WO2024135365A1 - Feuille d'acier laminée à chaud - Google Patents

Feuille d'acier laminée à chaud Download PDF

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WO2024135365A1
WO2024135365A1 PCT/JP2023/043668 JP2023043668W WO2024135365A1 WO 2024135365 A1 WO2024135365 A1 WO 2024135365A1 JP 2023043668 W JP2023043668 W JP 2023043668W WO 2024135365 A1 WO2024135365 A1 WO 2024135365A1
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steel sheet
hot
content
rolled steel
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PCT/JP2023/043668
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Japanese (ja)
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睦海 吉武
洋志 首藤
洵 安藤
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日本製鉄株式会社
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    • 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

  • the present invention relates to hot-rolled steel sheets.
  • Patent Document 1 describes a hot-rolled steel sheet having a predetermined chemical composition, a structure containing, in terms of area ratio, a total of 80-98% ferrite and bainite, and 2-10% martensite, and, when the boundaries in the structure where the misorientation is 15° or more are defined as grain boundaries, and the regions surrounded by the grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, the proportion of the crystal grains where the misorientation within the grains is 5-14° is 10-60% in terms of area ratio.
  • Patent Document 1 also teaches that by setting the proportion of the above crystal grains where the misorientation within the grains is 5-14° to an area ratio of 10-60%, it is possible to improve stretch flangeability and ductility while maintaining high strength, and further teaches that by controlling the total area ratio of ferrite and bainite in the structure and the area ratio of martensite within a predetermined range, it is possible to improve notch fatigue properties.
  • Patent Document 1 teaches that the Si content in a hot-rolled steel sheet should be limited to 0.100% or less to suppress the decrease in corrosion resistance after painting caused by the formation of such a scale pattern.
  • the present invention aims to provide a hot-rolled steel sheet that, despite its high strength, has improved stretch flangeability, ductility, and notch fatigue properties, and also has excellent corrosion resistance after painting.
  • the inventors conducted research, focusing particularly on the metal structure and surface properties of hot-rolled steel sheet.
  • the inventors discovered that by configuring the metal structure of a hot-rolled steel sheet having a specified chemical composition to contain at least one of ferrite and bainite, and martensite in specific proportions, and further controlling the proportion of crystal grains within a specified range, it is possible to improve stretch flangeability, ductility, and notch fatigue properties, while in addition, by controlling the surface roughness Ra of the steel sheet and its variation within a specified range, it is possible to improve corrosion resistance after painting, and thus completed the present invention.
  • the present invention which has achieved the above object is as follows. (1) In mass%, C: 0.020-0.070%, Si: more than 0.100 to 2.000%, Mn: 0.60-2.00%, Ti: 0.015-0.200%, sol. Al: 0.010-1.000%, P: 0.100% or less, S: 0.030% or less, N: 0.0060% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0-0.300%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.000%, B: 0 to 0.0100%, Sb: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, REM: 0-0.1000%, Bi: 0 to 0.0100%, As: 0 to 0.0100%, Zr: 0 to 1.00%, Co: 0-1.00%, Zn: 0 to 1.00%, W:
  • the metal structure has a ratio of crystal grains having an intragranular misorientation of 5 to 14° of 10 to 60% by area, where a boundary having a misorientation of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 ⁇ m or more is defined as a crystal grain, A hot-rolled steel sheet having a surface roughness Ra of less than 1.50 ⁇ m and a difference between a maximum value and a minimum value of the surface roughness Ra of 0.50 ⁇ m or less.
