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

• This disclosure relates to steel sheets, and more specifically to steel sheets that can be used as materials for mechanical parts, such as automotive parts.
• Steel plate with a high C content (high carbon steel plate) is used as steel plate for machine parts, such as automobile parts.
• Machine parts can have complex shapes and require high strength.
• the method for manufacturing machine parts using steel plate for machine parts is as follows: The steel plate is cold worked to form it into the shape of the machine part. After cold working, the steel plate is quenched and tempered. High-strength machine parts are manufactured using these manufacturing processes. As described above, steel plate for machine parts is formed into the shape of the machine part by cold working. Therefore, excellent cold workability is required.
• Patent Document 1 Technology for improving the cold workability of steel sheets is proposed, for example, in International Publication No. 2015/146173 (Patent Document 1).
• the steel sheet disclosed in Patent Document 1 contains, by mass%, 0.20 to 0.40% C, 0.10% or less Si, 0.50% or less Mn, 0.03% or less P, 0.010% or less S, 0.10% or less sol. Al, 0.0050% or less N, and 0.0005 to 0.0050% B. It also contains 0.002 to 0.030% in total of one or more of Sb, Sn, Bi, Ge, Te, and Se, with the balance being Fe and unavoidable impurities. In this steel sheet, the proportion of dissolved B in the B content is 70% or more. Furthermore, the microstructure is composed of ferrite and cementite. Furthermore, the cementite density within the ferrite grains is 0.08 particles/ ⁇ m2 or less . In the steel sheet of Patent Document 1, the density of cementite within ferrite grains is adjusted to improve cold workability.
• Patent Document 1 has sufficient cold workability. However, the cold workability of the steel sheet may be improved by other means.
• the purpose of this disclosure is to provide a steel sheet with excellent cold workability.
• the steel sheet disclosed herein has a chemical composition, in mass%, of C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, acid-soluble Al: 0.005-0.070%, N: 0.0200% or less, Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035 %, Cu: 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%, with the balance consisting of Fe and impurities.
• the total area ratio of ferrite and cementite particles is 95% or more, the average grain size of the ferrite grains is 5.0-30.0 ⁇ m, and the GOS (Grain Orientation Spread) value of the ferrite grains is 2.0° or less.
• the steel sheet disclosed herein provides excellent cold workability.
• the inventors conducted research into steel sheets that offer excellent cold workability. As a result, the inventors discovered the following:
• the inventors first investigated a chemical composition suitable for steel sheets for use in machine parts, such as automobile parts. As a result, the inventors determined a composition, in mass%, of C: 0.20-0.70%, Si: 0.07-1.00%, Mn: 0.20-3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010-1.500%, acid-soluble Al: 0.005-0.070%, N: 0.0200% or less, Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035%, Cu: 0-0.20%, W It was believed that a chemical composition containing the following elements would be suitable for steel sheet used in machine parts: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La:
• the inventors therefore investigated ways to improve the cold workability of steel sheets that satisfy the above chemical composition.
• the microstructure of steel sheet having the above-mentioned chemical composition is essentially composed of ferrite and cementite particles.
• the inventors focused on the strain distribution within the steel sheet. If strain is present locally within the steel sheet, regions with different amounts of plastic deformation will be generated locally during cold working. In this case, uniform plastic deformation is not possible, and non-uniform plastic deformation occurs. As a result, cold workability deteriorates. Therefore, the inventors focused on ferrite grains, which are the main component of the macrostructure of steel sheet. If there is variation in the amount of strain within the ferrite grains, there is a high possibility that the amount of plastic deformation will also vary from ferrite grain to ferrite grain.
• the inventors investigated the relationship between the GOS (Grain Orientation Spread) of steel sheet as an index of the amount of strain in each ferrite grain and cold workability.
• GOS indicates the average difference in orientation between each crystal grain and can be used as an index of the amount of strain in each crystal grain.
• the inventors discovered for the first time that, in a steel sheet with the above-mentioned chemical composition, when the total area ratio of ferrite and cementite particles is 95% or more, excellent cold workability can be achieved if the GOS value of the ferrite grains is 2.0° or less.
• the steel plate of this embodiment was completed based on the above technical concept and has the following configuration.
