WO2023026582A1 - 熱間圧延鋼板 - Google Patents

熱間圧延鋼板 Download PDF

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WO2023026582A1
WO2023026582A1 PCT/JP2022/017417 JP2022017417W WO2023026582A1 WO 2023026582 A1 WO2023026582 A1 WO 2023026582A1 JP 2022017417 W JP2022017417 W JP 2022017417W WO 2023026582 A1 WO2023026582 A1 WO 2023026582A1
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content
pearlite
steel sheet
hot
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PCT/JP2022/017417
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English (en)
French (fr)
Japanese (ja)
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耕平 中田
武 豊田
駿介 小林
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日本製鉄株式会社
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    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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

Definitions

  • the present invention relates to a hot-rolled steel sheet, and more particularly, to a hot-rolled steel sheet used for structural members of automobiles and the like, which has high strength, uniform elongation, and excellent punched end face fatigue properties. .
  • Patent Document 1 it has a predetermined chemical composition, and the metal structure has an area ratio of pearlite: 90 to 100%, pseudo pearlite: 0 to 10%, and proeutectoid ferrite: 0 to 1%, and the pearlite is 0.20 ⁇ m or less, and the average pearlite block diameter of the pearlite is 20.0 ⁇ m or less.
  • Patent Document 1 describes that with the above configuration, it is possible to obtain a hot-rolled steel sheet having a high tensile strength of 980 MPa or more and excellent ductility, hole expansibility, and punchability. .
  • Patent Document 2 in relation to the pearlite-based structure as described in Patent Document 1, in Patent Document 2, in the case of a hot-rolled steel sheet, if the growth time during pearlite transformation is sufficient, it is possible to have an elongated form of lamellar cementite. stated to be common. Further, Patent Document 3 describes a hot-rolled steel sheet having a pearlite structure as a main phase, a ferrite structure in the residual structure of 20% or less, and a lamellar spacing of the pearlite structure of 500 nm or less.
  • Patent Document 1 teaches that a total elongation of 13% or more can be achieved in relation to ductility, it does not specifically show improvement in uniform elongation.
  • Patent Document 1 in relation to punchability, although it is specifically disclosed that the occurrence of cracks on the end face during punching is suppressed, improvement in fatigue characteristics on the punched end face is not necessarily sufficient. No consideration has been made. Therefore, the hot-rolled steel sheet described in Patent Document 1 still has room for improvement in terms of uniform elongation and fatigue properties of punched edge faces.
  • an object of the present invention is to provide a hot-rolled steel sheet with high strength and excellent uniform elongation and punched end face fatigue properties by a novel configuration.
  • the present inventors improved the strength of the steel sheet by utilizing the precipitation strengthening by Cu, and appropriately controlled the C and Cr contents in the steel sheet. It was found that uniform elongation can be improved by increasing the percentage of high pearlite, and on the other hand, fatigue characteristics of the punched end face can be improved by making the cementite in the pearlite layered (lamellar). perfected the invention.
  • the present invention that has achieved the above object is as follows.
  • the microstructure is the area ratio, Perlite: 70% or more, ferrite: 0-30%, and bainite: 0-30%, Less than 10 pieces of cementite per 10 ⁇ m 2 having a major
  • the hot-rolled steel sheet according to (1) above comprising one or more selected from the group consisting of: (3) The hot-rolled steel sheet according to (1) or (2) above, which has a thickness of 1.0 to 6.0 mm.
  • a hot-rolled steel sheet is The chemical composition, in mass %, C: 0.20 to 0.30%, Si: 0.01 to 2.00%, Mn: 0.50-3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.005 to 3.000%, N: 0.0100% or less, O: 0.0100% or less, Cr: more than 1.00 to 3.00%, Cu: more than 1.00 to 3.00%, Ti: 0 to 0.10%, Nb: 0 to 0.10%, V: 0 to 0.10%, Ni: 0 to 2.00%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, REM: 0-0.005%, and balance: Fe and impurities,
  • the microstructure is the area ratio, Perlite: 70% or more, ferrite: 0-30%, and bainite: 0-30%, Less than 10 pieces of cementite per 10 ⁇
  • the present inventors first focused on pearlite, which has the highest work hardening ability among microstructures other than retained austenite, and appropriately controlled the contents of C and Cr in the steel sheet to make the pearlite an area ratio 70% or more, and if there is a residual structure, it is mainly composed of ferrite and / or bainite, thereby significantly improving the uniform elongation and improving the fatigue characteristics of the punched end face.
