preferably 40 g/ m2 or less, and most preferably 30 g/ m2 or less per side.
• the adhesion amount of the Fe-based plating layer is measured as follows. A sample of 10 x 15 mm size is taken from the steel sheet after the zinc-based plating layer is formed and embedded in resin to obtain a cross-section embedded sample. Three arbitrary locations on the cross section are observed using a scanning electron microscope (SEM) to determine the thickness of the Fe-based plating layer. The acceleration voltage for the SEM observation is set to 15 kV. The magnification during the SEM observation is set to 2000 to 10000 times depending on the thickness of the Fe-based plating layer. The average thickness measured at the three locations is multiplied by the specific gravity of iron to convert it into the adhesion amount of the Fe-based plating layer per side.
• SEM scanning electron microscope
• the steel plate of the present invention has both a tensile strength of 1600 MPa or more and a high L-shaped tensile strength after resistance welding, and also has excellent delayed fracture resistance after resistance welding. More specifically, a high L-shaped tensile strength of 1.5 kN or more after resistance spot welding can be achieved. Furthermore, even if the resistance spot welded joint produced using the steel plate of the present invention is subjected to the following treatment and hydrogen charging, delayed fracture does not occur. - Leave it in the air at room temperature (20°C) for 24 hours. Cathodic electrolytic charging was performed while immersed in a 3% NaCl + 0.5% NH 4 SCN aqueous solution at a current density of 0.06 mA/cm 2 for 72 hours.
• spot welding method In a spot welding method according to an embodiment of the present invention, a plate assembly including at least one of the above-described steel plates is clamped between a pair of welding electrodes and joined by passing current through the electrodes while applying pressure.
• the conditions for spot welding are not particularly limited, and general welding conditions can be adopted.
• two steel sheets are overlapped to form a sheet assembly.
• the sheet assembly is clamped from above and below with a pair of welding electrodes, and current is passed through the sheet assembly while applying pressure and controlling it to achieve the specified welding conditions. This joins the steel sheets that make up the sheet assembly, making it possible to form a resistance spot welded member.
• multiple steel sheets can be overlapped so that the surface of the zinc-based plated steel sheet having the zinc-based plating layer faces the cold-rolled steel sheet.
• the resistance spot welding method in one embodiment of the present invention can include a main current application process in which the plate assembly is clamped using the pair of welding electrodes and current is applied while applying pressure to form a nugget.
• the energization conditions and pressure conditions for forming the nugget in the main energization process are not particularly limited. From the perspective of application to structural parts of automobiles, etc., it is preferable to adjust the energization conditions and pressure conditions to the following ranges.
• the current value in the main current flow process can be preferably set to 3.0 to 15.0 kA in order to obtain a stable nugget diameter.
• Nugget diameters of 3.0 ⁇ t to 6.0 ⁇ t are generally used in spot welds of automotive steel plates, and if the current value is too small, the target nugget diameter cannot be obtained stably.
• the current value in the main current flow process is larger than the above range, the nugget diameter may become too large, or the degree of melting of the steel plate may increase, causing the molten weld to splash out outside the gap between the plates, resulting in a smaller nugget diameter.
• the current flow time of the main current flow process is preferably 0.18 to 1.0 s. This is the time required to obtain the target nugget diameter, similar to the current value of the main current flow process. If the current flow time of the main current flow process is less than 0.18 s, it will be difficult to form a nugget. On the other hand, if the current flow time of the main current flow process exceeds 1.0 s, the nugget diameter may become large, and there is a concern that workability may decrease. However, as long as the required nugget diameter is obtained, the current flow time tw of the main current flow process may be shorter or longer than the above preferred range.
• the pressure force in the main current process is preferably 2.0 kN to 9.0 kN. If the pressure force in the main current process is too large, the current diameter will increase, making it difficult to ensure the nugget diameter. On the other hand, if the pressure force in the main current process is too small, the current diameter will decrease and splashing will be more likely to occur. Therefore, it is preferable that the pressure force F in the main current process be within the above-mentioned preferred range. Note that the pressure force may be limited by the equipment capacity. However, as long as the pressure force is sufficient to obtain the required nugget diameter, the pressure force F in the main current process may be lower or higher than the above-mentioned preferred range.
• a post-current may be applied after the main current process.