  • the chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001-0.300%, Cr: 0.01-2.00%, Ni: 0.01-2.00%, Cu: 0.01-2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sb: 0.01 to 1.00%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001-0.0100%, REM: 0.0001-0.1000%, Bi: 0.0001-0.0100%, As: 0.0001 to 0.0100%, Zr: 0.01 to 1.00%, Co: 0.01 to 1.00%, Zn: 0.01-1.00%, W: 0.01 to 1.00%, and Sn: 0.01 to 1.00%
  • the hot-rolled steel sheet according to the above (1) characterized in that it contains at least one of the following: (3)
  • the present invention makes it possible to provide a hot-rolled steel sheet that, despite its high strength, has improved stretch flangeability, ductility, and notch fatigue properties, and also has excellent corrosion resistance after painting.
  • FIG. 2 is a diagram showing the shape of a saddle-shaped molded product used in a saddle-shaped stretch flange test method.
  • FIG. 2 is a diagram showing the shape of a fatigue test specimen used to evaluate notch fatigue properties.
  • the hot-rolled steel sheet according to the embodiment of the present invention has, in mass%, C: 0.020-0.070%, Si: more than 0.100 to 2.000%, Mn: 0.60-2.00%, Ti: 0.015-0.200%, sol.
  • the chemical composition satisfies 0.110 ⁇ [Si]+[sol.
  • the metal structure has a ratio of crystal grains having an intragranular misorientation of 5 to 14° of 10 to 60% by area, where a boundary having a misorientation of 15° or more is defined as a grain boundary, and a region surrounded by the grain boundary and having a circle equivalent diameter of 0.3 ⁇ m or more is defined as a crystal grain,
  • the surface roughness Ra is less than 1.50 ⁇ m, and the difference between the maximum and minimum values of the surface roughness Ra is 0.50 ⁇ m or less.
  • the metal structure of a hot-rolled steel sheet having a predetermined chemical composition is configured to contain at least one of ferrite and bainite and martensite in a specific ratio, more specifically, by configuring it to contain at least one of ferrite and bainite: 80 to 98% in total and martensite: 2 to 10% by area percentage, it is possible to improve strength, stretch flangeability, ductility, and notch fatigue properties in a well-balanced manner.
  • a crystal grain with an intragranular orientation difference of 5 to 14° is effective in improving strength, stretch flangeability, and ductility. Therefore, by appropriately controlling the proportion of these crystal grains, more specifically by controlling it to within the range of 10 to 60% by area, it is possible to further improve the balance between strength, stretch flangeability, and ductility.
  • steel sheets are generally painted after being subjected to a chemical conversion treatment such as zinc phosphate treatment.
  • a chemical reaction is caused between the treatment liquid and Fe eluted from the steel sheet, and a chemical conversion coating is formed on the steel sheet surface by chemical conversion crystals, thereby improving corrosion resistance after painting.
  • oxides resulting from the formation of Si scale patterns remain on the steel sheet surface, the elution of Fe is hindered, and areas where the chemical conversion coating is not formed, called "ske", appear, or a chemical conversion coating that does not contain Fe that would not normally be formed is formed due to the lack of elution of Fe, which can result in a decrease in corrosion resistance after painting.
  • the present inventors conducted a study focusing particularly on the surface properties of hot-rolled steel sheets, in order to improve corrosion resistance after painting by suppressing the formation of Si scale patterns in addition to improving stretch flangeability, ductility, and notch fatigue properties.
  • the inventors discovered that by controlling the surface roughness Ra of hot-rolled steel sheet to less than 1.50 ⁇ m and controlling the difference between the maximum and minimum values of the surface roughness Ra to 0.50 ⁇ m or less, it is possible to suppress the formation of Si scale patterns, thereby significantly improving the post-painting corrosion resistance of the hot-rolled steel sheet.
  • Si oxides are formed at the interface between the steel and Fe oxides (also called scale) that form on the steel sheet surface during the hot rolling process. Since the Si oxides firmly adhere the Fe oxides to the steel, the scales may not be sufficiently removed even by subsequent descaling using high-pressure water or the like. In such cases, the scale that has not been sufficiently removed is pushed into the steel sheet surface by subsequent rolling such as finish rolling, resulting in unevenness on the steel sheet surface and deterioration of the surface properties. This poor descaling and deterioration of the surface properties due to subsequent rolling result in the formation of Si scale patterns in the hot-rolled steel sheet after pickling.
