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  • B23K11/00—Resistance welding; Severing by resistance heating
  • B23K11/10—Spot welding; Stitch welding
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  • 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
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • C21D8/0221—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 characterised by the working steps
  • C21D8/0236—Cold rolling
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  • 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0263—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 characterised by the heat treatment following hot rolling
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  • 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/50—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for welded joints
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • 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
  • C25D5/48—After-treatment of electroplated surfaces
  • C25D5/50—After-treatment of electroplated surfaces by heat-treatment
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  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0614—Strips or foils
• • 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
  • B23K2101/00—Articles made by soldering, welding or cutting
  • B23K2101/34—Coated articles ; Surface treated articles
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  • 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
  • B23K2103/00—Materials to be soldered, welded or cut
  • B23K2103/02—Iron or ferrous alloys
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
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  • C25D3/00—Electroplating: Baths therefor
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  • C25D3/20—Electroplating: Baths therefor from solutions of iron
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  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/22—Electroplating: Baths therefor from solutions of zinc

Definitions

• the method proposed in Patent Document 1 can suppress the occurrence of shrinkage defects and cracks in the welded metal parts. However, the method does not improve the appropriate current range or cross tensile strength. In addition, the method requires pressure and current application under specific conditions after welding current is applied, which increases the time required for welding and reduces productivity. In addition, the method proposed in Patent Document 1 adjusts the welding conditions and does not contribute to improving the formability of the steel plate.
• Patent Document 2 Furthermore, according to the method proposed in Patent Document 2, a certain degree of improvement in formability and weldability is observed.
• the tensile strength of the steel plate obtained by the method in Patent Document 2 is less than 1000 MPa, and a tensile strength of 1450 MPa or more cannot be achieved.
• excellent hole expandability is required for manufacturing components with complex shapes.
• Patent Document 2 uses elongation (El) as an index of the formability of the steel plate, and does not take hole expandability into consideration.
• the present invention was made in consideration of the above situation, and aims to provide a steel plate that combines a tensile strength of 1450 MPa or more with high levels of formability (hole expandability) and weldability (wide range of appropriate current, cross tensile strength).
• a resistance spot welded component including 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 480° C. or less.
• the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the cold-rolled steel sheet is annealed in an atmosphere having a dew point of more than ⁇ 20° C., In the annealing, the cold-rolled steel sheet is Heat to an annealing temperature of 780 to 980°C at an average heating rate of 3 to 30°C/s. Holding at the annealing temperature for a holding time of 15 to 360 seconds; Cooling from the annealing temperature to room temperature at an average cooling rate of 3 ° C./s or more; Manufacturing method of steel plate.
• the steel slab is cooled at an average cooling rate of 100° C./h or more in a temperature range up to 600° C.,
• the cooled steel slab is reheated by heating it to a heating temperature of 1250 to 1450°C and holding it at the heating temperature for 60 minutes or more;
• the reheated steel slab is hot-rolled under a finish rolling end temperature of 850 to 950°C to obtain a hot-rolled steel sheet.
• the hot-rolled steel sheet is cooled at an average cooling rate of 80° C./s or more to a cooling stop temperature of 480° C.
• the cooled hot-rolled steel sheet is coiled at a coiling temperature of 480° C. or less.
• the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the cold-rolled steel sheet is annealed in an atmosphere having a dew point of more than ⁇ 20° C.
• the annealed steel sheet is subjected to hot-dip galvanization by immersing the steel sheet in a hot-dip galvanizing bath to form a zinc-based plating layer on at least one surface of the steel sheet;
• the method for producing a steel sheet includes cooling the steel sheet after the 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 Heat to an annealing temperature of 780 to 980°C at an average heating rate of 3 to 30°C/s. Holding at the annealing temperature for a holding time of 15 to 360 seconds; Cooling from the annealing temperature to the immersion in the hot-dip galvanizing bath at an average cooling rate of 3 ° C./s or more; Manufacturing method of steel plate.
• the present invention makes it possible to provide steel sheets that combine a tensile strength of 1,450 MPa or more with high levels of formability (hole expansion ability) and weldability (wide range of appropriate current, cross tensile strength).
• the steel sheet of the present invention has the above-mentioned composition. The reasons for limiting the composition will be explained below.
