Patent ID: 12203148

EXAMPLES

Next, examples of the present invention will be described. Conditions in the examples are examples of the conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to these examples of the conditions. The present invention is capable of adopting a variety of conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steels having a chemical composition shown for Steel Nos. 1 to 42 in Tables 1 and 2 were made from melting, and slabs having a thickness of 240 to 300 mm were manufactured by continuous casting. Hot-rolled steel sheets were obtained under manufacturing conditions shown in Tables 3 and 4 using the obtained slabs. The “average cooling rate between FT and CT” in Tables 3 and 4 indicates the average cooling rate from the start of cooling after hot rolling to coiling (stop of cooling). In addition, before finish rolling, descaling was performed by a normal method (the collision pressure of water to be sprayed was less than 3.0 MPa). Only for No. 41, descaling was performed such that the collision pressure of water to be sprayed became 3.5 MPa.

TABLE 1SteelChemical composition, mass % (remainder: Fe and impurities)No.CSiMnPSAlNTiBCrNbV10.0821.301.920.0820.0030.030.00230.1100.00170.6120.0710.811.650.0480.0090.030.00320.1100.00120.630.0230.0690.711.750.0590.0080.030.00390.1100.00140.680.0540.0650.771.820.0550.0070.030.00240.1100.00150.980.020.0550.0910.621.230.0510.0020.030.00340.1100.00140.6660.0970.671.790.0480.0020.030.00360.1200.00160.5270.1121.111.650.0630.0080.030.00150.0420.00170.4080.1421.192.300.0340.0020.030.00270.0220.00210.7190.1370.751.800.0660.0070.030.00390.0320.00230.62100.1440.702.310.0630.0100.030.00290.0230.00200.30110.0800.751.800.0810.0010.030.00400.0710.00120.620.02120.0610.751.700.0600.0090.030.00250.0870.00130.620.05130.0770.811.720.0250.0090.030.00190.0980.00210.65140.0710.711.850.0780.0030.030.00340.1120.00150.58150.0670.751.900.0630.0060.030.00220.1270.00250.62160.0640.751.700.0650.0040.030.00360.1100.00150.720.02170.0640.751.700.0380.0090.030.00190.1100.00150.710.02180.1600.751.700.0270.0020.030.00170.1100.00150.62190.0350.751.700.0940.0040.030.00220.1100.00150.62200.1100.201.480.0540.0080.030.00340.0530.00140.88SteelChemical composition, mass % (remainder: Fe and impurities)No.MoCuNiSbCaREMMgNote:1Present Invention Steel2Present Invention Steel3Present Invention Steel4Present Invention Steel50.10Present Invention Steel60.100.05Present Invention Steel70.008Present Invention Steel80.002Present Invention Steel90.004Present Invention Steel100.003Present Invention Steel110.10Present Invention Steel120.100.10Present Invention Steel13Present Invention Steel14Present Invention Steel15Present Invention Steel16Present Invention Steel170.10Present Invention Steel18Comparative Steel19Comparative Steel20Comparative SteelUnderlines indicate that values are outside the scope of the present invention.

TABLE 2SteelChemical composition, mass % (remainder: Fe and impurities)No.CSiMnPSAlNTiBCrNbVMoCuNiSbCaREMMgNote210.0951.701.730.0230.0020.030.00370.1150.00050.46ComparativeSteel220.0920.530.800.0580.0040.030.00300.0960.00420.90ComparativeSteel230 0651.092.600.0550.0020.030.00210.1450.00370 30ComparativeSteel240.0920.521.570.0740.0030.030.00210.0000.00500.99ComparativeSteel250.1031.492.130.0280.0020.030.00380.2000.00330.55ComparativeSteel260.0610.731.500.0210.0070.030.00320.0730.00000.25ComparativeSteel270.0400.602.400.0760.0090.030.00270.1070.00210.05ComparativeSteel280.1091.401.970.0510.0020.030.00320.1400.00181.20ComparativeSteel290.1001.462.380.0290.0070.030.00160.0970.00310.16PresentInventionSteel300.1031.451.190.0450.0050.030.00260.0590.00210.98PresentInventionSteel310.0410.611.820.0850.0030.030.00150.0230.00160.63PresentInventionSteel320.0450.631.920.0360.0090.030.00240.04.20.00180.61PresentInventionSteel330.1300.541.800.0660.0020.030.00370.0310.00130.73PresentInventionSteel340.0550.911.730.0570.0100.030.00200.0200.00120.33PresentInventionSteel350.0480.811.650.0840.0030.030.00180.1210.00190.32PresentInventionSteel360.0710.521.840.0630.0020.030.00260.1010.00230.67PresentInventionSteel370.0820.561.820.0650.0050.030.00250.0910.00210.27PresentInventionSteel380.0910.781.540.0350.0020.030.00240.0760.00170.91PresentInventionSteel390.0630.992.120.0980.0050.030.00290.0810.00230.87PresentInventionSteel400.0670.882.230.0610.0010.030.00310.0810.00130.43PresentInventionSteel410.0710.711.820.0510.0020.030.00360.0420.00210.71PresentInventionSteel420.0551.201.850.0070.0050.030.00210.1200.00150.65PresentInventionSteelUnderlines indicate that values are outside the scope of the present invention.