  • the inventors have found that, as will be described in detail later in relation to the manufacturing method of hot-rolled steel sheet, by appropriately controlling the temperature conditions between rough rolling and finish rolling, the scale can be sufficiently or completely removed by descaling before finish rolling, and in relation thereto, the scale can be significantly suppressed from being pressed into the steel sheet surface during the subsequent finish rolling to generate unevenness on the steel sheet surface.
  • the inventors have further studied the relationship between the surface properties of the steel sheet and the generation of Si scale patterns.
  • the inventors have found that the generation of Si scale patterns can be significantly suppressed by sufficiently or completely removing the scale by descaling before finish rolling so that the surface roughness Ra of the finally obtained hot-rolled steel sheet is less than 1.50 ⁇ m and the difference between the maximum and minimum values of the surface roughness Ra is 0.50 ⁇ m or less, and in relation thereto, a hot-rolled steel sheet having excellent corrosion resistance after painting can be obtained. That is, according to the embodiment of the present invention, despite the high strength, for example, tensile strength of 540 MPa or more, it is possible to improve stretch flangeability, ductility, and notch fatigue properties, and achieve excellent corrosion resistance after painting. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve the conflicting properties of high strength and excellent workability, and further has excellent corrosion resistance after painting, so it is particularly useful in the automotive field where these properties are required.
  • 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 exerts precipitation strengthening based on the formed precipitates and the like. It also contributes to refining the structure by the pinning effect of precipitates.
  • the C content is set to 0.020% or more. However, if the C content is excessive, stretch flangeability and weldability may be deteriorated.
  • the C content may be 0.065% or less, 0.060% or less, 0.055% or less, or 0.050% or less.
  • Silicon is an effective element for increasing strength as a solid solution strengthening element.
  • the silicon content is set to more than 0.100%.
  • the silicon content is set to 0.102% or more, 0.105% or more, 0.108% or more, 0.110% or more, 0.120% or more, 0.150% or more, 0.200% or more, 0.300% or more, 0.500% or more, 0.
  • the Si content is set to 2.000% or less.
  • the Si content is set to 1.800% or less. , 1.600% or less, 1.400% or less, or 1.200% or less.
  • Mn is an element that is effective in increasing strength as an element for hardenability and solid solution strengthening. In order to fully obtain these effects, the Mn content is set to 0.60% or more. The Mn content is set to 0.70%. The Mn content may be 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, if the Mn content is excessive, the stretch flangeability may be deteriorated. The Mn content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.
  • Ti is an element that finely precipitates in steel as carbide (TiC) and improves the strength of steel by precipitation strengthening.
  • Ti fixes C by forming carbides, which is detrimental to stretch flangeability.
  • the Ti content is set to 0.015% or more.
  • the Ti content is set to 0.200% or less.
  • the Ti content may be 0.180% or less, 0.170% or less, 0.150% or less, or 0.120% or less.
  • sol. Al is an element that acts as a deoxidizer for molten steel. In order to fully obtain this effect, the sol. Al content is set to 0.010% or more. On the other hand, if the sol. Al content is excessive, coarse oxides are formed, which reduces toughness and ductility and causes fatigue during rolling. Therefore, the sol. Al content is set to 1.000% or less. The sol. Al content is set to 0.800% or less, 0.600% or less, or 0.400% or less.
  • the term "solubilized Al" means acid-soluble Al, and indicates solute Al that is present in the steel in a solid solution state.
  • the P content is set to 0.100% or less.
  • the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. may be 0.001% or more, 0.003% or more, or 0.005% or more.
  • the Si content is set to 0.030% or less.
  • the S content is set to 0.020% or less, 0.010% or less.