  • the inventors have found that increasing the Cr content to over 1.00% can extend the region of pearlite formation to the low carbon side, resulting in a relatively low carbon content of 0.20-0.30%. It was found that a high pearlite fraction of 70% or more in terms of area ratio can be achieved in spite of the low C content.
  • the present inventors have found that in such a hot-rolled steel sheet having a relatively low C content of 0.20 to 0.30% and a pearlite-based microstructure with a Cr content of more than 1.00%, It was found that the fatigue characteristics of the punched end face can be improved by substantially not including martensite with high hardness and low toughness and retained austenite that generates the martensite by deformation-induced transformation.
  • the present inventors have found such a relatively low C content of 0.20 to 0.30% and a Cr content of more than 1.00%, and an area ratio of 70% or more
  • studies were conducted focusing on the form of cementite in the pearlite.
  • minute voids may occur starting from cementite or the interface between cementite and ferrite on the punched end face. These voids can be the cause of deterioration in the fatigue properties of the punched end face after punching.
  • the present inventors have found that the amount of coarse spheroidal cementite in pearlite should be reduced, more specifically pearlite having a long axis length of more than 0.3 ⁇ m and an aspect ratio of less than 3.0
  • the number density of cementite By limiting the number density of cementite to less than 10 per 10 ⁇ m 2 , the occurrence of voids during punching can be suppressed, and as a result, the fatigue properties at the punched end face can be significantly improved.
  • Ti, Nb and V are elements that generally contribute to the improvement of steel sheet strength through carbide precipitation.
  • the hot-rolled steel sheet according to the embodiment of the present invention contains only a relatively low content of 0.20 to 0.30% of C, which is an element effective in improving the strength of the steel sheet, as described above. is. Therefore, it is generally difficult to achieve a high strength, specifically a tensile strength of 900 MPa or more, while suppressing the contents of these elements to relatively low contents. Therefore, the present inventors have found that, despite such a relatively low C content, Cu is contained in the steel sheet in an amount exceeding 1.00%, so that the steel sheet is strengthened by precipitation strengthening by Cu. It has been found that the strength can be maintained at a high level.
  • the present inventors mainly combined the following four findings to develop a hot-rolled steel sheet having a high tensile strength of 900 MPa or more, uniform elongation, and excellent punching edge fatigue characteristics.
  • the combination of these findings and the fact that a hot-rolled steel sheet having high strength, uniform elongation and excellent punched end face fatigue characteristics can be obtained by this combination has not been known in the past, and this time, the present invention
  • the residual structure is composed of ferrite and/or bainite to improve the fatigue properties of the punched edge by being substantially free of martensite in the microstructure and retained austenite that produces said martensite by strain-induced transformation.
  • the hot-rolled steel sheet according to the embodiment of the present invention will be described in more detail.
  • the unit of content of each element means “% by mass” unless otherwise specified.
  • the term “to” indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.
  • C is an essential element for ensuring the strength of the hot-rolled steel sheet.
  • the C content is made 0.20% or more.
  • the C content may be 0.21% or more, 0.22% or more, or 0.23% or more.
  • an excessive C content may deteriorate the weldability of the steel sheet. Therefore, the C content should be 0.30% or less.
  • C content may be less than 0.30%, 0.29% or less, 0.28% or less, 0.27% or less, 0.26% or less, 0.25% or less or 0.24% or less .
  • the C content since the C content is relatively low as described above, it is advantageous from the viewpoint of weldability. Is possible.
  • Si is an element used for deoxidizing steel.
  • the Si content is set to 0.01% or more.
  • the Si content may be 0.10% or more, 0.20% or more, 0.30% or more, or 0.50% or more.
  • the Si content is set to 2.00% or less.
  • the Si content may be 1.85% or less, 1.70% or less, 1.50% or less, or 1.40% or less.
  • Mn is an element effective for delaying phase transformation of steel and preventing phase transformation from occurring during cooling.
  • the Mn content is made 0.50% or more.
  • the Mn content may be 0.60% or more, 0.80% or more, 1.00% or more, 1.30% or more, or 1.50% or more.
  • the Mn content should be 3.00% or less.