TABLE 3FinishAverageRollingrollingCoolingcooling rateCoilHeatingstartcompletionstartbetween FTCoilingcoolingTestSteeltemperaturetemperaturetemperaturetimeand CTtemperaturerateNo.No.° C.° C.° C.Seconds° C./sec° C.° C./hourNote11126411379550.66148133Present Invention Example22129511139650.78042137Present Invention Example33125011869620.87743228Present Invention Example44128711089710.65644136Present Invention Example55128511309830.55945135Present Invention Example66127711609850.75746233Present Invention Example77126411229880.65347131Present Invention Example88129111869920.76148938Present Invention Example99125311019720.84649532Present Invention Example1010129211869810.97643536Present Invention Example1111130011339810.18147531Present Invention Example1212128811049990.18342232Present Invention Example1313127911889820.29143234Present Invention Example1414128711439750.110244131Present Invention Example1515127311649610.112245932Present Invention Example1616126511769650.211142733Present Invention Example1717127511369810.19547931Present Invention Example1818127511669720.49840529Comparative Example1912126111299720.29845028Comparative Example2020129511579720.79546335Comparative ExampleUnderlines indicate that values are outside the scope of the present invention.

TABLE 4FinishAverageRollingrollingCoolingcooling rateCoilHeatingstartcompletionstartbetween FTCoilingcoolingTestSteeltemperaturetemperaturetemperaturetimeand CTtemperaturerateNo.No.° C.° C.° C.Seconds° C./sec° C.° C./hourNote2121128711599720.49948529Comparative Example2222127311439720.710142631Comparative Example2323126711029720.710143733Comparative Example2424129011019510.99945141Comparative Example2525126411399610.910141950Comparative Example2626128611859630.810149280Comparative Example2727126511109830.89943495Comparative Example2828125611749720.8101463100Comparative Example2929127711008700.6101451100Comparative Example30301287119610610.39944277Comparative Example31311273117510301.210143287Comparative Example32321271112310201.610144135Comparative Example3333127811759830.91047541Comparative Example3434129711239720.516049455Comparative Example3535127311759800.49838551Comparative Example3636129111239510.59935026Comparative Example3737128211759710.69746510Comparative Example3838125611239820.810242620Comparative Example3939128711759110.710143831Comparative Example4040127711239820.92545432Comparative Example4141128711309850.75144233Comparative Example4242127311769500.48345173Comparative ExampleUnderlines indicate that values are outside the scope of the present invention.

For the obtained hot-rolled steel sheets, the microstructural fraction at the ¼ position of the sheet thickness in the sheet thickness direction from the surface, the average grain size of the secondary phase, the average grain size of the particles having grain diameters that are largest 10% or less out of all of the particles in the secondary phase, the pole density in the (110)<112> orientation, the average interval between precipitates having a diameter of 20 nm or less, and the pole density in the (110)<1-11> orientation in the microstructure from the surface to the 1/16 position of the sheet thickness in the sheet thickness direction from the surface were obtained by the above-described methods. In Test Nos. 18, 33, 35, and 36, the secondary phase particles were connected, and it was not possible to measure the grain diameters as particles.

The obtained results are shown in Tables 5 and 6. In examples where the total of the area ratios of bainite and the secondary phase did not reach 100%, the remainder of the microstructure was ferrite. In addition, in Test No. 24, no precipitates having a diameter of 20 nm or less were observed.