  • the lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the S content is set to 0.001% or less. % or more, 0.002% or more, or 0.003% or more.
  • N forms precipitates with Ti preferentially over C, and may reduce the amount of Ti available for fixing C. Therefore, the N content is set to 0.0060% or less.
  • the lower limit of the N content is not particularly limited and may be 0%, but an excessive reduction in the N content leads to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.
  • O is an element that is mixed in during the manufacturing process. If O is contained in excess, coarse inclusions may form, reducing the toughness of the steel plate. Therefore, the O content is set to 0.0100% or less.
  • the O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • the lower limit of the O content is not particularly limited and may be 0%, but is preferably 0.0001% or less. %, refining takes time, which leads to a decrease in productivity. Therefore, the O content may be 0.0001% or more, or 0.0005% or more.
  • the basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.
  • Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to refining the structure through a pinning effect, and thus to increasing the strength of the steel sheet. It is also an element that fixes C by forming carbonitrides and suppresses the formation of cementite that is detrimental to stretch flangeability.
  • the Nb content may be 0%, but in order to obtain these effects, In the present invention, the Nb content is preferably 0.001% or more.
  • the Nb content may be 0.005% or more, 0.010% or more, or 0.015% or more.
  • the Nb content is set to 0.050% or less.
  • the Nb content is set to 0.040% or less, 0.050% or less. It may be 0.030% or less or 0.020% or less.
  • V is an element that contributes to improving strength through precipitation strengthening, etc.
  • the V content may be 0%, but in order to obtain such an effect, the V content must be 0.001% or more.
  • the V content may be 0.010% or more, 0.030% or more, or 0.050% or more.
  • the V content is preferably 0.300% or less.
  • the V content may be 0.200% or less, 0.100% or less, or 0.080% or less. good.
  • Cr is an element that improves 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 should be 0.01% or less. % or more.
  • the Cr content may be 0.03% or more, or 0.05% or more.
  • the Cr content is preferably 2.00% or less.
  • the Cr content is preferably 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, It may be 0.15% or less, or 0.10% or less.
  • Ni and Cu are elements that contribute to improving strength by precipitation strengthening or solid solution strengthening.
  • the Ni and Cu contents may be 0%, but in order to obtain such effects, these elements are The content of each of these elements is preferably 0.01% or more, and may be 0.03% or more or 0.05% or more. On the other hand, even if these elements are contained in excess, the effect becomes saturated. Therefore, the Ni and Cu contents are preferably 2.00% or less, more preferably 1.50% or less, 1.00% or less, 0.50% or less, and 0.50% or less. .30% or less, 0.15% or less, or 0.10% or less.
  • Mo is an element that improves 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 should be 0.001% or less. % or more.
  • the Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more.
  • the Mo content is preferably 1.000% or less.
  • the Mo content is preferably 0.800% or less, 0.500% or less, 0. It may be 200% or less, 0.100% or less, or 0.080% 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 should be 0%.
  • 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.
  • an excessive B content is not effective. There is a risk that the B content will saturate, resulting in an increase in manufacturing costs. Therefore, the B content is preferably 0.0100% or less.
  • the B content is preferably 0.0050% or less, 0.0030% or less, 0.0015% or less It may be 0.0010% or less.
  • Sb is an element effective in improving corrosion resistance.
  • the Sb content may be 0%, but in order to obtain such an effect, the Sb content is preferably 0.01% or more.
  • the Sb content may be 0.02% or more, or 0.05% or more.
  • excessive Sb content may cause a decrease in toughness. Therefore, the Sb content is set to 1.00
  • the Sb content 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 content of each of these elements is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the Ca, Mg and Hf contents are each preferably 0.0100% or less, and more preferably 0. It may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • REM is an element that can control the morphology of nonmetallic inclusions.
  • the REM content may be 0%, but to obtain such an effect, the REM content should be 0.0001%.
  • the REM content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the REM content is preferably 0.1000% or less.