• the total area ratio of ferrite and cementite particles is 95% or more, the average grain size of the ferrite grains is 5.0-30.0 ⁇ m, and the GOS (Grain Orientation Spread) value of the ferrite grains is 2.0° or less.
• the steel sheet of this embodiment satisfies the following characteristics.
• the chemical composition is, in mass%, C: 0.20 to 0.70%, Si: 0.07 to 1.00%, Mn: 0.20 to 3.00%, P: 0.030% or less, S: 0.0080% or less, Cr: 0.010 to 1.500%, acid-soluble Al: 0.005 to 0.070%, N: 0.0200% or less, Ti: 0 to 0.500%, V: 0 to 0.500%, Nb: 0 to 0.500%, B: 0 to 0.0035%, Cu : 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca:
• C 0.20-0.70% Carbon (C) improves the hardenability of steel sheets. As a result, when a mechanical part is manufactured using the steel sheet as a raw material and hardening is performed, the strength of the mechanical part is increased. If the C content is less than 0.20%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.70%, the cold workability of the steel sheet will be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.20 to 0.70%.
• the lower limit of the C content is preferably 0.22%, more preferably 0.25%, even more preferably 0.28%, and still more preferably 0.30%.
• the upper limit of the C content is preferably 0.68%, more preferably 0.65%, and even more preferably 0.60%.
• Si 0.07 ⁇ 1.00%
• Silicon (Si) deoxidizes steel during the steelmaking process of steel sheet production. Furthermore, Si enhances the temper softening resistance of steel sheets when tempering is performed in a process for manufacturing mechanical parts using the steel sheets as raw materials. As a result, the strength of the mechanical parts is increased. If the Si content is less than 0.07%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 1.00%, the strength of the steel sheet becomes excessively high due to solid solution strengthening, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.07 to 1.00%.
• Mn 0.20-3.00%
• Manganese (Mn) improves the hardenability of steel sheets.
• Mn content is less than 0.20%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment.
• Mn content exceeds 3.00%, the strength of the steel sheet becomes excessively high due to solid solution strengthening, and therefore the cold workability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.20 to 3.00%.
• the lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, even more preferably 0.35%, and still more preferably 0.40%.
• the upper limit of the Mn content is preferably 2.90%, more preferably 2.80%, even more preferably 2.50%, even more preferably 2.00%, even more preferably 1.90%, even more preferably 1.70%, and even more preferably 1.50%.
• S 0.0080% or less Sulfur (S) is an unavoidable impurity. That is, the S content is greater than 0%. If the S content exceeds 0.0080%, sulfides are formed in excess. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability of the steel sheet is reduced. Therefore, the S content is 0.0080% or less.
• the S content is preferably as low as possible. However, excessive reduction in the S content significantly increases the production cost. Therefore, in consideration of industrial production, the lower limit of the S content is preferably 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and still more preferably 0.0005%.
• the upper limit of the S content is preferably 0.0075%, more preferably 0.0070%, even more preferably 0.0065%, even more preferably 0.0060%, even more preferably 0.0055%, and even more preferably 0.0050%.
• the lower limit of the Cr content is preferably 0.015%, more preferably 0.030%, even more preferably 0.050%, even more preferably 0.080%, and still more preferably 0.100%.
• the upper limit of the Cr content is preferably 1.480%, more preferably 1.450%, even more preferably 1.400%, even more preferably 1.300%, even more preferably 1.100%, even more preferably 0.900%, and even more preferably 0.700%.
• Acid soluble Al 0.005-0.070%
• Aluminum (Al) deoxidizes steel during the steelmaking process of steel sheet production. If the acid-soluble Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the acid-soluble Al content exceeds 0.070%, Al nitrides are formed in excess, which refines the austenite grains, and therefore the hardenability of the steel sheet deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the acid-soluble Al content is 0.005 to 0.070%.
• the lower limit of the acid-soluble Al content is preferably 0.010%, more preferably 0.012%, even more preferably 0.015%, and still more preferably 0.020%.
• the upper limit of the acid-soluble Al content is preferably 0.065%, more preferably 0.060%, even more preferably 0.055%, and still more preferably 0.050%.