• the post-current may be applied under any conditions without particular limitations, but it is preferable that the current value in the post-current is higher than the current value in the main current process. Specifically, it is preferable that the current value is 1.1 times or more the current value in the main current process. In addition, it is preferable that the welding time in the post-current is 1.0 second or less.
• the post-current may be applied in multiple stages, in which case it is preferable that the total current time in the post-current is 1.0 second or less.
• a tempering process may be performed to temper the periphery of the nugget.
• the conditions of the tempering process are not particularly limited, but it is preferable that the current value in the tempering process is lower than the current value in the main current application process, and specifically, it is preferable that the current value in the tempering process is 0.9 times or less the current value in the main current application process. Furthermore, it is preferable that the current application time in the tempering process is 2.0 seconds or less.
• a resistance spot welded component according to an embodiment of the present invention is a resistance spot welded component including at least one of the above-mentioned steel plates in a plate set.
• the resistance spot welded component can be manufactured by a general resistance spot welding method as described above.
• the steel sheet of the present invention may be a cold-rolled steel sheet having no plating layer on its surface, or a zinc-based plated steel sheet having a zinc-based plating layer on its surface.
• the zinc-based plated steel sheet may be any of an electroplated steel sheet, a hot-dip plated steel sheet, and an alloyed hot-dip plated steel sheet. Therefore, a suitable manufacturing method will be described below for each case.
• a steel sheet satisfying the above-mentioned conditions can be manufactured by using molten steel having the above-mentioned composition as a starting material and sequentially carrying out the following steps. Note that if no plating is performed after annealing, a steel sheet (cold-rolled steel sheet) without a plating layer on the surface can be obtained.
• Continuous casting (2) Cooling (3) Reheating (4) Hot rolling (5) Cooling (6) Coiling (7) Pickling (8) Heat treatment (9) Cold rolling (10) Annealing
• molten steel having the above-mentioned composition is continuously cast to produce a steel slab.
• the continuous casting method has higher production efficiency than the mold casting method.
• the continuous casting can be performed using any continuous casting machine, but it is preferable to use a vertical bending type continuous casting machine.
• the vertical bending type continuous casting machine has an excellent balance between equipment cost and the surface quality of the obtained steel slab.
• the vertical bending type continuous casting machine is also excellent in the effect of suppressing surface cracks.
• the cooling stop temperature it is sufficient to control the average cooling rate in the temperature range up to 600°C, and there is no particular limitation on the cooling stop temperature.
• it can be cooled to any temperature below 600°C.
• it may be cooled to room temperature and then reheated and hot rolled, or cooling may be stopped at a temperature higher than room temperature to form a hot piece, which is then reheated and hot rolled.
• ⁇ Heating temperature 1280-1400°C If the heating temperature in the reheating is less than 1280°C, the precipitates cannot be sufficiently redissolved, and coarse precipitates remain even after final annealing. As a result, the delayed fracture resistance after resistance welding deteriorates. Therefore, the heating temperature is set to 1280°C or higher. On the other hand, if the heating temperature is higher than 1400°C, the crystal grains become coarse. As a result, the desired crystal grain size cannot be obtained after final annealing, and the resistance weldability deteriorates. Therefore, the heating temperature is set to 1400°C or lower, preferably 1350°C or lower.
• Holding time 60 minutes or more If the holding time in the reheating is less than 60 minutes, the precipitates cannot be sufficiently redissolved, and coarse precipitates remain even after the final annealing. As a result, resistance weldability is reduced. Therefore, the holding time is set to 60 minutes or more.
• the upper limit of the holding time is not particularly limited, but from the viewpoint of productivity, it is preferably set to 180 minutes or less, and more preferably set to 150 minutes or less.
• the reheated steel slab is hot rolled to obtain a hot rolled steel sheet.
• the structure in the steel sheet is made uniform and the anisotropy of the material is reduced, thereby improving the L-shaped tensile strength after resistance welding.
• Finish rolling end temperature 850 to 950°C
• the finish rolling end temperature is set to 850°C or higher.
• the finish rolling end temperature is set to 950°C or lower.
• the hot-rolled steel sheet is cooled.
• the steel sheet structure of the hot-rolled steel sheet is controlled by quenching to a temperature range where bainite transformation occurs without ferrite transformation.