• Si 0.2 to 1.8% Si is an element that has the effect of improving weldability. This is because the addition of Si alleviates Mn segregation, and as a result, the variation in hardness in the plate thickness direction is reduced. In order to obtain the above effect, the Si content is set to 0.2% or more, preferably 0.3% or more. On the other hand, if the Si content exceeds 1.8%, liquid metal embrittlement occurs during resistance spot welding. Therefore, the Si content is set to 1.8% or less, preferably 1.6% or less.
• Mn 2.4 to 3.5%
• Mn is an element that contributes to improving the strength of steel sheets through solid solution strengthening and the generation of hard phases.
• Mn is an element that stabilizes austenite and is an element necessary for controlling the fraction of hard phases.
• the Mn content is set to 2.4% or more.
• the Mn content is set to 3.5% or less, preferably 3.2% or less.
• the P content is set to 0.03% or less, preferably 0.02% or less.
• the lower limit of the P content is not particularly limited and may be 0%. However, since excessive reduction of the P content increases the steelmaking cost, the P content is preferably set to 0.005% or more.
• the S content is set to 0.003% or less, preferably 0.002% or less.
• the lower limit of the S content is not particularly limited and may be 0%. However, since excessive reduction of the S content increases the steelmaking cost, the S content is preferably set to 0.0002% or more.
• Al 0.01 to 0.50% 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 0.50%, the ferrite phase is excessively generated during annealing, making it difficult to ensure strength. Therefore, the Al content is set to 0.50% or less, preferably 0.45% or less.
• N 0.008% or less
• N is an element that deteriorates hole expandability by forming coarse nitrides. If the N content is higher than 0.008%, the deterioration of hole expandability becomes significant. Therefore, the N content is set to 0.008% or less, preferably 0.007% or less.
• the lower limit of the N content is not particularly limited and may be 0%. However, since excessive reduction of the N content increases the steelmaking cost, it is preferable that the N content is 0.001% or more.
• Sb 0.001 to 0.012%
• Sb is an element that has the effect of strengthening grain boundaries by segregating at the grain boundaries, and adding Sb can improve the cross tensile strength.
• the Sb content is set to 0.001% or more, preferably 0.002% or more.
• the Sb content is set to 0.012% or less, preferably 0.010% or less.
• Cr 0.50% or less Cr is an element that has the effect of further improving 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.
• Ca 0.0050% or less
• Ca is an element that has the effect of further improving hole expandability by making the shape of sulfides spherical. Ca also contributes to improving delayed fracture resistance after resistance welding. However, when the Ca content exceeds 0.0050%, the effect is saturated, so when Ca is added, the Ca content is set to 0.0050% or less. On the other hand, from the viewpoint of enhancing the effect of adding Ca, it is preferable that the Ca content is 0.0005% or more.
• the composition of the steel sheet of the present invention further has an A value defined by the following formula (1) of 0.9 to 6.0.
• A ([C] + [Si] / 8 + [Mn] / 20) / (2 ⁇ ([Ti] + [Nb]) + 85 ⁇ ([B] + [Sb] / 20) ...
• the parentheses in the above formula (1) represent the content (mass %) of the element in the parentheses, and if the element is not contained, the value is set to 0.
• the A value is set to 0.9 or more, preferably 1.0 or more.
• the A value is set to 6.0 or less, preferably 5.8 or less, and more preferably 5.0 or less.
• 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. Furthermore, the above-mentioned “tempered martensite” includes not only martensite that is generated by self-tempering during cooling in annealing, from cooling below the martensitic transformation start point (Ms point) to cooling to room temperature, but also martensite that is generated by further tempering thereafter.
• Ms point martensitic transformation start point
• the volume fraction of ferrite (F) exceeds 5%, the amount of voids generated during punching increases, resulting in a decrease in hole expandability.
• the volume fraction of ferrite at the 1/4 position of the plate 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 is set to 0%.
• Microstructure in the range from the surface to a depth of 7 ⁇ m In the present invention, it is important to control the microstructure in the range from the surface of the steel sheet to a depth of 7 ⁇ m, the reason for which will be described below.
• 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, it is preferably 25 g/ m2 or more per one side of the steel sheet, while from the viewpoint of adhesion of the coating layer, it 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 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 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.