• N 0.0200% or less Nitrogen (N) is an unavoidably contained impurity. In other words, the N content is greater than 0%. N combines with Al to form Al nitrides. Al nitrides refine austenite grains during heating in quenching in the process of manufacturing mechanical parts using steel sheet as a raw material. Refinement of austenite grains reduces the hardenability of the steel sheet. If the N content exceeds 0.0200%, austenite grains are excessively refined during heating in quenching. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hardenability of the steel sheet is significantly reduced. Therefore, the N content is 0.0200% or less.
• the remainder of the chemical composition of the steel plate according to this embodiment consists of Fe and impurities.
• impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, during the industrial production of steel plate, and are acceptable to the extent that they do not adversely affect the steel plate according to this embodiment.
• the chemical composition of the steel sheet of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti: 0-0.500%, V: 0-0.500%, Nb: 0-0.500%, B: 0-0.0035%, Cu: 0-0.20%, W: 0-0.03%, Ta: 0-0.03%, Sn: 0-0.030%, Sb: 0-0.030%, Co: 0-0.030%, As: 0-0.030%, Mg: 0-0.030%, Y: 0-0.030%, Zr: 0-0.030%, La: 0-0.030%, Ce: 0-0.030%, and Ca: 0-0.030%. All of these elements are optional and may not be included. These optional elements will be explained below.
• the chemical composition of the steel sheet according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, V, Nb, and B. All of these elements are optional elements and may not be contained. When contained, Ti, V, Nb, and B increase the strength of the steel sheet.
• Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%.
• Ti When Ti is contained, that is, when the Ti content is more than 0%, Ti forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Ti also bonds with N to suppress the formation of nitrides by solute B. Even if Ti is contained even a small amount, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.500%, excessive precipitates are formed, resulting in an excessively high strength of the steel sheet, which in turn reduces the cold workability of the steel sheet even if the contents of other elements are within the ranges of this embodiment.
• the Ti content is 0 to 0.500%.
• the lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, even more preferably 0.003%, and still more preferably 0.005%.
• the upper limit of the Ti content is preferably 0.400%, more preferably 0.300%, even more preferably 0.200%, even more preferably 0.100%, and still more preferably 0.080%.
• V 0-0.500%
• Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%.
• V When V is contained, that is, when the V content exceeds 0%, V forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Even if even a small amount of V is contained, the above effect can be obtained to some extent.
• the V content exceeds 0.500%, excessive precipitates are formed, resulting in an excessively high strength of the steel sheet, which in turn reduces the cold workability of the steel sheet even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.500%.
• the lower limit of the V content is preferably 0.001%, more preferably 0.002%, even more preferably 0.003%, and still more preferably 0.005%.
• the upper limit of the V content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.300%, even more preferably 0.200%, even more preferably 0.100%, and even more preferably 0.080%.
• Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%.
• Nb When Nb is contained, that is, when the Nb content is more than 0%, Nb forms precipitates such as carbides. Therefore, the strength of the steel sheet is increased by precipitation strengthening. Nb also bonds with N to prevent solute B from forming nitrides. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.500%, excessive precipitates are formed, resulting in an excessively high strength of the steel sheet, which in turn reduces the cold workability of the steel sheet even if the contents of other elements are within the ranges of this embodiment.
• the Nb content is 0 to 0.500%.
• the lower limit of the Nb content is preferably 0.001%, more preferably 0.002%, even more preferably 0.003%, and still more preferably 0.005%.
• the upper limit of the Nb content is preferably 0.480%, more preferably 0.450%, even more preferably 0.400%, even more preferably 0.350%, and still more preferably 0.300%.
• B 0-0.0035%
• Boron (B) is an optional element and may not be contained, that is, the B content may be 0%.
• B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel sheet and increases the strength of the steel sheet. Even if even a small amount of B is contained, the above effects can be obtained to some extent.
• the B content exceeds 0.0035%, B compounds are formed, which results in an excessively high strength of the steel sheet, and therefore reduces the cold workability of the steel sheet even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0035%.
• the lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, and still more preferably 0.0005%.
• the upper limit of the B content is preferably 0.0032%, more preferably 0.0028%, even more preferably 0.0025%, even more preferably 0.0020%, and still more preferably 0.0015%.
• the chemical composition of the steel sheet according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cu, W, Ta, Sn, Sb, Co, As, Mg, Y, Zr, La, Ce, and Ca. All of these elements are optional elements and may not be contained. In other words, the content of these elements may be 0%.
