Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• Patent Document 2 describes a high-strength hot-rolled steel sheet that satisfies, by mass%, C: 0.015 to 0.10%, Si: 2% or less, Mn: 2% or less, Ti: 0.08 to 0.2%, has a granular bainitic ferrite structure, and has 5 ⁇ 10 6 precipitates/mm 2 or more with a circle equivalent diameter of less than 0.03 ⁇ m and 2 ⁇ 10 6 precipitates/mm 2 or less with a circle equivalent diameter of 0.03 ⁇ m or more.
• Patent Document 2 also teaches that the above configuration can provide a high-strength hot-rolled steel sheet with excellent formability by improving both stretch flangeability and elongation.
• C is an element effective in increasing the strength of steel plate.
• C forms carbides and/or carbonitrides with Ti and Nb in steel, and also contributes to precipitation strengthening based on the formed precipitates.
• the C content is set to 0.030% or more.
• the C content may be 0.040% or more, 0.050% or more, 0.060% or more, or 0.070% or more.
• the C content is set to 0.150% or less.
• the C content may be 0.140% or less, 0.120% or less, 0.100% or less, or 0.080% or less.
• Si 0.01 to 1.00%
• Si is an element that is effective in increasing strength as a solid solution strengthening element.
• Si also has the effect of suppressing the precipitation of cementite. Therefore, by containing Si, it is possible to suppress the consumption of C in the steel to form cementite, and thereby it is possible to promote the formation of TiC precipitates during cooling after hot rolling.
• the Si content is set to 0.01% or more.
• the Si content may be 0.02% or more, 0.03% or more, 0.04% or more, or 0.06% or more.
• Si is contained excessively, a surface quality defect called Si scale may occur, or martensite may be excessively generated.
• the Si content is set to 1.00% or less.
• the Si content may be 0.90% or less, 0.80% or less, 0.60% or less, 0.40% or less, 0.20% or less, 0.10% or less, or 0.08% or less.
• Mn is an element that is effective in increasing strength as a hardenability and solid solution strengthening element. In order to fully obtain these effects, the Mn content is set to 0.50% or more. The Mn content may be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, if Mn is contained excessively, a large amount of MnS may be generated, which may reduce toughness. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
• Ti 0.05-0.20%
• TiC carbide
• TiC carbide
• Ti also forms carbides to fix C, and is an element that suppresses the formation of cementite, which is harmful to hole expansion.
• Ti is an element that contributes to suppressing recrystallization.
• the Ti content is set to 0.05% or more. The Ti content may be 0.08% or more, 0.10% or more, 0.11% or more, 0.12% or more, 0.13% or more, or 0.14% or more.
• the Ti content is set to 0.20% or less.
• the Ti content may be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.
• Al is an element that acts as a deoxidizer for molten steel.
• the Al content is set to 0.01% or more.
• the Al content may be 0.05% or more, 0.06% or more, 0.07% or more, 0.08% or more, 0.09% or more, 0.10% or more, or 0.15% or more.
• Al also has the effect of promoting ferrite transformation and bainite transformation.
• the Al content is preferably set to 0.20% or more.
• the Al content may be 0.22% or more, 0.25% or more, or 0.28% or more.
• the Ti/Al ratio may be 0.14 or more, 0.15 or more, 0.16 or more, 0.18 or more, 0.20 or more, 0.25 or more, 0.30 or more, 0.40 or more, or 0.50 or more.
• the Ti/Al ratio may be 16.00 or less, 14.00 or less, 12.00 or less, 10.00 or less, 8.00 or less, 6.00 or less, 5.00 or less, or 3.33 or less.
• Cr is an element that enhances the hardenability of steel and contributes to improving strength.
• the Cr content may be 0%, but in order to obtain such an effect, the Cr content is preferably 0.001% or more.
• the Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more.
• the Cr content is preferably 2.00% or less.
• the Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
• Mo is an element that enhances the hardenability of steel and contributes to improving strength.