TABLE 5Pole densityAverageAverage grainPole densityin (110)<1-11>intervalAveragesize of particlesin (110)<112>orientation frombetweengrainhaving grainorientation atsurface to sheetprecipitatessize ofdiameters thatsheet thicknessthickness 1/16havingSecondarysecondaryare largest¼ positionpositiondiameter of 20TestSteelBainitephasephase10% or lessfrom Surfacefrom surfacenm or lessNo.No.Area %Area %μmμm——nmNote1197.92.11.42.02.32.245Present Invention Example2296.8221.32.01.91.842Present Invention Example3393.96.11.42.22.32.543Present Invention Example4491.18.91.32.21.81.838Present Invention Example5595.44.61.42.22.22.442Present Invention Example6691.78.31.42.12.22.345Present Invention Example7796.43.61.42.11.81.831Present Invention Example8890.79.31.32.42.01.935Present Invention Example9997.12.91.42.42.11.837Present Invention Example101098.02.01.32.02.52.246Present Invention Example111194.75.31.32.02.12.549Present Invention Example121294.85.21.32.12.52.245Present Invention Example131395.84.21.32.12.11.9111Present Invention Example141491.09.01.42.11.82.1152Present Invention Example151590.99.11.32.22.12.098Present Invention Example161694.15.91.42.02.21.985Present Invention Example171797.82.21.42.32.41.9201Present Invention Example181812.088.0——2.42.235Comparative Example191299.01.01.42.32.22.037Comparative Example202092.08.01.42.02.32.421Comparative ExampleUnderlines indicate that values are outside the scope of the present invention.

TABLE 6Pole densityAverageAverage grainPole densityin (110)<I-11>intervalAveragesize of particlesin (110)<112>orientation frombetweengrainhaving grainorientation atsurface to sheetprecipitatessize ofdiameters thatsheet thicknessthickness 1/16havingSecondarysecondaryare largest¼ positionpositiondiameter of 20TestSteelBainitephasephase10% or lessfrom surfacefrom surfacenm or lessNo.No.Area %Area %μmμm——nmNote212195.24.81.32.22.42.022Comparative Example222232.068.01.32.12.52.430Comparative Example222386.913.11.42.12.21.933Comparative Example242493.86.21.32.42.12.4—Comparative Example252297 82.21.42.13.44.329Comparative Example262629.80.21.32.01.92.234Comparative Example272783.03.11.42.31.82.123Comparative Example282887.812.21.32.42.02.235Comparative Example292993.46.61.42.13.14.238Comparative Example303094.25.81.62.22.51.641Comparative Example213193.26.81.42.61.82.549Comparative Example323297.22.81.42.82.41.848Comparative Example223334.26.2——1.82.235Comparative Example343489.910.11.32.02.83.136Comparative Example253515.085.0——2.22.135Comparative Example36360.0100.0——2.21.949Comparative Example373798.51.51.32.32.42.148Comparative Example383898.11.91.32.12.02.035Comparative Example393992.37.71.32.22.53.240Comparative Example404087.34.41.42.22.52.115Comparative Example414191.28.81.42.01.93.538Comparative Example424293.16.91.32.13.23.435Comparative ExampleUnderlines indicate that values are outside the scope of the present invention.

For the obtained hot-rolled steel sheets, the tensile strengths TS, the total elongations El, the hole expansion rates λ, the limit bend radii R, and the ductile brittle, transition temperatures vTrs were obtained by the following methods.

Tensile Strength TS and Total Elongation El

The tensile strength TS and the total elongation El were obtained by performing a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241: 2011. The cross-head speed was set to 10 mm/min. Cases where the tensile strength TS was 980 MPa or more were regarded as being excellent in terms of strength and determined as pass, and cases where the tensile strength was less than 980 MPa were regarded as being poor in strength and determined as fail. Cases where the total elongation El was 13.0% or more were regarded as being excellent in terms of ductility and determined as pass, and cases where the total elongation El was less than 13.0% were regarded as being poor in ductility and determined as fail.

Hole Expansion Rate 7

The hole expansibility was evaluated with the hole expansion rate λ that was obtained by punching a circular hole with a diameter of 10 mm using a 60° conical punch under a condition where the clearance became 12.5% and performing a hole expansion test such that burrs were formed on the die side. For each test number, a hole expansion test was performed five times, and the average value thereof was calculated, thereby obtaining the hole expansion rate λ. Cases where the hole expansion rate was 60% or more were regarded as being excellent in terms of hole expansibility and determined as pass, and cases where the hole expansion rate was less than 60% were regarded as being poor in hole expansibility and determined as fail.

Limit Bend Radius R

The bendability was evaluated with the limit bend radius R that was obtained by performing a V-bending test. The limit bend radius R was obtained by performing a V-bending test using a No. 1 test piece in accordance with JIS Z 2248: 2014 such that a direction perpendicular to a rolling direction became the longitudinal direction (the bend ridge line coincided with the rolling direction).