• 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 strength and hole expandability are reduced. Therefore, the average cooling rate is set to 3°C/s or more.
• the upper limit of the average cooling rate is not particularly limited, but it is preferably less than 100°C/s, more preferably 50°C/s or less, and even more preferably 20°C/s or less.
• 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 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.
• the cold-rolled steel sheet was first annealed under the conditions shown in Table 2, and then cooled to the point of immersion in the hot-dip galvanizing bath at the average cooling rate shown in Table 2. After that, 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.
• the average crystal grain size of ferrite and martensite was determined by image analysis of the SEM images obtained by the above SEM observation. Specifically, first, the area of each crystal grain of ferrite and martensite in the SEM image was determined by image analysis. Next, the circle equivalent diameter of the crystal grain was calculated from the area, and the average value was taken as the average crystal grain size. Image-Pro from Media Cybernetics was used for the image analysis.
• the integrated intensities of X-ray diffraction lines of the ⁇ 200 ⁇ , ⁇ 211 ⁇ , and ⁇ 220 ⁇ planes of ferrite and the ⁇ 200 ⁇ , ⁇ 220 ⁇ , and ⁇ 311 ⁇ planes of austenite were measured, and the volume fraction of retained austenite was calculated from the obtained integrated intensities.
• a calculation formula described in p. 26, 62-64 of "X-ray Diffraction Handbook" (2000, Rigaku Denki Co., Ltd.) was used.
• the number density of Ti-based precipitates and Nb-based precipitates was determined by TEM-EDS (energy dispersive X-ray analysis). Specifically, first, the L-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 5 to 100 ⁇ m deep from the surface of the steel plate were obtained. Next, Ti-based precipitates and Nb-based precipitates in the TEM image were identified by EDS. The TEM image was subjected to image analysis using Image-Pro, and the area of each Ti-based precipitate and Nb-based precipitate was determined. From the area, the circle equivalent diameter of each particle was calculated.
• TEM-EDS energy dispersive X-ray analysis
• the number of Ti-based precipitates and Nb-based precipitates having a circle equivalent diameter of 0.005 ⁇ m or more and less than 0.10 ⁇ m was counted, and the number density of the precipitates was determined by dividing by the area of the observed range.
• the number densities of precipitates were calculated in the same manner for the TEM images obtained by observing the above 10 locations, and the average values were taken as the number densities of Ti-based precipitates and Nb-based precipitates.
• the welding current was increased from 3.0 kA in 0.1 kA increments, and the maximum current value at which no spatter (dust) was generated was recorded.
• the nugget diameter was measured by observing the cross section of the welded part of the test piece after welding, and the difference between the minimum current at which the nugget diameter was 4.0 ⁇ t (mm) or more relative to the plate thickness t (mm) and the maximum current value at which no spatter was generated was determined to be the appropriate current range for welding.
• An appropriate current range of 1.2 kA or more was rated as "good," and one below 1.2 kA was rated as "poor.”
• Cross tensile strength The cross tensile strength after resistance spot welding was measured based on the cross tensile test method (JIS Z 3137). For the measurement, a 50 x 150 mm cross tensile test piece cut out from a steel plate was used, and resistance spot welding was performed on a plate set consisting of two of the same type of test pieces.
• the resistance welding conditions were a servo motor pressure type single-phase AC (50 Hz) resistance welding machine attached to a welding gun, and a tensile test piece having a resistance weld was produced.
• the pair of electrode tips used were alumina-dispersed copper DR type electrodes with a tip curvature radius R40 mm and tip diameter 6 mm.
• the welding conditions were a pressure force of 4000 N, current flow time of 20 cycles (50 Hz), and hold time of 5 cycles (50 Hz).
• the current value was set to a current value at which the nugget diameter was 4.5 ⁇ t (mm) from the above-mentioned test in the appropriate current range. If the strength was 5.5 kN or more, the cross tensile strength after resistance spot welding was rated as "good", and if the strength was less than 5.5 kN, the cross tensile strength after resistance spot welding was rated as "poor".
• steel sheets that meet the conditions of the present invention have a tensile strength of 1,450 MPa or more, as well as high levels of formability (hole expansion ability) and weldability (wide range of appropriate current, cross tensile strength).

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