  • the REM content is preferably 0.0500% or less, 0.0100% or less, It may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides with atomic numbers 57, 58, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78 ) and the REM content is the total content of these elements.
  • Bi and As are elements effective for improving corrosion resistance.
  • the Bi and As contents may be 0%, but in order to obtain such effects, the contents of these elements should each be 0.
  • the content of these elements is preferably 0.0001% or more, and may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the Bi and As contents are preferably 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0050% or less. It may be 0.0020% or less.
  • Zr is an element that can control the morphology of nonmetallic inclusions.
  • the Zr content may be 0%, but in order to obtain such an effect, the Zr content should be 0.01%.
  • the Zr content may be 0.05% or more, or 0.10% or more.
  • the Zr content is preferably 1.00% or less.
  • the Zr content is preferably 0.80% or less, 0.50% or less, 0.30% or less, or It may be 0.20% or less.
  • Co is an element that contributes to improving hardenability and/or heat resistance.
  • the Co content may be 0%, but in order to obtain these effects, the Co content should be 0.01% or more.
  • the Co content may be 0.05% or more, or 0.10% or more.
  • the Co content is preferably 1.00% or less.
  • the Co content is preferably 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less. may be also possible.
  • Zn is an element effective in controlling the shape of inclusions.
  • the Zn content is preferably 0.01% or more. % or more, or 0.10% or more.
  • the Zn content is 1.00% or less.
  • the Zn content may be not more than 0.80%, not more than 0.50%, not more than 0.30%, or not more than 0.20%.
  • W is an element that improves the hardenability of steel and contributes to improving strength.
  • the W content may be 0%, but in order to obtain such an effect, the W content should be 0.01% or less. % or more.
  • the W content may be 0.05% or more, or 0.10% or more.
  • the W content is preferably 1.00% or less.
  • the W content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.
  • Sn is an element effective in improving corrosion resistance.
  • the Sn content may be 0%, but in order to obtain such an effect, the Sn content is preferably 0.01% or more.
  • the Sn content may be 0.02% or more, or 0.05% or more.
  • excessive Sn content may cause a decrease in toughness. Therefore, the Sn content is set to 1.00
  • the Sn content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.
  • the remainder other than the above elements consists of Fe and impurities.
  • Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing hot-rolled steel sheets.
  • [0.110 ⁇ [Si]+[sol. Al] ⁇ 2.500] The chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention must satisfy the following formula. 0.110 ⁇ [Si]+[sol. Al] ⁇ 2.500
  • [Si] and [sol. Al] are the contents (mass%) of each element.
  • the grains are surrounded by a boundary with an orientation difference of 15° or more and have a circle equivalent diameter of 0.3 ⁇ m or more.
  • crystal grains having an intragranular misorientation of 5 to 14° are effective in improving strength and stretch flangeability.
  • the ratio of the crystal grains is controlled within the range of 10 to 60% by area to improve the balance between strength and stretch flangeability.
  • sol. Al is also an element effective in controlling the ratio of crystal grains having an intragranular misorientation of 5 to 14 degrees within the range of 10 to 60%. This is believed to be due to the fact that the temperature of the Ar3 point is increased by the inclusion of Si and sol. Al, and the transformation strain introduced into the grains is reduced.
  • the chemical composition of the hot rolled steel sheet according to the embodiment of the present invention contains Si and sol. In order to further enhance these effects, the total content of Si and sol.
  • Al is controlled to be more than 0.110%, that is, to satisfy [Si] + [sol. Al] > 0.110.
  • the total content of Si and sol. Al is preferably 0.120% or more, and may be 0.150% or more, 0.200% or more, or 0.300% or more. If the content of Si is too high, the formation of ferrite may be promoted, and the strength may decrease. Therefore, the total content of Si and sol. Al is 2.500% or less, that is, [Si] + [sol. Al]
  • the total content of Si and sol. Al may be 2.000% or less, 1.500% or less, 1.000% or less, or 0.800% or less.