• This homogenized hot-rolled structure control has the effect of refining the microstructure of the finally obtained steel sheet, mainly ferrite and martensite.
• Average cooling rate 80° C./s or more
• the average cooling rate is set to 80° C./s or more. If the average cooling rate is less than 80° C./s, ferrite transformation will start, resulting in a non-uniform microstructure and reduced resistance weldability.
• the upper limit of the average cooling rate is not particularly limited, but it is preferably set to 200° C./s or less.
• Cooling stop temperature 460°C or less Similarly, in order to obtain the above effect, the cooling stop temperature is set to 460°C or less. If the cooling stop temperature is higher than 460°C, pearlite is generated excessively, the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous, and the resistance weldability is deteriorated. On the other hand, the lower limit of the cooling stop temperature is not particularly limited, but it is preferably set to 250°C or more.
• the cooled hot-rolled steel sheet is coiled at a coiling temperature of 460°C or less. If the coiling temperature is higher than 460°C, pearlite is excessively generated, and the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous, resulting in a decrease in resistance weldability. Therefore, the coiling temperature is set to 460°C or less, preferably 440°C or less. There is no particular limit to the lower limit of the coiling temperature, but if the coiling temperature is too low, hard martensite is excessively generated, and the cold rolling load increases. Therefore, the coiling temperature is preferably set to 250°C or more.
• Heat treatment temperature 300 to 700°C
• the pickled hot-rolled steel sheet is subjected to a heat treatment at a heat treatment temperature of 300 to 700° C. to obtain a heat-treated hot-rolled steel sheet.
• the precipitation state of carbides in the finally obtained steel sheet can be improved, and the delayed fracture resistance after resistance welding can be improved.
• the heat treatment temperature is set to 300°C or higher.
• the heat treatment temperature is set to 700°C or lower.
• heat treatment time is not particularly limited, but if it exceeds 96 hours, productivity will drop significantly. Therefore, from the perspective of further improving productivity, it is preferable that the heat treatment time be 96 hours or less.
• the heat-treated hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the conditions for the cold rolling are not particularly limited, and the cold rolling can be performed according to a conventional method.
• the dew point in the temperature range of 600 to 980°C is set to be over -15°C, preferably -10°C or higher, and more preferably -5°C or higher.
• the upper limit of the dew point is not particularly limited, but from the viewpoint of improving the adhesion when a zinc-based plating layer is provided on the steel sheet surface, it is preferable to set it to 30°C or lower.
• the reaction between C in the surface layer of the steel sheet and moisture mainly proceeds at a temperature of 600°C or higher. Therefore, the dew point during annealing may be controlled in the temperature range of 600 to 980°C, and the dew point in the temperature range below 600°C is not particularly limited.
• heating, holding, and cooling are performed in the following order. Average heating rate: 12°C/s or more up to 650°C. Average heating rate: less than 12°C/s and annealing temperature: heated to 830-980°C. Hold (soak) at the annealing temperature for a holding time of 20 to 360 seconds. From the annealing temperature, cooling is performed at an average cooling rate of 3° C./s or more to room temperature.
• heating to the annealing temperature is performed in two stages, and the average heating rate in each stage is controlled within a specific range.
• first heating stage will be referred to as the “first heating stage”
• second heating stage will be referred to as the “second heating stage.”
• Average heating rate 12°C/s or more
• the cold-rolled steel sheet is heated to 650°C (first heating). If the average heating rate to 650°C is less than 12°C/s, the structure of the steel sheet becomes coarse, and the desired average crystal grain size cannot be obtained. Therefore, in the annealing, the cold-rolled steel sheet is heated to 650°C at an average heating rate of 12°C/s or more.
• the upper limit of the average heating rate is not particularly limited, if the steel sheet is heated too rapidly, recrystallization is difficult to proceed. Therefore, the average heating rate is preferably 30°C/s or less.
• the cold-rolled steel sheet is heated to the annealing temperature (second heating).
• second heating by heating to the annealing temperature at an average heating rate of less than 12 ° C./s, decarburization near the surface layer of the steel sheet can be promoted, and as a result, a ferrite phase can be generated.
• the average heating rate is 12 ° C./s or more, decarburization becomes insufficient, and the volume fraction of ferrite in the region 7 to 12 ⁇ m from the steel sheet surface in the sheet thickness direction cannot be set to the desired range. Therefore, the average heating rate is set to less than 12 ° C./s.