  • the Mn content may be 2.80% or less, 2.70% or less, 2.50% or less, or 2.20% or less.
  • P is an element mixed in during the manufacturing process.
  • the P content may be 0%, but excessive reduction leads to increased costs. Therefore, the P content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.005% or more.
  • the S content is preferably 0.0090% or less, more preferably 0.0085% or less or 0.0070% or less.
  • the S content may be 0%, but excessive reduction leads to an increase in cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.
  • Al is an element used for deoxidizing steel.
  • the Al content is made 0.005% or more.
  • the Al content may be 0.010% or more, 0.030% or more, 0.050% or more, 0.100% or more, or 0.300% or more.
  • an excessive Al content may increase inclusions and deteriorate the workability of the steel sheet. Therefore, the Al content is set to 3.000% or less.
  • the Al content may be 2.800% or less, 2.500% or less, 2.000% or less, 1.800% or less, 1.500% or less, or 1.200% or less.
  • N 0.0100% or less
  • N combines with Al in the steel to form AlN, which prevents the pearlite block diameter from increasing due to the pinning effect, thereby improving the toughness of the steel.
  • an excessive N content saturates the effect and may rather cause a decrease in toughness. Therefore, the N content is set to 0.0100% or less.
  • the N content is preferably 0.0090% or less, 0.0080% or less or 0.0070% or less. From such a point of view, the N content may be 0%, but excessive reduction leads to an increase in cost. Therefore, the N content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.
  • O is an element mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed to lower the toughness of the steel sheet. Therefore, the O content is 0.0100% or less.
  • the O content may be 0.0090% or less, 0.0080% or less, 0.0070% or less, or 0.0060% or less.
  • the O content may be 0%, but excessive reduction leads to increased costs. Therefore, the O content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.
  • Cr more than 1.00 to 3.00%
  • Cr is an element that contributes to improving the strength of the steel sheet, and also has the effect of suppressing spheroidization of cementite. Therefore, in order to reduce the number density of coarse spheroidal cementite in the pearlite and suppress the generation of voids during punching, it is necessary to contain a certain amount or more of Cr. Furthermore, since Cr stabilizes cementite, the inclusion of Cr can expand the pearlite formation region toward the low carbon content side. Therefore, by including Cr in an appropriate amount, ie, an amount exceeding 1.00%, it is possible to achieve a pearlite fraction of 70% or more even with a relatively low C content.
  • Cr content is 1.01% or more, 1.02% or more, 1.03% or more, 1.05% or more, 1.10% or more, 1.30% or more, 1.50% or more, or 1.70% or more.
  • Cr content is set to 3.00% or less.
  • the Cr content may be 2.80% or less, 2.70% or less, 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.
  • Cu is an element effective in improving strength by precipitation strengthening. Also, unlike Ti, Nb and V, Cu can improve the strength of the steel sheet without forming carbides. Therefore, Cu does not consume the carbon required to form the desired pearlite structure in which coarse spherical cementite is reduced. Therefore, Cu is an extremely important element for achieving both strength improvement and punch end face fatigue properties. In order to sufficiently obtain these effects, the Cu content should be more than 1.00%. Cu content is 1.01% or more, 1.02% or more, 1.03% or more, 1.05% or more, 1.10% or more, 1.20% or more, 1.30% or more, 1.50% or more or 1.70% or more.
  • the Cu content is set to 3.00% or less.
  • the Cu content may be 2.80% or less, 2.70% or less, 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.
  • the hot-rolled steel sheet may contain at least one of the following optional elements in place of part of the remaining Fe, if necessary.
  • hot-rolled steel sheets contain Ti: 0-0.10%, Nb: 0-0.10%, V: 0-0.10%, Ni: 0-2.00% and Mo: 0-1. It may contain at least one selected from the group consisting of 00%.
  • the hot rolled steel sheet may contain B: 0 to 0.0100%.
  • the hot-rolled steel sheet may contain at least one selected from the group consisting of Ca: 0 to 0.0050% and REM: 0 to 0.005%.
  • Ti, Nb and V are elements that contribute to the improvement of steel sheet strength by precipitation of carbides.
  • the Ti, Nb and V contents may be 0%, but in order to obtain the above effect, one selected from these may be contained alone or two or more may be combined as needed. good.
  • the Ti, Nb and V contents are each preferably 0.001% or more, and may be 0.01% or more, 0.02% or more, or 0.03% or more. .