  • the chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention may be measured by a general analytical method.
  • the chemical composition of the hot-rolled steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
  • C and S may be measured using the combustion-infrared absorption method
  • N may be measured using the inert gas fusion-thermal conductivity method
  • O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
  • the metal structure of the hot-rolled steel sheet according to the embodiment of the present invention includes, in area %, at least one of ferrite and bainite: 80 to 98% in total, and martensite: 2 to 10%.
  • the total area ratio of at least one of ferrite and bainite is 80% or more, and may be, for example, 82% or more, 85% or more, 88% or more, or 90% or more.
  • the area ratio of martensite may be 10% or less, and may be, for example, 9% or less, 8% or less, 7% or less, or 6% or less.
  • the total area ratio of at least one of ferrite and bainite is high or the area ratio of martensite is low, the balance between strength and notch fatigue properties may be reduced, and desired properties may not be obtained.
  • the total area ratio of at least one of ferrite and bainite is 98% or less, and may be, for example, 96% or less, 94% or less, or 92% or less.
  • the area ratio of martensite is 2% or more, and may be, for example, 3% or more, 4% or more, or 5% or more.
  • the metal structure of the hot-rolled steel sheet may contain either ferrite or bainite, and preferably contains both ferrite and bainite. Therefore, the area ratio of either ferrite or bainite may be 0%, or may be, for example, 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, or 40% or more, respectively. Similarly, the area ratio of ferrite and bainite may be, for example, 90% or less, 80% or less, 70% or less, 60% or less, or 50% or less, respectively. From the viewpoint of improving the ductility of the hot-rolled steel sheet, the area ratio of bainite is preferably 80% or less, and more preferably 70% or less.
  • the remaining structure other than ferrite, bainite, and martensite may be 0% by area, but when the remaining structure is present, the remaining structure may be at least one of retained austenite and pearlite.
  • the area ratio of the remaining structure is not particularly limited, but may be, for example, 1% or more, 2% or more, or 3% or more. From the viewpoint of further improving the stretch flangeability, the area ratio of the remaining structure is preferably, for example, 10% or less, and may be 8% or less, 6% or less, or 5% or less.
  • Identification of the metal structure and calculation of the area ratio in the hot-rolled steel sheet are performed by optical microscope observation after etching with a Nital reagent or Lepera solution and X-ray diffraction method.
  • the structure observation by optical microscope is performed on the plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface. Specifically, first, a sample is taken from the hot-rolled steel sheet, and the observation surface of the sample is etched with Nital.
  • image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope, and the area ratios of ferrite and pearlite, as well as the total area ratio of bainite and martensite are calculated. Those with equiaxed crystal grains and no substructure can be identified as ferrite, and those containing substructure can be identified as bainite and martensite.
  • image analysis is performed on a structure photograph obtained at a 1/4 depth position of the plate thickness in a field of view of 300 ⁇ m ⁇ 300 ⁇ m using an optical microscope to calculate the total area ratio of retained austenite and martensite.
  • the image analysis is performed using the "Analyze” function of the image analysis software "ImageJ", which allows the above-mentioned area ratios and total area ratio to be calculated.
  • ImageJ is an open source, public domain image processing software that is widely used among those skilled in the art.
  • the area ratio of the martensite is calculated by subtracting the obtained area ratio of the retained austenite from the total area ratio of the retained austenite and martensite calculated previously.
  • the area ratio of the bainite is calculated by subtracting the obtained area ratio of the martensite from the total area ratio of the bainite and martensite calculated previously.
  • an increase in the dislocation density in the grain improves the strength while decreasing the workability.
  • the strength can be improved without decreasing the workability in the crystal grains in which the orientation difference in the grain is controlled to 5 to 14°.
  • crystal grains with an orientation difference in the grain of less than 5° are excellent in workability but difficult to increase in strength.