• the Cu content is 0-0.20%
• the W content is 0-0.03%
• the Ta content is 0-0.03%
• the Sn content is 0-0.030%
• the Sb content is 0-0.030%
• the Co content is 0-0.030%
• the As content is 0-0.030%
• the Mg content is 0-0.030%
• the Y content is 0-0.030%
• the Zr content is 0-0.030%
• the La content is 0-0.030%
• the Ce content is 0-0.030%
• the Ca content is 0-0.030%.
• the lower limit of the Cu content is preferably 0.01%, and more preferably 0.03%.
• the upper limit of the Cu content is preferably 0.15%, and more preferably 0.10%.
• the lower limit of the W content is preferably 0.01%.
• the preferred upper limit of the W content is 0.02%.
• the preferred lower limit of the Ta content is 0.01%.
• the preferred upper limit of the Ta content is 0.02%.
• the lower limit of the Sn content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Sn content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the Sb content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Sb content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the Co content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Co content is preferably 0.025%, more preferably 0.020%, and still more preferably 0.015%.
• the lower limit of the As content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the As content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the Mg content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Mg content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the Y content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Y content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the Zr content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Zr content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the La content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the La content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the lower limit of the Ca content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.005%.
• the upper limit of the Ca content is preferably 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the chemical composition of the steel sheet of this embodiment can be measured by a known elemental analysis method. Specifically, chips are collected from the interior of the steel sheet to a depth of 0.1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) is performed on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method.
• ICP-AES Inductively Coupled Plasma Atomic Emission Spectrometry
• the contents of elements other than the C content of the steel plate of this embodiment are determined by rounding the measured value to the smallest digit specified in this embodiment, and the value obtained is the content of that element. Note that rounding means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.
• the total area ratio of ferrite and cementite particles is 95% or more, that is, the microstructure of the steel sheet of this embodiment is substantially composed of ferrite and cementite particles.
• the structure other than ferrite and cementite particles is, for example, one or more types selected from the group consisting of bainite, martensite, and pearlite.
• the total area ratio of ferrite and cementite particles in the microstructure is 96% or more, more preferably 97% or more, even more preferably 98% or more, and even more preferably 99% or more.
• the microstructure may be a structure consisting of ferrite and cementite particles.
• the preferred range for the total area ratio of ferrite and cementite particles is 96 to 100%, more preferably 97 to 100%, even more preferably 98 to 100%, and even more preferably 99 to 100%.
• the total area ratio of ferrite and cementite particles in the microstructure can be measured by the following method.
• a test piece having a cross section (L cross section) parallel to the rolling direction and thickness direction of the steel plate is taken from the center position of the steel plate width.
• the size of the test piece is not particularly limited as long as it includes the observation field described below.
• the L cross section of the steel plate is used as the observation surface.
• the observation surface of the test piece is mirror-polished.
• the mirror-polished observation surface is etched using 3% nitric acid alcohol (Nital etchant).
• the etching time is 120 seconds.
• Five rectangular observation fields are selected from the etched observation surface, each of which includes the center position of the plate thickness and is 100 ⁇ m in the thickness direction of the steel plate and 120 ⁇ m in the direction perpendicular to the plate thickness direction.
• the five observation fields are arranged consecutively in the direction perpendicular to the plate thickness, and the center position of the central observation field of the five arranged observation fields is the center position of the plate width of the steel plate.
• ferrite and cementite particles exhibit different contrast and morphology from other structures (bainite, martensite, pearlite, etc.). Therefore, ferrite and cementite particles within the observation field are identified based on their contrast and morphology.
• ferrite is a white region without any substructure such as lath within the grain.
• Bainite and martensite are regions that contain substructure.
• Pearlite is a striped region with a lamellar structure.
• Cementite particles are black regions that are less bright than ferrite. In other words, cementite appears darker than ferrite.
• the total area ratio (%) of ferrite and cementite particles is calculated based on the total area of ferrite and cementite particles in the five observation fields and the total area of the five observation fields.
• the total area ratio is calculated as an integer value obtained by rounding the obtained value to one decimal place.
• the average grain size of the ferrite grains is 5.0 to 30.0 ⁇ m.