• the lower limit of the average heating rate is not particularly limited, but in order to ensure tensile strength, it is preferable that the average heating rate is set to more than 2 ° C./s.
• Annealing temperature 830 to 980°C If the annealing temperature is less than 830°C, the ferrite fraction becomes too high, making it difficult to achieve both tensile strength and resistance weldability. Therefore, the annealing temperature is set to 830°C or higher, preferably 840°C or higher, and more preferably 850°C or higher. On the other hand, if the annealing temperature is too high, the crystal grain growth of austenite becomes significant, and the crystal grains become coarse, resulting in a decrease in resistance spot weldability. Therefore, the annealing temperature is set to 980°C or lower, and preferably 950°C or lower.
• Holding time 20 to 360 seconds Holding at the annealing temperature promotes recrystallization and transforms a part or all of the structure into austenite. If the holding time at the annealing temperature is less than 20 seconds, the desired microstructure cannot be obtained. Therefore, the holding time is set to 20 seconds or more. On the other hand, if the holding time is longer than 360 seconds, the crystal grains become coarse, which reduces the resistance weldability. Therefore, the holding time is set to 360 seconds or less, preferably 300 seconds or less.
• Average cooling rate 3° C./s or more If the average cooling rate in the cooling is less than 3° C./s, the desired martensite volume fraction cannot be obtained, and the tensile strength decreases. Therefore, the average cooling rate is set to 3° C./s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but it is preferably set to less than 100° C./s.
• temper rolling may be performed.
• the temper rolling may be performed under any conditions, but it is preferable to set the elongation rate to 0.05% to 2.0%.
• the annealed steel sheet may be electroplated to form a zinc-based plating layer on at least one surface of the steel sheet.
• the electroplating can be carried out under any conditions without any particular limitation. That is, in the present invention, the desired characteristics are achieved by controlling the microstructure and precipitates of the base steel sheet, so that the plating treatment conditions are not limited and the plating can be carried out according to a conventional method.
• a steel sheet satisfying the above-mentioned conditions can be manufactured by using molten steel having the above-mentioned composition as a starting material and sequentially carrying out the following steps. According to this method, a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface can be obtained.
• Continuous casting (2) Cooling (3) Reheating (4) Hot rolling (5) Cooling (6) Coiling (7) Pickling (8) Heat treatment (9) Cold rolling (10) Annealing (11) Hot dip plating (12) Cooling
• each of the steps (1) to (10) above can be carried out under the same conditions as in the first embodiment.
• the cooling step (10) of the annealing instead of cooling to room temperature, cooling can be performed until the steel enters the hot-dip galvanizing bath.
• the average cooling rate in the cooling step is 3°C/s or more, as in the first embodiment described above.
• Hot-dip galvanization the annealed steel sheet is immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer on at least one surface of the steel sheet.
• a hot-dip galvanized steel sheet can be obtained.
• the hot-dip plating can be carried out by any method.
• the desired characteristics are achieved by controlling the microstructure and precipitates of the base steel sheet, so the plating conditions are not particularly limited and can be carried out according to conventional methods.
• any hot-dip plating bath can be used without any particular limitation, but it is preferable to use a hot-dip plating bath having a composition consisting of Al, Zn, and unavoidable impurities.
• the Al concentration in the plating bath is not particularly limited, but may be, for example, 0.05% or more and 0.25% or less. If the Al concentration is 0.05% or more, the generation of bottom dross is suppressed, and it is possible to prevent the dross from adhering to the steel sheet and causing defects.
• the Al concentration is 0.25% or less, the increase in top dross is suppressed, and it is possible to prevent the dross from adhering to the steel sheet and causing defects.
• material costs can be reduced by lowering the Al concentration.
• the temperature of the hot-dip plating bath is preferably 440 to 500°C, which is the bath temperature in general hot-dip plating.
• the temperature of the steel sheet when entering the hot-dip plating bath is preferably 440 to 550°C.
• the coating weight may be adjusted.
• the coating weight is adjusted by gas wiping.
• the coating weight is adjusted by adjusting the gas wiping conditions, such as the gas pressure and the distance between the wiping nozzle and the steel sheet.