  • the Ti, Ni and V contents are each preferably 0.10% or less.
  • the Ti, Ni and V contents may each be 0.08% or less, 0.06% or less or 0.05% or less.
  • Ni is an element that dissolves in steel to increase strength without impairing toughness.
  • the Ni content may be 0%, the Ni content is preferably 0.001% or more in order to obtain the above effects.
  • the Ni content may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.50% or more.
  • Ni is an expensive element, and excessive addition causes an increase in cost. Therefore, the Ni content is preferably 2.00% or less.
  • the Ni content may be 1.70% or less, 1.50% or less, or 1.20% or less.
  • Mo is an element that increases the strength of steel.
  • the Mo content may be 0%, the Mo content is preferably 0.001% or more in order to obtain the above effects.
  • the Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more.
  • the Mo content is preferably 1.00% or less.
  • Mo content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less .
  • B segregates at grain boundaries and has the effect of increasing grain boundary strength.
  • the B content may be 0%, the B content is preferably 0.0001% or more in order to obtain the above effect.
  • the B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more.
  • the B content is preferably 0.0100% or less.
  • the B content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.
  • Ca is an element that improves workability by controlling the form of non-metallic inclusions that act as starting points for fracture and cause deterioration in workability.
  • the Ca content may be 0%, the Ca content is preferably 0.0001% or more in order to obtain the above effect.
  • the Ca content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more.
  • the Ca content is preferably 0.0050% or less.
  • the Ca content may be 0.0045% or less or 0.0040% or less.
  • REM is an element that improves the toughness of weld zones by adding a small amount of REM.
  • the REM content may be 0%, the REM content is preferably 0.0001% or more in order to obtain the above effects.
  • the REM content may be 0.0003% or greater, 0.0005% or greater, or 0.001% or greater.
  • an excessive REM content may reduce weldability. Therefore, the REM content is preferably 0.005% or less.
  • the REM content may be 0.004% or less or 0.003% or less.
  • REM herein refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoids lanthanum with atomic number 57 (La) to lutetium with atomic number 71 (Lu ), and the REM content is the total content of these elements.
  • the balance other than the above elements consists of Fe and impurities.
  • impurities refers to components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when hot-rolled steel sheets are industrially manufactured. Examples of impurities include Sn: 0.02% or less, Sb: 0.02% or less, W: 0.015% or less, and Co: 0.015% or less.
  • the microstructure of the hot-rolled steel sheet contains 70% or more pearlite in terms of area ratio.
  • Pearlite has the highest work hardening ability among microstructures other than retained austenite. It becomes possible to For example, if ferrite is excessively generated and as a result the area ratio of pearlite is less than 70%, sufficient strength may not be ensured.
  • the area ratio of pearlite is 70% or more, and may be 72% or more, 75% or more, 77% or more, 80% or more, 85% or more, or 90% or more.
  • the upper limit of the area ratio of pearlite is not particularly limited, and may be 100%.
  • the perlite area ratio may be 99% or less, 98% or less, 96% or less, 94% or less, 92% or less, 90% or less, 87% or less, 83% or less, or 79% or less.
  • the remaining structure other than pearlite may have an area ratio of 0%, but if there is a remaining structure, it is composed of at least one of ferrite and bainite. Therefore, the area ratio of ferrite and bainite is set to 0 to 30%. Ferrite and bainite may each have an area ratio of 1% or more, 2% or more, 4% or more, or 6% or more. Similarly, ferrite and bainite may each have an area ratio of 25% or less, 20% or less, 15% or less, or 10% or less.
  • the residual structure is composed of ferrite and/or bainite, that is, the residual structure does not contain or substantially does not contain high-hardness, low-toughness martensite or retained austenite that forms the martensite by deformation-induced transformation. Therefore, it is possible to secure good punching end face fatigue characteristics.
  • the expression "substantially absent” or “substantially free of” means that the total area fraction of martensite and retained austenite in the residual structure is less than 0.5%. is. It is difficult to accurately measure the total amount of such microstructures, and the effect thereof can be ignored.
  • the total area ratio of ferrite and bainite is 30% or less, 25% or less, 21% or less, 17% or less, 13% or less, 10% or less, 8% or less, 6% or less, 4% or less, 2 % or less, 1% or less, or 0%.