• the average grain size of ferrite grains affects cold workability. If the average grain size of ferrite grains is less than 5.0 ⁇ m, the ferrite grains are excessively fine. In this case, sufficient cold workability cannot be obtained. If the average grain size of ferrite grains is 5.0 ⁇ m or more, the steel sheet will have excellent cold workability, provided that Features 1, 2, and 4 are satisfied.
• the preferred lower limit of the average particle size of the ferrite grains is 5.2 ⁇ m, more preferably 5.5 ⁇ m, even more preferably 5.7 ⁇ m, even more preferably 6.0 ⁇ m, even more preferably 6.5 ⁇ m, and even more preferably 7.0 ⁇ m.
• the upper limit to the average grain size of ferrite grains is 30.0 ⁇ m.
• the average grain size of the ferrite grains can be measured by the following method.
• the average grain size ( ⁇ m) of the ferrite grains is determined from the obtained grain size number.
• the average grain size of the ferrite grains is a value obtained by rounding off the obtained value to one decimal place (i.e., the value to one decimal place).
• the GOS value of the ferrite grains is further 2.0° or less.
• the GOS value indicates the magnitude of the crystal orientation misorientation (variation in crystal orientation) occurring within each crystal grain, and can be used as an index of the amount of strain for each crystal grain.
• the larger the GOS value the more ferrite grains there are with large misorientation within the crystal grains. In this case, the amount of strain accumulated within the ferrite grains becomes excessively large, and as a result, the steel sheet cannot achieve sufficient cold workability.
• the GOS value is 2.0° or less, the variation in crystal orientation within the ferrite grains is sufficiently suppressed, and there are a sufficiently large number of ferrite grains with a small amount of accumulated strain within the grains. Therefore, provided that Features 1 to 3 are satisfied, the steel sheet can have excellent cold workability.
• the upper limit of the GOS value is preferably 1.9°, more preferably 1.8°, and even more preferably 1.7°.
• the lower limit of the GOS value is not particularly limited. A lower GOS value is preferable. However, excessive reduction of the GOS value increases production costs. Therefore, a preferred lower limit of the GOS value is 0.1°, more preferably 0.2°, and even more preferably 0.3°.
• the method for measuring GOS in this embodiment is as follows.
• a test piece for measuring the KAM value is taken from the steel sheet.
• the size of the test piece is not particularly limited as long as it has a cross section (hereinafter referred to as the observation surface) including three observation fields of 100 ⁇ m in the thickness direction and 100 ⁇ m in the rolling direction, which do not overlap with each other, and is centered at the center of the sheet thickness.
• the observation surface of the test specimen is mirror-polished. After mirror-polishing, three 100 ⁇ m x 100 ⁇ m measurement fields as described above are set within the observation surface so that they do not overlap each other. Electron backscatter diffraction (EBSD) measurements are performed for each observation field. For EBSD measurements, the acceleration voltage is 20 kV and the probe current is 28 nA. The measurement points for each observation field are set as follows:
• Each observation field is divided into regular hexagonal pixel units.
• the distance between the centers of adjacent pixels is 0.3 ⁇ m.
• the measurement point is the center position of a pixel that contains the entire regular hexagonal pixel within the observation field. In other words, pixels that are partly outside the observation field are excluded from measurement.
• the crystal orientation obtained by EBSD measurement at each measurement point is taken as the crystal orientation of the pixel containing that measurement point. Note that measurement points for which the Confidence Index (CI value), which indicates the likelihood of the obtained crystal orientation, is 0.1 or less will not be used in subsequent calculations. Furthermore, pixels containing measurement points for which the CI value is 0.1 or less will be treated as not existing.
• CI value Confidence Index
• the crystal structure of the pixel containing each measurement point is determined from the diffraction pattern (Kikuchi line pattern) obtained at each measurement point. If the crystal structure at the measurement point is a bcc (body-centered cubic) structure, the pixel containing that measurement point is determined to be ferrite.
• a region of multiple consecutively arranged pixels is determined to be a ferrite region.
• the orientation difference between the target pixel and each of the six adjacent pixels is calculated.
• the arithmetic mean of the six obtained orientation differences is defined as the orientation difference of the pixel in question.
• a group of multiple consecutively arranged pixels with an orientation difference of 5° or less is defined as a group of pixels within the same ferrite grain.