• the steel sheet after the hot dip coating is cooled to room temperature at an average cooling rate of 3°C/s or more. If the average cooling rate in the cooling is less than 3°C/s, the desired martensite volume fraction cannot be obtained, and the tensile strength decreases. Therefore, the average cooling rate is set to 3°C/s or more. On the other hand, the upper limit of the average cooling rate is not particularly limited, but it is preferably less than 100°C/s.
• the average cooling rate is important to set the average cooling rate to 3°C/s or more in both the annealing process and the cooling after hot-dip plating in order to obtain the desired microstructure.
• a fourth embodiment of the present invention after the hot-dip galvanization, an alloying treatment is performed prior to the cooling to room temperature.
• the hot-dip galvanized layer is alloyed by the alloying treatment, and a galvannealed steel sheet can be obtained.
• the alloying treatment can be carried out under any conditions without any particular limitation. That is, in the present invention, the desired properties are achieved by controlling the microstructure and precipitates of the base steel sheet, so that the alloying treatment conditions are not limited and can be carried out according to a conventional method.
• the alloying process is preferably carried out at a temperature of 450°C or higher and 600°C or lower.
• 450°C or higher it is possible to provide a steel sheet with excellent press formability without leaving any ⁇ phase in the plating layer.
• 600°C or lower good plating adhesion can be obtained.
• the alloying time is preferably 5 to 60 seconds.
• pre-plating may be optionally further performed to form a pre-plated layer on the steel sheet surface.
• the timing of the pre-plating is not particularly limited, and may be any timing prior to plating for forming the zinc-based plating layer.
• the zinc-based plating layer is formed by electroplating
• the zinc-based plating layer is formed by hot-dip plating, it is preferable to perform the pre-plating after the cold rolling and before the annealing. That is, the processes may be performed in the order of cold rolling, pre-plating, annealing, and hot-dip zinc-based plating. Note that when alloying is performed, it may be performed after the hot-dip zinc plating as usual.
• any pre-plating layer can be formed, but it is preferable to form an Fe-based plating layer.
• the Fe-based plating layer is preferably formed by electroplating. Below, we will explain the case where an Fe-based plating layer is formed as a pre-plating layer by performing an Fe-based electroplating process.
• the Fe-based electroplating process method is not particularly limited.
• any bath can be used as the Fe-based electroplating bath, such as a sulfuric acid bath, a hydrochloric acid bath, or a sulfuric acid + hydrochloric acid bath.
• the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 1.0 mol/L or more as Fe 2+ . If the Fe ion content in the Fe-based electroplating bath is 1.0 mol/L or more as Fe 2+ , a sufficient Fe deposition amount can be obtained.
• the Fe-based electroplating bath may contain Fe ions, as well as alloying elements such as B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, and may also contain conductivity aids such as sodium sulfate and potassium sulfate as additives or impurities.
• the metal elements may be contained as metal ions, and the nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acid, etc.
• the iron sulfate plating solution may contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.
• the bath temperature is preferably 30° C. or higher in consideration of the ability to maintain a constant temperature.
• the pH of the Fe-based electroplating bath is not particularly specified, but is preferably 3.0 or lower in consideration of the electrical conductivity of the Fe-based electroplating bath.
• the current density is not particularly limited, but is usually 10 to 150 A/ dm2 .
• the sheet passing speed may be 5 mpm or more and 150 mpm or less. This is because a sheet passing speed of less than 5 mpm results in poor productivity, while a sheet passing speed of 150 mpm or more makes it difficult to stably control the coating weight.
• the cold-rolled steel sheet Before the Fe-based electroplating treatment, the cold-rolled steel sheet may be subjected to a degreasing treatment and water washing to clean the surface, and further to a pickling treatment and water washing to activate the surface. It is preferable to carry out the Fe-based electroplating treatment after these pretreatments.
• a degreasing treatment and water washing There are no particular limitations on the method of degreasing and water washing, and ordinary methods can be used.
• various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures of these can be used. Among these, sulfuric acid, hydrochloric acid, or mixtures of these are preferred.
• the pickling treatment solution may also contain an antifoaming agent, a pickling promoter, a pickling inhibitor, etc.
• steel sheets were produced using the following procedure, and their properties were evaluated.