• the Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more.
• the Mo content may be 0.01% or more, 0.02% or more, or 0.05% or more.
• the Mo content is preferably 1.00% or less.
• the Mo content may be 0.80% or less, 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.
• [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness.
• the B content may be 0%, but in order to obtain such an effect, the B content is preferably 0.0001% or more.
• the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.
• Sn and Sb are elements effective for improving corrosion resistance.
• the Sn and Sb contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements may lead to a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.00% or less, and may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
• Ca, Mg and Hf are elements capable of controlling the morphology of nonmetallic inclusions.
• the Ca, Mg and Hf contents may be 0%, but in order to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more or 0.0010% or more.
• the Ca, Mg and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• Bi is an element effective in improving corrosion resistance.
• the Bi content may be 0%, but in order to obtain such an effect, the Bi content is preferably 0.001% or more.
• the Bi content may be 0.002% or more.
• the Bi content is preferably 0.010% or less.
• the Bi content may be 0.005% or less or 0.003% or less.
• REM is an element capable of controlling the form of nonmetallic inclusions.
• the REM content may be 0%, but in order to obtain such an effect, the REM content is preferably 0.0001% or more.
• the REM content may be 0.0005% or more or 0.0010% or more.
• the REM content is preferably 0.0100% or less.
• the REM content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• the As content may be 0%, but in order to obtain such an effect, the As content is preferably 0.001% or more.
• the As content may be 0.002% or more or 0.003% or more.
• the As content is preferably 0.010% or less.
• the As content may be 0.008% or less or 0.005% or less.
• the remaining structure includes at least one of pearlite and retained austenite or is at least one of them, and is preferably pearlite.
• the area ratio of the remaining austenite is 2% or less or 1% or less, preferably 0.5% or less.
• the structure observation is performed with a scanning electron microscope. Prior to the observation, the sample for structure observation is polished by wet polishing with emery paper and diamond abrasive grains having an average particle size of 1 ⁇ m, and the observation surface is mirror-finished, and then the structure is etched with a 3% nitric acid alcohol solution. The magnification of the observation is 2000 times, and 10 random images of a 30 ⁇ m x 40 ⁇ m field of view at a position 1/4 of the plate thickness from the surface are taken. The ratio of the structure is obtained by a point count method.
• a total of 225 lattice points are set at intervals of 3 ⁇ m vertically and 4 ⁇ m horizontally for the obtained structure image, and the structure present under the lattice points is identified, and the structure ratio contained in the steel material is obtained from the average value of the 10 sheets.
• Ferrite is a blocky crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more inside.
• Bainite is a collection of lath-shaped crystal grains, and does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction.
• the group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by 5° or less.
• Bainite is counted as one bainite grain when it is surrounded by grain boundaries with an orientation difference of 15° or more.
• martensite which contains a large amount of dissolved carbon, can be distinguished from other structures because it appears brighter than other structures.
• the area ratio of the remaining structure is determined by subtracting the total area ratio of ferrite, bainite, and martensite from 100%.
• pearlite has a unique structure in which cementite is precipitated in a lamellar form, and therefore can be identified by a scanning electron microscope.
• the volume fraction of the retained austenite can be calculated by X-ray diffraction measurement, and since the volume fraction of the retained austenite is equivalent to the area fraction, this can be regarded as the area fraction of the retained austenite.
• the number density of TiC precipitates with a diameter of less than 10.0 nm is 1.0 ⁇ 10 14 pieces/cm 3 or more
• the number density of TiC precipitates with a diameter of 10.0 to less than 30.0 nm is 1.0 ⁇ 10 10 to 1.0 ⁇ 10 14 pieces/cm 3.
• TiC precipitates include not only TiC but also composite carbides containing Ti and other elements other than Ti, such as V and Nb, that is, (Ti, Nb, V)C, and further NbC.