• the GOS is calculated as follows. An arbitrary pixel (measurement point) within the same ferrite grain is selected. The crystal orientation difference between that pixel and each of the other pixels (measurement points) within the same ferrite grain is calculated. This operation is performed for all pixels (measurement points) within the ferrite grain. The average value of the orientation differences between all of the obtained pixels is calculated. The calculated value is defined as the GOS value of the crystal grain.
• GOS value is defined by the following formula:
• n is substituted with the number of pixels in the ferrite grain
• ⁇ i is substituted with the crystal orientation of the i-th pixel in the ferrite grain
• ⁇ ave is substituted with the arithmetic mean value of the crystal orientations of all pixels in the ferrite grain.
• the arithmetic mean value of the GOS values of all ferrite grains within the observation field is taken as the GOS value of the ferrite grains of the steel plate.
• the GOS value can be determined by analysis using well-known analysis software, manufactured by TSL under the trade name OIM Analysis.
• the steel sheet of this embodiment is a material for use in mechanical parts, such as automobile parts.
• mechanical parts include automobile springs, washers, and bicycle gears.
• the steel sheet of this embodiment may also be used for applications other than mechanical parts that require excellent cold workability.
• An example of the method for manufacturing the steel sheet according to this embodiment includes the following steps. (Step 1) Hot rolling step (Step 2) Annealing step Each step will be described below.
• Hot rolling step hot rolling is performed on a slab that satisfies Feature 1.
• the slab is produced, for example, by the following method.
• Molten steel that satisfies Feature 1 and whose chemical composition has an element content within the range of this embodiment is produced.
• the molten steel is used to produce a slab by a casting method.
• the molten steel is used to produce a slab by a well-known continuous casting method.
• the chemical composition of the produced slab satisfies Feature 1.
• Step 11 Rough rolling step
• the heated slab is rough rolled using a rough rolling mill to produce an intermediate steel plate (rough bar).
• the rough rolling mill is, for example, a reverse rolling mill.
• Step 12 Finish rolling step
• a tandem rolling mill is used to perform finish rolling on an intermediate steel plate (rough bar) to produce a hot-rolled steel plate.
• the tandem rolling mill includes a plurality of rolling stands 1 to L (L is an integer) arranged in a row.
• the number of rolling stands n constituting the tandem rolling mill is not particularly limited, but may be, for example, 4 to 7 stands.
• the finish rolling process is performed without reheating the intermediate steel plate after rough rolling.
• Step 13 Winding step
• the hot-rolled steel sheet that has been finish-rolled in the finish rolling process of the hot rolling process is cooled and coiled.
• Step 2 Annealing step In the annealing step, the coiled hot-rolled steel sheet after the coiling step is subjected to annealing treatment, which spheroidizes cementite to form cementite particles.
• the above manufacturing process further satisfies the following conditions: (Condition 1)
• the heating temperature T11 in the heating step is set to 1100 to 1350°C.
• the final finish rolling start temperature T12 which is the surface temperature of the steel sheet at the inlet side of the most downstream rolling stand L that applies the final reduction in the finish rolling step, is set to 850°C or higher.
• the finish rolling temperature FT in the finish rolling step is set to 850 to 1000°C.
• ⁇ L-1 defined by formula (1) is set to 5 to 20.
• ⁇ L defined by formula (1) is set to 30 to 70.
• ⁇ n exp(0.753+3000/(T n +273)) ⁇ n 0.21 ⁇ v n 0.13 (1)
• n is a number between 1 and L
• ⁇ n is the equivalent plastic strain imparted to the rough bar in the corresponding rolling stand n
• v n is the strain rate (s -1 ) of the rough bar when it passes through the corresponding rolling stand n.
• T n is the surface temperature (°C) of the steel plate at the inlet side of rolling stand n.
• the surface temperature of the intermediate steel plate (rough bar) at the entry side of the most downstream rolling stand L is defined as the final finish rolling start temperature T 12 (°C).
• the final finish rolling start temperature T 12 is measured by a thermometer arranged at the entry side of the most downstream rolling stand L that applies the final reduction to the intermediate steel plate. If the final finish rolling start temperature T12 is less than 850°C, the ferrite in the produced steel sheet will be excessively small. As a result, the average grain size of the ferrite grains will be less than 5.0 ⁇ m. Therefore, the final finish rolling start temperature T12 is 850°C or higher.