  • the total area ratio of ferrite and bainite is 1% or more, 2% or more, 4% or more, 6% or more, 8% or more, 10% or more, 13% or more, 17 % or more or 21% or more.
  • the number density of coarse spherical cementite among the cementites constituting pearlite is limited within a predetermined range.
  • cementite having an aspect ratio of less than 3.0 is less than 10 per 10 ⁇ m 2 .
  • the aspect ratio of cementite refers to a value obtained by dividing the length of the major axis of cementite appearing on the viewing surface by the length of the minor axis.
  • Cementite having a major axis length of more than 0.3 ⁇ m and an aspect ratio of less than 3.0 is defined herein as coarse spheroidal cementite.
  • coarse spheroidal cementite may become a starting point of void generation during punching of a steel plate, and these voids can be a cause of deterioration of fatigue properties at the punched end face after punching. Therefore, it is extremely important to reduce the amount of such coarse spheroidal cementite in order to improve the fatigue properties of the punched end face.
  • the number density of the coarse spherical cementite is limited to less than 10 per 10 ⁇ m 2 , the generation of voids during punching can be reliably suppressed. As a result, it is possible to remarkably improve the fatigue properties of the punched end faces.
  • the number density of coarse spherical cementites may be 8 or less, 6 or less, or 4 or less per 10 ⁇ m 2 in pearlite.
  • the number density of the coarse spherical cementite may be 0 per 10 ⁇ m 2 in the pearlite, but may be, for example, 1 or more or 2 or more.
  • the aspect ratio refers to the ratio of the length of the major axis to the length of the minor axis of an ellipsoid when ellipsoid approximation processing is performed on individual cementites by image processing.
  • the area ratio of the microstructure is determined as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). The measurement area is an area of 12,000 ⁇ m 2 (for example, an area of 80 ⁇ m ⁇ 150 ⁇ m), and the area ratios of pearlite and ferrite are calculated from a structure photograph at a magnification of, for example, about 5000 times using the point counting method.
  • SEM scanning electron microscope
  • pearlite is a region surrounded by grain boundaries where the ferrite crystal orientation difference is 15° or more, where the ferrite phase and the cementite phase are mixed, and the cementite has a lamellar and/or spherical form. and certify. Therefore, for example, pearlite has a layered (lamellar) dispersed structure of ferrite phase and cementite, as well as a structure mainly composed of cementite dispersed in clusters. It also includes a structure containing more than 50% in terms of area ratio with respect to the total amount of cementite.
  • it is an aggregate of lath-shaped crystal grains, and has a plurality of iron-based carbides having a major axis of 20 nm or more inside the laths, and these carbides are a single variant, that is, an iron-based carbide elongated in the same direction. Those belonging to the group are identified as bainite. Inclusions observed in the pearlite structure are basically cementite, and using a scanning electron microscope with an energy dispersive X-ray spectroscope (SEM-EDS), etc., individual inclusions are identified as cementite or iron-based carbides. need not be identified.
  • SEM-EDS energy dispersive X-ray spectroscope
  • the inclusion is cementite or iron-based carbide
  • the inclusion may be analyzed using SEM-EDS or the like separately from SEM observation, if necessary.
  • Retained austenite has a cementite area fraction of less than 1% inside, and if such a structure exists, it is analyzed using electron back scatter diffraction (EBSD) after observing the structure with SEM. , fcc structure is determined as retained austenite.
  • EBSD electron back scatter diffraction
  • the number density of coarse spherical cementite is determined as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • the measurement area is a 12,000 ⁇ m 2 (for example, an area of 80 ⁇ m ⁇ 150 ⁇ m) SEM photograph of about 5000 times, and the image of the area recognized as pearlite is binarized, and the dark part is ferrite and the bright part is cementite. .
  • ellipsoid approximation is performed by image processing, and the length of the major axis and the length of the minor axis of the ellipsoid are calculated as the length of the major axis and the length of the minor axis of each cementite. and the aspect ratio of each cementite is defined by the following equation.
  • a hot-rolled steel sheet having the above chemical composition and structure can achieve a high tensile strength, specifically a tensile strength of 900 MPa or more.
  • the tensile strength is preferably 910 MPa or higher or 920 MPa or higher, more preferably 940 MPa or higher or 980 MPa or higher, and most preferably 1000 MPa or higher or 1080 MPa or higher.
