Patent ID: 12247293

EXAMPLES

A cold-rolled steel sheet (0.2% C-2.0% Si-2.3% Mn) having a sheet thickness of 0.8 mm was prepared as a steel sheet to be plated.

After this steel sheet is cut into a size of 100 mm×200 mm, then annealing and hot-dip plating were performed using a batch-type hot-dip plating tester.

In the annealing, annealing was performed at 860° C. for 120 seconds in an atmosphere that includes gas containing 5% H2gas and N2gas as a remainder and has a dew point of 0° C., in a furnace having oxygen concentration of 20 ppm or lower.

After the annealing, the steel sheet was air-cooled with N2gas, and when the steel sheet temperature reached the bath temperature+20° C., it was immersed in a plating bath having a bath temperature shown in Table 1 for about 3 seconds. The composition of the plating bath and the composition of the formed plating layer were as shown in Table 1.

The plated base sheet with the plating layer formed thereon was cooled to a temperature of 20° C. or lower under the conditions shown in Table 1, and subjected to post-heat treatment to obtain a plated steel sheet (a hot-dip Zn-based plated steel sheet). The retention time of the post-heat treatment was set to 100 seconds. The time from the completion of the cooling step to the start of the post-heat treatment was set as shown in Table 1.

TABLE 1Manufacturing methodCooling conditionAverageAverageAveragecooling ratecooling ratecoolingfrom bathfrom bathrate fromBathtemperaturetemperature270° C.Post-Heat treatmenttemperatureto 20° C.to 270° C.to 20° C.Presence orNo.Classification(° C.)(° C./s)(° C./s)(° C./s)absence1Comparative48020.020.020.0Presenceexample2Comparative48020.020.020.0Presenceexample3Invention42020.020.020.0Presenceexample4Invention42020.020.030.0Presenceexample5Comparative46020.020.020.0Presenceexample6Invention46020.020.020.0Presenceexample7Invention46024.720.030.0Presenceexample8Invention47020.020.020.0Presenceexample9Invention48020.020.020.0Presenceexample10Invention48024.420.030.0Presenceexample11Invention48020.020.020.0Presenceexample12Comparative52020.020.020.0Presenceexample13Invention50020.020.020.0Presenceexample14Invention50024.220.030.0Presenceexample15Invention50020.020.020.0Presenceexample16Invention55020.020.020.0Presenceexample17Comparative52020.020.020.0Presenceexample18Comparative5005.05.05.0Presenceexample19Comparative58020.020.020.0Presenceexample20Comparative51020.020.020.0Presenceexample21Invention54020.020.020.0Presenceexample22Invention58015.015.015.0Presenceexample23Comparative54020.020.020.0Presenceexample24Comparative54020.020.020.0Absenceexample25Invention55020.020.020.0Presenceexample26Invention55023.720.030.0Presenceexample27Invention55020.020.020.0Presenceexample28Invention56020.020.020.0Presenceexample29Invention56023.620.030.0Presenceexample30Invention56015.015.015.0Presenceexample31Invention56023.620.030.0Presenceexample32Invention56020.020.020.0Presenceexample33Invention58020.020.020.0Presenceexample34Comparative60020.020.020.0PresenceexampleManufacturing methodPost-Heat treatmentOne sideTime toChemical composition (mass %) of plating layerplatingstart ofRemainder: Zn and impuritieslayerheatOtheradhesiontreatmentTemperatureTotalamountNo.(h)(° C.)AlMgSiCaFeKindsvalue(g/m2)1<0.51204.505.000.050.100.20—0.00402<0.51206.000.000.050.100.10Pb:0.01400.013<0.51206.003.000.050.030.20—0.00394<0.51206.003.000.050.030.20—0.00395<0.512010.003.000.050.000.10—0.00406<0.517010.003.000.200.100.00—0.004571.017010.003.000.200.100.00—0.00508<0.512010.005.000.100.100.20—0.00409<0.512015.006.000.100.100.10Mn:0.001600.001102.012015.006.000.100.100.10—0.005511<0.512015.006.000.100.100.20Sr:0.01500.0112245020.005.000.200.200.20—0.005013<0.512020.007.000.200.200.20Ni:0.20400.20143.012020.007.000.200.200.20Ni:0.20400.20155.012020.007.500.200.200.20—0.004016<0.512020.0010.000.000.200.10Sb:0.05400.0517<0.55020.005.000.200.200.20—0.004618<0.512020.007.000.200.200.10—0.004019<0.512020.0013.000.200.200.10—0.004520<0.512021.008.003.500.200.30—0.004521<0.522023.008.000.300.400.10Sn:0.01450.0122<0.512023.008.001.000.500.20Ti:0.01400.0123<0.512023.008.100.202.500.20—0.004524<0.5—23.008.000.200.500.10—0.004525<0.512026.006.500.300.400.30Pb:0.05300.0526<0.512026.006.500.300.400.30—0.0030272212026.005.500.300.300.10Cu:0.005350.00528<0.512030.006.000.300.400.10—0.003529<0.512030.006.000.300.400.10—0.003530<0.512030.005.700.400.300.20—0.003531<0.512030.006.000.400.300.20—0.005032<0.512030.002.000.400.300.20Cr:0.0151050.01533<0.512035.008.000.450.100.20—0.004034<0.512040.003.000.500.560.20—0.0045The underline indicates a value outside the range of the present invention.

With respect to the obtained plated steel sheet, the area ratio of each phase in the plating phase, the number density of the Ca-based intermetallic compound having a circle equivalent diameter of 0.10 μm or smaller and the Ca-based intermetallic compound having a circle equivalent diameter of 0.07 μm or smaller in the MgZn2phase, the thickness of the alloy layer, and the thickness of the internal oxidation layer were measured by the methods described above.