• NbC can precipitate in addition to (Ti, Nb)C, but the amount of NbC precipitates is extremely small compared to (Ti, Nb)C precipitates. Therefore, even if NbC precipitates are included in the TiC precipitates, the effect of the NbC precipitates on the number density of the TiC precipitates is substantially negligible.
• the number density of TiC precipitates with a diameter of less than 10.0 to 30.0 nm may be 5.0 x 10 10 pieces/cm 3 or more, 1.0 x 10 11 pieces/cm 3 or more, 5.0 x 10 11 pieces/cm 3 or more, or 1.0 x 10 12 pieces/cm 3 or more.
• the number density of TiC precipitates having a diameter of 10.0 to less than 30.0 nm may be 8.0 ⁇ 10 13 particles/cm 3 or less, 6.0 ⁇ 10 13 particles/cm 3 or less, 5.0 ⁇ 10 13 particles/cm 3 or less, 3.0 ⁇ 10 13 particles/cm 3 or less, or 1.0 ⁇ 10 13 particles/cm 3 or less.
• electrolysis is performed under the conditions of 500 mA and 450 C (coulombs) with a 0.05% SDS-10% AA-based electrolyte in order to remove dirt and oxide layers on the sample surface.
• the test material is removed from the electrolyte, the surface is washed with methanol, and then the test material is immersed in a separately prepared 0.05% SDS-10% AA electrolyte, and the second stage of electrolytic extraction (main electrolysis) is performed under conditions of 500 mA and 3600 C (coulombs), and about 1 g of the test material is electrolyzed.
• HSP Hansen Solubility Parameter
• FFF-ICP-MS analysis field flow fractionation
• the area ratio of ferrite in which TiC precipitates with diameters of 10.0 to less than 30.0 nm exist is determined using a transmission electron microscope (TEM) as follows. First, a sample taken from a steel sheet is processed into a 0.1 mm-thick slice including the 1/4 position of the sheet thickness, and then the sample is prepared by electrolytic polishing. This sample is observed using a 200 kV transmission electron microscope, with the electron beam incidence direction on the ferrite adjusted so that sufficient contrast is obtained within a range of 10 degrees from the [001] crystal zone axis, and TiC precipitates are observed.
• TEM transmission electron microscope
• a lattice-like region of 100 nm x 100 nm is set on a transmission electron microscope photograph.
• the number of regions in which TiC precipitates of less than 10.0 to 30.0 nm are observed inside ferrite grains (if even one TiC precipitate of less than 10.0 to 30.0 nm is observed inside a ferrite grain, it is determined that the TiC precipitate is observed in that region) and the number of regions in which ferrite grains are located among the lattice-like regions are counted.
• the ratio of these is regarded as the area ratio of ferrite in which TiC precipitates of less than 10.0 to 30.0 nm in equivalent circle diameter are observed.
• the TiC precipitates located on the lines of the lattice frame are not taken into consideration, and only the TiC precipitates located within the lattice frame are taken into consideration.
• the area ratio of ferrite in which TiC precipitates with diameters of 10.0 to less than 30.0 nm exist is determined by arithmetically averaging the obtained 10 ferrite area ratios. Ferrite is identified in TEM observation by using brightness. Specifically, the average brightness of the entire image is calculated, and the area with a brightness higher than the average brightness is determined as ferrite.
• TiC precipitates are also identified by analyzing the crystal structure from the electron diffraction image of the TEM. Specifically, if the presence of TiC can be confirmed from the electron diffraction image of two or three precipitates present in a ferrite grain, the precipitates observed in the ferrite grain are determined as TiC precipitates.
• the ratio of the average hardness of bainite and martensite to the average hardness of ferrite i.e., (average hardness of bainite and martensite)/(average hardness of ferrite) is 0.80 to 1.20.
• the ratio of the average hardness of bainite and martensite to the average hardness of ferrite may be 0.82 or more, 0.85 or more, 0.88 or more, or 0.90 or more, and may be 1.15 or less, 1.10 or less, 1.05 or less, or 1.00 or less.