  • the tensile strength may be 1500 MPa or less or 1400 MPa or less.
  • the hot-rolled steel sheet having the above chemical composition and structure can achieve high uniform elongation, more specifically 7.0% or more, preferably 7.5% or more, and more Preferably, a uniform elongation of 8.0% or more can be achieved.
  • the uniform elongation may be 20.0% or less or 15.0% or less.
  • the tensile strength and uniform elongation are measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the direction perpendicular to the rolling direction of the hot-rolled steel sheet and performing a tensile test in accordance with JIS Z2241:2011. be.
  • Uniform elongation means plastic elongation (%) at maximum test force defined in JIS Z2241:2011.
  • a hot-rolled steel sheet having the chemical composition and structure described above can achieve high punch end face fatigue properties. More specifically, in the punching fatigue limit ratio corresponding to the value ( ⁇ F /TS) obtained by dividing the fatigue limit ⁇ F (MPa) determined based on the fatigue test of the punched end face by the tensile strength TS (MPa) A punched edge fatigue property of 0.28 or higher can be achieved.
  • the punching fatigue limit ratio is preferably 0.30 or more, more preferably 0.32 or more. Although there is no need to specify the upper limit, for example, the punching fatigue limit ratio may be 0.42 or less or 0.40 or less.
  • the punching fatigue limit ratio is determined by the following method using a plate-shaped test piece having the dimensions shown in FIG.
  • a plate-shaped test piece (30 mm ⁇ 90 mm) sampled from a hot-rolled steel plate so that the rolling direction is the long side is punched in the center with a punch diameter of 10 mm and a punching clearance of 12%, and then a constant stress amplitude.
  • a double-sided plane bending fatigue test is performed at ⁇ (MPa).
  • the plate-shaped test piece may be chamfered at the corners of the plate end surface as shown in FIG.
  • a double-sided plane bending fatigue test was performed until the number of repetitions N reached 10 7 times, and the maximum stress amplitude among the tests that did not lead to fracture was defined as the fatigue limit ⁇ F (MPa), and the fatigue limit ⁇ F (MPa ) divided by the tensile strength TS (MPa) ( ⁇ F /TS) is determined as the punching fatigue limit ratio.
  • a hot rolled steel sheet according to an embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm.
  • the plate thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or may be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less. .
  • a preferred method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes: heating a slab having the chemical composition described above in relation to hot rolled steel to 1150° C. or above; A hot rolling step including finish rolling of a heated slab, wherein the delivery side temperature of the finish rolling is 820 to 920 ° C.; The obtained steel sheet is primary cooled from the finish rolling delivery side temperature to the primary cooling end temperature of 720 ° C. or less at an average cooling rate of 50 ° C./sec or more, and then to the coiling temperature at an average cooling rate of 10 ° C./sec or less.
  • a cooling step comprising secondary cooling with coiling the steel sheet at a coiling temperature of 580-650°C; and an additional cooling step comprising cooling the coiled steel sheet to a steel sheet temperature of 550°C or less, wherein the steel sheet temperature is 550°C after coiling. It is characterized by including an additional cooling step with a time to °C of 30-180 minutes. Each step will be described in detail below.
  • a slab having the chemical composition described above in relation to hot rolled steel is heated prior to hot rolling.
  • the heating temperature of the slab is set to 1150° C. or higher so that Ti carbonitrides and the like are fully dissolved again.
  • the upper limit is not particularly specified, it may be 1250° C., for example.
  • the heating time is not particularly limited, but may be, for example, 30 minutes or more and/or 120 minutes or less.
  • the slab to be used is preferably cast by continuous casting from the viewpoint of productivity, but may be produced by ingot casting or thin slab casting.
  • the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness or the like.
  • Conditions for the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
  • finish rolling The heated slab or the slab that has been rough rolled as necessary is then subjected to finish rolling, and the delivery side temperature in the finish rolling is controlled at 820 to 920°C. If the delivery-side temperature of finish rolling exceeds 920°C, the accumulation of work strain in austenite during cooling is insufficient, pearlite transformation is delayed, and a pearlite fraction of 70% or more cannot be achieved. For this reason, the upper limit of the outlet temperature of the finishing temperature is 920°C, preferably 915°C, more preferably 910°C.
  • the lower limit of the delivery side temperature of finish rolling is set to 820°C.