• the amount of strain imparted to the steel sheet is insufficient. In this case, unrecrystallized grains remain. As a result, the average grain size of ferrite grains becomes less than 5.0 ⁇ m. Furthermore, the GOS value becomes excessively large and exceeds 2.0°. On the other hand, if ⁇ L exceeds 70, the amount of strain imparted to the steel sheet is excessive. In this case, the average grain size of ferrite grains becomes less than 5.0 ⁇ m. Furthermore, the GOS value becomes excessively large and exceeds 2.0°.
• t0 is the thickness (mm) of the slab
• t1 is the thickness (mm) of the hot-rolled steel sheet after finish rolling.
• the cumulative reduction rate R is less than 60%, the reduction in the hot rolling process is insufficient. In this case, recrystallization does not occur sufficiently. As a result, the average grain size of ferrite grains exceeds 30.0 ⁇ m. Furthermore, the strain within the grains is not sufficiently relieved, and the GOS value exceeds 2.0°. Therefore, the cumulative rolling reduction R is 60% or more.
• the surface temperature of the steel sheet at the start of coiling is defined as the coiling temperature CT (°C). If the coiling temperature CT exceeds 650° C., the ferrite grains become coarse. As a result, the average grain size of the ferrite grains exceeds 30.0 ⁇ m. On the other hand, if the coiling temperature CT is less than 550° C., the ferrite grains become excessively fine, and as a result, the average grain size of the ferrite grains becomes less than 5.0 ⁇ m. Therefore, the coiling temperature CT is 650 to 550°C.
• the annealing temperature T3 in the annealing step is set to 600 to 730° C.
• the holding time t3 at the annealing temperature T3 is set to 20 hours or more.
• the preferred upper limit of the holding time t3 is 50 hours. If the annealing temperature T3 is less than 600°C or the holding time t3 is less than 20 hours, the annealing is insufficient. In this case, the ferrite grains become excessively fine. As a result, the average grain size of the ferrite grains becomes less than 5.0 ⁇ m. On the other hand, if the annealing temperature T3 exceeds 730°C, the ferrite grains become coarse, and as a result, the average grain size of the ferrite grains exceeds 30.0 ⁇ m.
• the steel plate of this embodiment is manufactured using the above manufacturing method.
• the effects of the steel plate of this embodiment will be explained in more detail below using examples.
• the conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel plate of this embodiment. Therefore, the steel plate of this embodiment is not limited to this one example of conditions.
• molten steel was continuously cast to produce a slab.
• a hot rolling process was carried out on the slab. Specifically, the slab was heated at a heating temperature T 11 (°C) (condition 1) shown in Table 2.
• the slab was then rough rolled using a reverse rolling mill to produce a rough bar (intermediate steel plate).
• the rough bar was finish rolled using a tandem rolling mill consisting of multiple rolling stands to produce a hot-rolled steel plate.
• the final finish rolling start temperature T 12 (°C) (condition 2), the finish rolling temperature FT (°C) (condition 3), ⁇ L-1 at the rolling stand L-1 preceding the most downstream rolling stand L and ⁇ L at the most downstream rolling stand L (condition 4), and the cumulative reduction rate R (%) of the hot rolling process (condition 5) were as shown in Table 2.
• the hot-rolled steel sheet after the hot rolling process was subjected to a coiling process.
• the coiling temperature CT (°C) (condition 6) was as shown in Table 2.
• the steel sheet after the coiling process was subjected to an annealing process.
• the annealing temperature T3 (°C) and the holding time t3 (hours) at the annealing temperature T3 (condition 7) were as shown in Table 2. Steel sheets of each test number were produced by the above production process.
• Test 1 Test for measuring the chemical composition of steel sheets
• Test 2 Test for measuring the total area ratio of ferrite and cementite particles in steel sheets
• Test 3 Test for measuring the average grain size of ferrite in steel sheets
• Test 4 Test for measuring the GOS value of steel sheets
• Vickers hardness test (Test 6) Ductility test Tests 1 to 6 will be described below.
• Test 5 Vickers hardness test
• HV Vickers hardness
• Test pieces were taken from 10 locations at the center of the steel plate width, arranged at 500 mm intervals in the rolling direction.
• the size of the test pieces was 15 mm in the rolling direction ⁇ 30 mm in the width direction ⁇ plate thickness.