• the ratio of the average nanohardness of ferrite to the average nanohardness of bainite is determined as follows. First, a sample is cut out from a steel plate so that a cross section perpendicular to the surface can be observed. The cross section of the sample is polished to a mirror finish by wet polishing with emery paper and diamond abrasive grains with an average particle size of 1 ⁇ m.
• the mirror-finished cross section is indented at a depth of 1/4 of the plate thickness from the surface with a microhardness tester at a test load of 1000 ⁇ N and measurement intervals of 5 ⁇ m, and the nanohardness is measured, obtaining a total of 100 measured values.
• the same sample is measured using a scanning electron microscope, and only measurement points with indentations inside the ferrite and inside the bainite and martensite are extracted with reference to the obtained structure analysis results.
• the arithmetic average of the nanohardnesses for the ten extracted ferrite particles is determined as the average hardness of the ferrite
• the arithmetic average of the nanohardnesses for the ten extracted bainite and martensite particles is determined as the average hardness of the bainite and martensite
• the ratio thereof (average hardness of bainite and martensite)/(average hardness of ferrite) is determined as the ratio of the average hardness of bainite and martensite to the average hardness of ferrite.
• the method is characterized by including an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 600 to 750°C at an average cooling rate of 30 to 200°C/sec, and then intermediate air-cooled at an average cooling rate of 15°C/sec or less for 3 to 15 seconds, and a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C/sec, and then coiled at a coiling temperature of 20 to 290°C.
• the alloy elements can be reliably dissolved in the slab, and in particular Ti can be sufficiently dissolved. If the holding time is short, Ti may remain in the final structure as coarse carbide (TiC).
• the upper limit of the holding time is not particularly limited, but is preferably 4000 seconds or less from the viewpoint of productivity, etc.
• holding in the temperature range of 1150 to 1300 ° C may be performed after rough rolling.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• the end temperature of the finish rolling is important in controlling the metal structure of the steel sheet. If the end temperature of the finish rolling is low, the degree of processing of austenite increases, the nucleation of ferrite is excessively promoted, and the ferrite transformation progresses, and the strength may decrease. In addition, due to the excessive progress of the ferrite transformation, the hardness difference between the ferrite and the hard phases of bainite and martensite increases, and the hole expansion property may decrease. For this reason, the end temperature of the finish rolling is set to 850 ° C. or more.
• hot rolling crushes the crystals in the steel, disrupting the orderly arrangement of Fe atoms in the crystals, resulting in numerous discontinuous structures called deformation bands, and numerous step-like irregularities (ledges) at the grain boundaries.
• deformation bands and ledges allow many new ferrite crystals to be generated from them, accelerating the ferrite transformation.
• austenite recrystallizes during hot rolling, and in this case, the only place where the arrangement of Fe atoms is disrupted is the grain boundaries. For this reason, new ferrite crystals can only be generated from the austenite grain boundaries, and the ferrite transformation cannot be sufficiently promoted in the subsequent intermediate air cooling process.
• the end temperature of the finish rolling is higher than 950°C, coarse TiC precipitates will precipitate during rolling, making it difficult to generate TiC precipitates with the desired diameter and number density in the next intermediate air cooling step. Therefore, in the hot rolling step, it is important to set the end temperature of the finish rolling to 950°C not only to suppress recrystallization and promote ferrite transformation, but also from the perspective of properly generating TiC precipitates.
• the end temperature of the finish rolling is 880 to 920°C.
• the finish-rolled steel sheet is primarily cooled on a run-out table (ROT) to an intermediate cooling temperature of 600 to 750 ° C. at an average cooling rate of 30 to 200 ° C. / sec, and then intermediate cooling is performed for 3 to 15 seconds at an average cooling rate of 15 ° C. / sec or less.