  • the steel sheet is primarily cooled from the finish rolling delivery side temperature to the primary cooling end temperature of 720° C. or less at an average cooling rate of 50° C./second or more. If the average cooling rate to the primary cooling end temperature is less than 50°C/sec or the primary cooling end temperature is more than 720°C, a large amount of ferrite is generated and a pearlite fraction of 70% or more is achieved. I can't do it.
  • the average cooling rate of primary cooling may be 52° C./second or more.
  • the upper limit of the average cooling rate is not particularly limited, for example, the average cooling rate of the primary cooling is preferably 200 ° C./sec or less in order to obtain the desired structure, and even if it is 100 ° C./sec or less. good.
  • Secondary cooling cooling to winding temperature at 10°C/sec or less
  • the steel sheet is cooled from the primary cooling end temperature to the coiling temperature (that is, the temperature range of 580 to 650° C.) at an average cooling rate of 10° C./sec or less. If the average cooling rate of the secondary cooling is higher than 10°C, temperature unevenness is likely to occur in the thickness direction and the width direction of the steel sheet, resulting in variations in the metal structure.
  • the average cooling rate of secondary cooling is preferably 9° C./sec or less.
  • the lower limit of the average cooling rate is not particularly limited, but from the viewpoint of productivity, the average cooling rate of secondary cooling is set to 1° C./second or more, and may be 2° C./second or more. In order to obtain the effect of dividing the cooling process into two stages, the secondary cooling is preferably performed immediately after the primary cooling is completed.
  • the coiling temperature may be 584°C or higher and/or may be 640°C or lower.
  • an additional cooling step that is, a cooling step after winding, is performed after the winding step, and in the cooling step after winding, the steel sheet is cooled from the winding temperature to a steel sheet temperature of 550 ° C. or less. .
  • a cooling step after winding it is important to control the time required for the temperature of the steel sheet to reach 550° C. within the range of 30 to 180 minutes after winding. If the steel sheet temperature reaches 550°C in less than 30 minutes, a large amount of bainite is generated and pearlite is not sufficiently generated, so that a pearlite fraction of 70% or more cannot be achieved.
  • the cementite in the produced pearlite will coarsen and become spherical.
  • the number density of cementite in which the length of the long axis in pearlite is more than 0.3 ⁇ m and the aspect ratio is less than 3.0 cannot be limited to less than 10 per 10 ⁇ m 2 , and voids are formed when punching a steel plate. It becomes impossible to sufficiently suppress the occurrence.
  • the cooling time until the steel plate temperature reaches 550 ° C. is controlled within the range of 30 to 180 minutes, so that the pearlite fraction is 70% or more.
  • the time required for the steel sheet temperature to reach 550° C. may be 35 minutes or more and/or may be 150 minutes or less.
  • the time required for the steel plate temperature to reach 550°C can be adjusted by any appropriate method. For example, if the coiling temperature is around 580°C, cover the coil with a heat insulating cover, etc., as necessary, in order to ensure that the steel sheet temperature reaches 550°C for at least 30 minutes. may
  • hot-rolled steel sheets according to embodiments of the present invention were produced under various conditions, and the mechanical properties of the obtained hot-rolled steel sheets were investigated.
  • a slab having the chemical composition shown in Table 1 was manufactured by continuous casting. Then, from these slabs, hot-rolled steel sheets with a thickness of 2.5 mm were produced under the heating, hot rolling, cooling, coiling and additional cooling conditions shown in Table 2. The balance other than the components shown in Table 1 is Fe and impurities.
  • the chemical composition obtained by analyzing the sample taken from the manufactured hot-rolled steel sheet is the same as the chemical composition of the slab shown in Table 1, especially the Sn and Sb contents in the impurities are 0.02% or less, The W and Co contents were 0.015% or less.
  • a No. 5 tensile test piece of JIS Z2241: 2011 was taken from the hot-rolled steel sheet thus obtained in a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241: 2011 to determine the tensile strength ( TS) and uniform elongation (uEl) were measured.
  • the fatigue properties of punched end faces were evaluated by the following method using plate-shaped test pieces having the dimensions shown in FIG. First, a plate-shaped test piece (30 mm ⁇ 90 mm) sampled from a hot-rolled steel plate so that the rolling direction is the long side is punched in the center with a punch diameter of 10 mm and a punching clearance of 12%, and then a constant stress amplitude.