• Four types of steel sheets were produced: cold-rolled steel sheet (CR), electrolytic galvanized steel sheet (EG), hot-dip galvanized steel sheet (GI), and galvannealed hot-dip galvanized steel sheet (GA).
• molten steel having the composition shown in Table 1 was continuously cast into a steel slab, which was then cooled.
• the average cooling rate in the temperature range up to 600°C during the cooling was as shown in Table 2.
• the cooled steel slab was reheated under the conditions shown in Table 2, and then hot rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet.
• the hot-rolled steel sheet was cooled to the coiling temperature under the conditions shown in Table 2, and wound into a coil. The hot-rolled steel sheet was then pickled.
• the pickled hot-rolled steel sheet was subjected to heat treatment at the heat treatment temperature shown in Table 2 to obtain a heat-treated hot-rolled steel sheet.
• the heat treatment time was 12 hours.
• the heat-treated hot-rolled steel sheet was cold-rolled to produce a cold-rolled steel sheet with a thickness of 1.4 mm.
• the cold-rolled steel sheet was annealed under the conditions shown in Table 2. In the annealing, the cold-rolled steel sheet was soaked at the annealing temperature and holding time shown in Table 2, and then cooled to room temperature at the average cooling rate shown in Table 2.
• the cold-rolled steel sheet was first annealed under the conditions shown in Table 2 and cooled to room temperature at the average cooling rate shown in Table 2. Then, electroplating was performed to form an electrogalvanized layer on the surface of the steel sheet. The electroplating was performed using a sulfuric acid bath containing 1.5 mol/L of Zn2 + . The temperature of the plating bath was 50°C and the pH was 1.5.
• hot-dip galvanized steel sheet In the manufacture of hot-dip galvanized steel sheet (GI), the cold-rolled steel sheet was first annealed under the conditions shown in Table 2, and then cooled at the average cooling rate shown in Table 2 until it entered the hot-dip galvanizing bath. The steel sheet was then immersed in the hot-dip galvanizing bath to form a hot-dip galvanized layer on the surface of the steel sheet.
• the hot-dip galvanizing bath used was a plating bath consisting of Al, Zn, and unavoidable impurities, with an Al concentration of 0.14% in the bath.
• the temperature of the hot-dip galvanizing bath was 460°C.
• the steel sheet was then cooled to room temperature at the average cooling rate shown in Table 2.
• the cold-rolled steel sheet was first annealed under the conditions shown in Table 2, and then cooled to the point of entering the hot-dip galvanizing bath at the average cooling rate shown in Table 2. Thereafter, an alloying process was performed to alloy the hot-dip galvanized layer.
• the hot-dip galvanizing bath used had the same composition as the galvanizing bath used in the manufacture of the hot-dip galvanized steel sheet (GI), and the temperature of the galvanizing bath was also the same.
• the alloying process was performed at a temperature of 550°C. Thereafter, the cold-rolled steel sheet was cooled to room temperature at the average cooling rate shown in Table 2.
• the average grain size of ferrite, martensite, and bainite was determined by image analysis of the above microscope image. Specifically, the area of each grain of ferrite, martensite, and bainite in the above microscope image was first determined by image analysis. Next, the circle equivalent diameter of the grain was calculated from the area, and the average value was used as the average grain size. Image-Pro from Media Cybernetics was used for the image analysis.
• the volume fraction of retained austenite was determined by X-ray diffraction. Specifically, the steel sheet was first polished to 1/4 of the sheet thickness direction, and the diffracted X-ray intensity at the 1/4 sheet thickness direction was measured by X-ray diffraction. The measurement was performed using an X-ray diffraction device RINT2200 manufactured by Rigaku Corporation, with Mo K ⁇ radiation as the radiation source and an acceleration voltage of 50 keV.
• the average grain size of the retained austenite grains was determined by polishing the cross section to 1/4 of the plate thickness, etching with 3% nital, and analyzing the TEM image obtained by TEM observation. Specifically, a TEM was first used to observe at a magnification of 15,000 times to obtain a TEM image of the microstructure at the 1/4 plate thickness position. The area of each retained austenite grain was determined by image analysis of the obtained TEM image. Next, the circle equivalent diameter of each grain was calculated from the area, and the average value was used as the average grain size of the retained austenite. Image-Pro from Media Cybernetics was used for the image analysis.