Further, with respect to the obtained plated steel sheets, the end surface corrosion resistance and the workability were evaluated by methods, which will be described later.

[End Surface Corrosion Resistance]

A sample of 50×100 mm was taken from the plated steel sheet, Zn phosphoric acid treatment (SD5350 system: Nippon Paint Industrial Coatings Co., Ltd. standard) was performed, and then electrodeposition coating (PN110 Powernics Gray: Nippon Paint Industrial Coatings Co., Ltd. standard) was performed so as to obtain a thickness of 20 μm, and baking was performed at a baking temperature of 150° C. for 20 minutes. This coated plated steel sheet (the plated steel sheet subjected to electrodeposition coating) was subjected to a combined cycle corrosion test in accordance with JASO (M609-91) to measure the maximum swelling width at three points from the sample end surface, and the average value is calculated to evaluate corrosion resistance after painting.

In a case where the number of cycles of JASO (M609-91) described above is 150 cycles and the coating film swelling width from the end surface is less than 1.0 mm, it was rated as “AAA”, in a case where the coating film swelling width is less than 1.5 mm, it was rated as “AA”, in a case where the coating film swelling width is in a range of 1.5 to 2.5 mm, it was rated as “A”, and in a case where the coating film swelling width exceeds 2.5 mm, it was rated as “B”.

[Workability]

The workability of the plating layer was evaluated by powdering resistance.

A plated steel sheet was cut into a size of 40 mm (C)×100 mm (L)×0.8 mm (t), and this was bent 60° at 5R with the C direction as a bending axis direction by using a V bending tester manufactured by Hoden Seimitsu Kako Kenkyusho Co., Ltd., and then evaluated from the 5-point average value of a peeling width of the plating layer generated by tape peeling.

Specifically, in a case where no peeling occurs, it was rated as “AA”, in a case where the average peeling width is in a range of 0.1 to 0.5 mm, it was rated as “A”, and in a case where the average peeling width exceeds 0.5 mm, it was rated as “B”.

The results are shown in Table 2.

TABLE 2Plating layerMgZn2phaseCa-basedCa-basedintermetallicintermetalliccompound phasecompound phaseMg2Si phaseof 0.10 μm or lessof 0.10 μm or lessTernaryCircle-Number densityNumber density(Al—Zn)eutecticequivalent(piece/(μm ×(piece/(μm ×phasestructurediameterNo.ClassificationArea %μm))μm))area %area %(μm)1Comparative25001065—example2Comparative0001288—example3Invention26601658—example4Invention25661659—example5Comparative1300087—example6Invention33303037—example7Invention33333136—example8Invention39803130—example9Invention431204116—example10Invention441294016—example11Invention441504313—example12Comparative40004416—example13Invention401304218—example14Invention4113104217—example15Invention491404110—example16Invention55130345—example17Comparative38004418—example18Comparative4500437—example19Comparative70110255—example20Comparative3500431212.0example21Invention4820457—example22Invention491204341.9example23Comparative830006—example24Comparative48001537—example25Invention44130497—example26Invention431310498—example27Invention46100504—example28Invention44110533—example29Invention42118535—example30Invention3980556—example31Invention4075555—example32Invention3270653—example33Invention20707010—example34Comparative2500750—examplePlating layerOtherintermetalliccompoundInternalEvaluationCircle-oxidationEndequivalentAlloy layerlayersurfaceMg2Si phasediameterThicknessThicknesscorrosionNo.Area %(μm)Area %(μm)(μm)resistanceWorkability10.0—0.00.21.1BA20.0—0.00.21.2BA30.00.00.10.21.5AA40.00.00.10.21.5AAA50.00.40.10.22.0BA60.0—0.00.31.5AAA70.0—0.00.31.5AAAA80.0—0.00.32.0AAA90.00.20.20.21.0AAAAA100.0—0.00.21.0AAAAA110.02.10.20.31.9AAAAA120.00.00.20.21.8BAA130.00.40.10.32.0AAAAA140.0—0.00.21.8AAAAA150.00.20.20.31.4AAAAA160.03.06.21.62.0AAAAA170.00.00.30.22.0BAA180.00.55.00.31.5BAA190.0—0.00.22.1AB2010.0—0.00.32.2BB210.0—0.00.22.0AAAA224.00.10.20.42.0AAAAA230.02.511.00.32.0AB240.00.00.20.32.0BA250.00.30.20.41.9AAAAA260.0—0.00.41.8AAAAA270.00.40.20.41.2AAAAA280.0—0.00.61.1AAAAA290.0—0.00.61.1AAAAA300.00.40.20.61.3AAAA310.0—0.00.71.4AAAAA320.00.60.20.81.4AAAA330.0—0.01.01.5AAAA340.0—0.02.12.2BAThe underline indicates a value outside the range of the present invention.

As can be seen from the results in Tables 1 and 2, in Nos. 3, 4, 6 to 11, 13 to 16, 21, 22, and 25 to 33, which are the present invention examples, the chemical composition and the area ratio of the MgZn2phase in the cross section in the thickness direction were within the range of the present invention, and the MgZn2phase contained a Ca-based intermetallic compound. Therefore, the end surface corrosion resistance and the workability were exceptional.

On the other hand, in Nos. 1, 2, 5, 12, 17 to 20, 23, 24, and 34, which are comparative examples, one or more of the chemical composition, the area ratio of the MgZn2phase in the cross section in the thickness direction, and the number density of the Ca-based intermetallic compound in the MgZn2phase were out of the range of the present invention. As a result, either the end surface corrosion resistance or the workability was inferior.