  • a double-sided plane bending fatigue test was performed at ⁇ (MPa).
  • the plate-shaped test piece may be chamfered at the corners of the plate end surface as shown in FIG.
  • a double-sided plane bending fatigue test was performed until the number of repetitions N reached 10 7 times, and the maximum stress amplitude among the tests that did not lead to fracture was defined as the fatigue limit ⁇ F (MPa), and the fatigue limit ⁇ F (MPa ) divided by the tensile strength TS (MPa) ( ⁇ F /TS) was determined as the punching fatigue limit ratio.
  • a hot-rolled steel sheet having a TS of 900 MPa or more, a uEl of 7.0% or more, and a punching fatigue limit ratio of 0.28 or more was evaluated as a hot-rolled steel sheet having high strength, uniform elongation, and excellent punching edge fatigue characteristics. .
  • the results are shown in Table 3 below.
  • Comparative Example 9 a large amount of ferrite was generated due to the low average cooling rate of the primary cooling in the cooling process, and a pearlite fraction of 70% or more could not be achieved. As a result, TS decreased. In Comparative Example 10, since the cooling end temperature of the primary cooling was high, a large amount of ferrite was similarly generated, and a pearlite fraction of 70% or more could not be achieved. As a result, TS decreased. In Comparative Example 11, since the coiling temperature was low, a large amount of bainite was generated, and a pearlite fraction of 70% or more could not be achieved. As a result, the TS improved, but the uEl decreased.
  • Comparative Example 12 a large amount of ferrite was generated due to the high coiling temperature, and a pearlite fraction of 70% or more could not be achieved. Moreover, in Comparative Example 12, the coiling temperature was high, so that the precipitated Cu particles became coarse, and it is considered that the precipitation strengthening ability of Cu was not sufficiently exhibited. As a result, TS decreased. In Comparative Example 13, a large amount of bainite was generated because the time until the temperature reached 550° C. after winding was short, and although the TS was improved, the uEl was lowered. In Comparative Example 14, since it took a long time to reach 550° C.
  • Examples 1-8, 15-18, 23, 25, 27, and 29-36 have a given chemical composition and microstructure, and in addition, coarse spheroids in pearlite at that microstructure.
  • a uEl of 7.0% or more and a punching fatigue limit ratio of 0.28 or more even though the TS has a high strength of 900 MPa or more, and therefore a high A hot-rolled steel sheet with high strength, uniform elongation, and excellent punched edge fatigue properties was obtained.
  • a similar fatigue test was performed on a test piece from a welded member obtained by arc welding the steel plate of each example. As a result, in all examples, it was possible to achieve high fatigue properties equivalent to those without such welds. It is believed that this is mainly due to the relatively low C content of 0.30% or less.

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20130068403A (ko) * 2011-12-15 2013-06-26 주식회사 포스코 고탄소 열연강판, 냉연강판 및 그 제조방법
JP2015515548A (ja) * 2012-04-10 2015-05-28 ポスコ 材質均一性に優れた高炭素熱延鋼板及びその製造方法
JP2020509190A (ja) * 2016-12-20 2020-03-26 ポスコＰｏｓｃｏ 高温伸び特性に優れた高強度鋼板、温間プレス成形部材、及びそれらの製造方法
WO2020179737A1 (ja) * 2019-03-06 2020-09-10 日本製鉄株式会社 熱間圧延鋼板およびその製造方法
WO2021176999A1 (ja) * 2020-03-02 2021-09-10 日本製鉄株式会社 熱間圧延鋼板

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
KR20130068403A (ko) * 2011-12-15 2013-06-26 주식회사 포스코 고탄소 열연강판, 냉연강판 및 그 제조방법
JP2015515548A (ja) * 2012-04-10 2015-05-28 ポスコ 材質均一性に優れた高炭素熱延鋼板及びその製造方法
JP2020509190A (ja) * 2016-12-20 2020-03-26 ポスコＰｏｓｃｏ 高温伸び特性に優れた高強度鋼板、温間プレス成形部材、及びそれらの製造方法
WO2020179737A1 (ja) * 2019-03-06 2020-09-10 日本製鉄株式会社 熱間圧延鋼板およびその製造方法
WO2021176999A1 (ja) * 2020-03-02 2021-09-10 日本製鉄株式会社 熱間圧延鋼板

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