• a Vickers hardness test was performed in accordance with JIS Z 2244-1 (2020) at five measurement points in the plate thickness direction from the surface of the steel plate: t/8 depth position, t/4 depth position, t/2 depth position, 3t/4 depth position, and 7t/8 depth position.
• the test force was 98 N.
• the arithmetic mean of the five hardness values obtained was taken as the Vickers hardness of the test piece.
• the arithmetic mean of the Vickers hardness of the obtained 10 test pieces was taken as the Vickers hardness (HV) of the test number.
• the Vickers hardness was an integer value obtained by rounding the obtained result to one decimal place.
• the obtained Vickers hardness (HV) is shown in the "Cold workability (Hardness) (HV)" column in Table 3.
• Test Nos. 1 to 29 satisfied Features 1 to 4. Therefore, the Vickers hardness was in the range of 110 to 160 HV and the elongation at break was 40% or more. Therefore, excellent cold workability was obtained.
• test number 30 the C content was too high. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not achieved.
• test number 31 the C content was too low. As a result, the Vickers hardness was less than 110 HV, and the strength was too low.
• test number 32 the Si content was too high. As a result, the Vickers hardness exceeded 160 HV, the elongation at break was less than 40%, and sufficient cold workability was not achieved.
• test number 33 the Si content was too low. As a result, the Vickers hardness was less than 110 HV, and the strength was too low.
• test number 37 the Cr content was too low. As a result, the Vickers hardness was less than 110 HV, and the strength was too low.
• test number 38 the heating temperature T11 in the heating step was too high. As a result, the average grain size of the ferrite grains exceeded 30.0 ⁇ m. As a result, the Vickers hardness was less than 110 HV, and the strength was too low.
• the heating temperature T11 in the heating step was too low. Therefore, the average grain size of the ferrite grains was less than 5.0 ⁇ m. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so sufficient cold workability was not obtained.
• the final finish rolling start temperature T12 was too low. Therefore, the average grain size of ferrite grains was less than 5.0 ⁇ m. As a result, the Vickers hardness exceeded 160 HV, and the fracture elongation was less than 40%, so sufficient cold workability was not obtained.
• test number 41 the finish rolling temperature FT was too low. As a result, the average grain size of ferrite grains was less than 5.0 ⁇ m. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, meaning sufficient cold workability was not achieved.
• test number 42 the finish rolling temperature FT was too high. As a result, the average grain size of ferrite grains exceeded 30.0 ⁇ m. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.
• ⁇ L-1 defined by formula (1) was less than 5 in the rolling stand L-1 preceding the most downstream rolling stand L. Therefore, the GOS value exceeded 2.0°. As a result, the elongation at break was less than 40%, and sufficient cold workability was not obtained.
• ⁇ L defined by Equation (1) was less than 30 in the most downstream rolling stand L. Therefore, the average grain size of ferrite grains was less than 5.0 ⁇ m. Furthermore, the GOS value exceeded 2.0°. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so sufficient cold workability was not obtained.
• the cumulative rolling reduction R was less than 60%.
• the average grain size of ferrite grains exceeded 30.0 ⁇ m.
• the GOS value exceeded 2.0°.
• the Vickers hardness was less than 110 HV, and the strength was excessively low.
• the elongation at break was less than 40%, and sufficient cold workability was not achieved.
• test number 48 the coiling temperature CT was high. As a result, the average grain size of ferrite grains exceeded 30.0 ⁇ m. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.
• test number 49 the coiling temperature CT was low. As a result, the average grain size of ferrite grains was less than 5.0 ⁇ m. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, meaning sufficient cold workability was not achieved.
• test number 50 the annealing temperature T3 was high. Therefore, the average grain size of ferrite grains exceeded 30.0 ⁇ m. As a result, the Vickers hardness was less than 110 HV, and the strength was excessively low.
• the annealing temperature T3 was low. Therefore, the average grain size of the ferrite grains was less than 5.0 ⁇ m. As a result, the Vickers hardness exceeded 160 HV, and the fracture elongation was less than 40%, so sufficient cold workability was not obtained.
• test number 52 the holding time t3 was short. Therefore, the average grain size of the ferrite grains was less than 5.0 ⁇ m. As a result, the Vickers hardness exceeded 160 HV, and the elongation at break was less than 40%, so sufficient cold workability was not obtained.

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