The V-bending test was performed by setting the angle between a die and a punch to 60° and changing the tip radii of the punches in 0.1 mm increments, and the maximum value of the tip radii of the punches that could be bent without cracking was obtained. The maximum value of the tip radii of the punches that could be bent without crack was regarded as the limit bend radius R. In a case where a value (R/t) obtained by dividing the limit bend radius R by the sheet thickness t of the test piece was 1.0 or less, the bendability was regarded as being excellent, determined as pass, and expressed as “Good” in Tables 7 and 8. On the other hand, in a case where a value (R/t) obtained by dividing the limit bend radius R by the sheet thickness t of the test piece was more than 1.0, the bendability was regarded as being poor, determined as fail, and expressed as “Bad” in Tables 7 and 8.

Ductile Brittle Transition Temperature vTrs

For the ductile brittle transition temperature vTrs, a Charpy impact test was performed using a V-notch test piece having a subsize of 2.5 mm regulated in JIS Z 2242: 2018. A temperature at which the brittle fracture surface ratio became 50% was obtained, and this was regarded as the ductile brittle transition temperature vTrs. In a case where the ductile brittle transition temperature vTrs was −40° C. or lower (−40° C. was included, negative values from −40° C.), the low temperature toughness was regarded as being excellent and determined as pass, and, in a case where the ductile brittle transition temperature vTrs was higher than −40° C. (−40° C. was not included, positive values from −40° C.), the low temperature toughness was regarded as being poor and determined as fail. In addition, in a case where the ductile brittle transition temperature vTrs was −70° C. or lower, the low temperature toughness was determined as more excellent.

The above-described test results are shown in Tables 7 and 8.

TABLE 7TensileTotalHoleDuctile brittlestrengthelongationexpansiontransitionTestSteelTSEIrate λtemperature vTrsNo.No.MPa%%Bendability° C.Note11102813.263Good−45Present Invention Example22103513.169Good−52Present Invention Example33102013.169Good−55Present Invention Example4499113.266Good−65Present Invention Example55105713.360Good−46Present Invention Example66103213.163Good−41Present Invention Example77107913.260Good−47Present Invention Example88101513.567Good−54Present Invention Example99100413.462Good−60Present Invention Example1010106613.164Good−58Present Invention Example1111100613.667Good−49Present Invention Example121298713.569Good−45Present Invention Example1313103413.360Good−82Present Invention Example1414102113.266Good−84Present Invention Example1515101213.465Good−77Present Invention Example1616101513.166Good−79Present Invention Example171799813.264Good−81Present Invention Example1818121010.862Good−53Comparative Example191990514.567Good−49Comparative Example202096513.363Good−42Comparative ExampleUnderlines indicate that values are outside the scope of the present invention or are not preferable characteristics.

TABLE 8TensileTotalHoleDuctile brittlestrengthelongationexpansiontransitionTestSteelTSEIrate λtemperature vTrsNo.No.MPa%%Bendability° C.Note2121102113.560Good−30Comparative Example2222102111.561Good−43Comparative Example2323107413.445Good−30Comparative Example242497113.561Good−47Comparative Example2525107713.255Bad−20Comparative Example262671219.069Good−52Comparative Example272787017.062Good−64Comparative Example2828104311.267Good−54Comparative Example2929102513.145Bad−51Comparative Example3030103413.161Good−21Comparative Example3131102513.347Good−10Comparative Example3232105513.752Good−5Comparative Example333378218.030Good−68Comparative Example3434103112.868Bad−69Comparative Example3535102010.064Good−65Comparative Example363610509.870Good−48Comparative Example373798214.240Good−41Comparative Example3838104913.158Good−68Comparative Example393999213.161Bad−69Comparative Example404092013.260Good10Comparative Example4141102213.565Bad−55Comparative Example4242100214.151Bad−65Comparative ExampleUnderlines indicate that values are outside the scope of the present invention or are not preferable characteristics.

From Tables 5 to 8, it is found that the present invention examples are excellent in terms of strength, ductility, bendability, hole expansibility, and low temperature toughness. In addition, it is found that the present invention examples in which the average interval between precipitates having a diameter of 20 nm or less was 50 nm or more have more excellent low temperature toughness.

On the other hand, it is found that the comparative examples are poor in one or more characteristics of strength, ductility, bendability and hole expansibility.

INDUSTRIAL APPLICABILITY

According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet being excellent in terms of strength, ductility, bendability, hole expansibility, and low temperature toughness and a manufacturing method thereof.