• the microstructure in the range of 7 to 12 ⁇ m deep from the surface of the steel sheet was observed with a microscope, and the volume fraction and average grain size of ferrite were calculated.
• the measurement target was a steel sheet having a plating layer (EG, GI, and GA)
• the measurement by glow discharge optical emission spectrometry was performed from the surface of the steel sheet, and the point where Fe exceeded 50 mass% was regarded as the surface of the steel sheet.
• the observation with the microscope and the calculation of the volume fraction and average grain size from the microscope image were performed in the same manner as the measurement of the microstructure at the 1/4 position of the sheet thickness.
• the average number density of carbides with a particle size of 0.10 ⁇ m or more in the region 50 to 100 ⁇ m from the surface in the plate thickness direction was obtained by TEM observation. Specifically, first, the L cross section of the steel plate was observed at a magnification of 10,000 times by TEM, and TEM images of 10 locations randomly selected from the range of 50 to 100 ⁇ m deep from the surface of the steel plate were obtained. Next, the TEM images were analyzed using Image-Pro to determine the area of each carbide, and the circle equivalent diameter of each particle was calculated from the area. The identification of carbides present in the TEM image was performed by EDS (Energy Dispersive X-ray Spectroscopy).
• the number of carbides with a circle equivalent diameter of 0.10 ⁇ m or more was counted, and the number density of the carbides was obtained by dividing by the area of the observed range.
• the number density of precipitates was calculated for the 10 TEM images in the same procedure, and the average value was taken as the average number density of the carbides.
• Tensile strength A JIS No. 5 tensile test piece was taken from the steel plate so that the direction perpendicular to the rolling direction was the longitudinal direction (tensile direction). A tensile test was then performed using the test piece to measure the tensile strength (TS) of the steel plate. The tensile test was performed in accordance with JIS Z2241 (1998).
• an L-shaped tensile test piece was prepared to evaluate the L-shaped tensile strength. Specifically, two test pieces of 50 x 150 mm were cut out from the above steel plate, and each of the two test pieces was bent 90° in a V-bend so that the size of the welded surface was 50 x 50 mm. The two bent test pieces were welded by applying resistance spot welding to the center of the welded surface to obtain an L-shaped tensile test piece.
• the resistance spot welding was performed using a servo motor pressure type single-phase AC (50 Hz) resistance welding machine.
• a DR type electrode made of alumina-dispersed copper with a tip curvature radius R of 40 mm and a tip diameter of 6 mm was used, and the welding conditions were as follows: Pressure: 4500N ⁇ Power supply time: 20 cycles (50Hz) Hold time: 5 cycles (50 Hz) ⁇ Nugget diameter: 5.0 ⁇ t (mm)
• t is the thickness (mm) of the steel plate used.
• the obtained L-shaped tensile test specimen was used to perform a tensile test at a tensile speed (longitudinal direction) of 10 mm/min to measure the L-shaped tensile strength.
• An L-shaped tensile strength of 1.5 kN or more was judged as "pass”, and an L-shaped tensile strength of less than 1.5 kN was judged as "fail”.
• the results are shown in Table 3.
• a welded joint used for evaluating delayed fracture resistance was prepared. Specifically, two test pieces of 50 x 150 mm were cut out from the above steel plate. Next, the two test pieces were overlapped with a spacer of 50 mm x 50 mm and 1.4 mm thick sandwiched between both ends of the test pieces, and pre-welded. Next, the center of the pre-welded test piece was welded to form a welded joint. The welding conditions were the same as those when preparing the above L-shaped tensile test piece, and the nugget diameter was 5.0 ⁇ t (mm).
• the obtained welded joint was left in the air at room temperature (20°C) for 24 hours.
• the right-turned joint was immersed in a 3% NaCl + 1.0% NH 4 SCN aqueous solution, and cathodic electrolytic charging was performed at a current density of 0.06 mA / cm 2 for 72 hours. After that, it was confirmed whether delayed fracture occurred in the welded joint. If delayed fracture did not occur in the welded joint, it was judged as "pass”, and if delayed fracture occurred, it was judged as "fail”. The judgment results are shown in Table 3.
• steel plates that meet the conditions of the present invention have a tensile strength of 1600 MPa or more, high L-shaped tensile strength after resistance welding, and also have excellent delayed fracture resistance after resistance welding.

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