• ROT run-out table
• ferrite transformation can be rapidly promoted due to such slow cooling, and TiC precipitates with a diameter of 10.0 to 30.0 nm or less can be mainly generated on the high temperature side of 700 ° C. or more, and similarly, TiC precipitates with a diameter of less than 10.0 nm can be mainly generated on the low temperature side below 700 ° C.
• the area ratio of ferrite in which TiC precipitates with diameters of less than 10.0 to 30.0 nm exist can also be controlled within a desired range.
• the average cooling rate in the primary cooling from after finish rolling to the intermediate air cooling temperature is set to 30 ° C./sec or more, thereby suppressing excessive generation of ferrite during primary cooling, and promoting ferrite transformation by the next intermediate air cooling at a high temperature, so that TiC precipitates are appropriately generated and grain growth is achieved. If the average cooling rate is less than 30°C/sec, ferrite may be excessively generated or a relatively large amount of coarse TiC precipitates may be generated, and desired properties such as elongation may not be obtained.
• the average cooling rate of the primary cooling is set to 200°C/sec or less, and preferably 150°C/sec or less.
• the intermediate cooling temperature is above 750°C or the intermediate cooling time is above 15 seconds, excessive ferrite is generated or TiC precipitates become coarse. If excessive ferrite is generated, the desired metal structure containing ferrite, bainite and martensite in a specific ratio cannot be formed in the final steel sheet. Furthermore, if the TiC precipitates become coarse, the TiC precipitates cannot be generated in the desired diameter and number density. As a result, the appropriate strength improvement effect and hardness improvement effect of the TiC precipitates cannot be obtained, and the strength, elongation and/or hole expansion property are reduced.
• the intermediate cooling temperature is below 600°C
• the generation of ferrite is suppressed, the area ratio of ferrite in the final metal structure is less than 20%, and properties such as elongation are reduced.
• the average cooling rate during intermediate cooling is more than 15°C/sec, or if the intermediate cooling time is less than 3 seconds
• the formation and grain growth of TiC precipitates are suppressed, and the TiC precipitates cannot be formed with the desired diameter and/or number density.
• the appropriate strength-improving effect and hardness-improving effect of the TiC precipitates cannot be obtained, and the strength, elongation, and/or hole expandability are reduced.
• the formation of ferrite may be excessively suppressed, and in such a case, the desired metal structure containing ferrite, bainite, and martensite in a specific ratio cannot be formed in the final steel sheet.
• intermediate cooling In contrast, in the intermediate cooling step, primary cooling is performed at an average cooling rate of 30 to 200° C./sec, preferably 50 to 150° C./sec, to an intermediate cooling temperature of 600 to 750° C., preferably 650 to 720° C., and then intermediate cooling is performed at an average cooling rate of 15° C./sec or less, preferably 10° C./sec or less, for 3 to 15 seconds, preferably 4 to 12 seconds, thereby precipitating ferrite at a desired rate and generating TiC precipitates with an appropriate diameter and number density, and finally forming TiC precipitates with a diameter of less than 10.0 nm at a number density of 1.0 ⁇ 10 14 precipitates/cm 3 or more and TiC precipitates with a diameter of 10.0 to 30.0 nm at a number density of 1.0 ⁇ 10 10 to 1.0 ⁇ 10 14 precipitates/cm It is possible to limit the area ratio of ferrite in which the TiC precipitates having a diameter of 10.0 to less than 30.0 nm are present to
• the average cooling rate of the secondary cooling is less than 50 ° C./s, C is concentrated in the austenite while being cooled to the transformation point, and bainite and/or martensite transformation may occur, and therefore the desired metal structure cannot be obtained in the finally obtained steel sheet.
• bainite and/or martensite are generated relatively in large amounts, and the desired elongation cannot be achieved.
• the ratio of the average hardness of bainite and martensite to the average hardness of ferrite may be large, and in such a case, the hole expansion property is reduced.

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