  • crystal grains with an intragranular misorientation of more than 14° do not necessarily contribute to improving the stretch flangeability because the deformability is different within the crystal grains.
  • the proportion of crystal grains with an intragranular misorientation of 5 to 14° by appropriately controlling the proportion of crystal grains with an intragranular misorientation of 5 to 14°, more specifically, by controlling it within the range of 10 to 60% in terms of area%, it is possible to improve the stretch flangeability while achieving the desired steel sheet strength, and to further improve the balance between strength and stretch flangeability. If the proportion of crystal grains with an intragranular misorientation of 5 to 14° is small, the stretch flangeability may be reduced. Therefore, from the viewpoint of improving the stretch flangeability, the proportion of crystal grains with an intragranular misorientation of 5 to 14° may be 15% or more, 18% or more, or 20% or more.
  • the proportion of crystal grains with an intragranular misorientation of 5 to 14° may be 55% or less, 50% or less, 45% or less, or 40% or less.
  • the proportion of crystal grains with an intragranular orientation difference of 5 to 14° is measured by electron backscattered diffraction (EBSD). More specifically, first, a sample is taken from the steel sheet so that the plate thickness cross section parallel to the rolling direction and perpendicular to the plate surface is the observation surface. Next, at a depth position of 1/4 of the plate thickness from the surface of the steel sheet, an area of 200 ⁇ m in the rolling direction of the steel sheet and 100 ⁇ m in the normal direction to the rolling surface is analyzed by EBSD analysis at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
  • EBSD electron backscattered diffraction
  • the EBSD analysis is performed at an analysis speed of 50 to 300 points/second using an apparatus consisting of a thermal field emission scanning electron microscope (e.g., JSM-7001F manufactured by JEOL) and an EBSD detector (HIKARI detector manufactured by TSL).
  • a thermal field emission scanning electron microscope e.g., JSM-7001F manufactured by JEOL
  • an EBSD detector HKARI detector manufactured by TSL.
  • regions with an orientation difference of 15° or more and a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains
  • the average orientation difference within the crystal grains is calculated
  • the ratio of crystal grains with an orientation difference of 5 to 14° within the grains is obtained.
  • the crystal grains and the average orientation difference within the grains defined as above can be calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device.
  • orientation difference within a grain refers to "Grain Orientation Spread (GOS)", which is the orientation dispersion within a crystal grain.
  • GOS Grain Orientation Spread
  • the value of the orientation difference within a grain is described in "Analysis of Misorientation in Plastic Deformation of Stainless Steel by EBSD Method and X-ray Diffraction Method", Hidehiko Kimura et al., Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 71, No. 712, 2005, p.
  • the GOS is calculated as an average value of the misorientation between a reference crystal orientation and all measurement points within the same crystal grain.
  • the reference crystal orientation is an average orientation of all measurement points within the same crystal grain.
  • the GOS value can be calculated using the software "OIM Analysis (registered trademark) Version 7.0.1" that comes with the EBSD analyzer.
  • the surface roughness Ra is controlled to less than 1.50 ⁇ m, and the difference between the maximum and minimum values of the surface roughness Ra is controlled to 0.50 ⁇ m or less.
  • the corrosion resistance after painting of the hot-rolled steel sheet will be reduced.
  • the surface roughness Ra is less than 1.50 ⁇ m
  • the difference between the maximum and minimum values of the surface roughness Ra exceeds 0.50 ⁇ m
  • the variation in the surface roughness is large, so it cannot be said that the generation of the Si scale pattern is sufficiently suppressed.
  • the surface roughness Ra and the difference between its maximum and minimum values are preferably as low as possible.
  • the surface roughness Ra is preferably 1.40 ⁇ m or less, more preferably 1.20 ⁇ m or less or 1.00 ⁇ m or less, and most preferably 0.80 ⁇ m or less.
  • the difference between the maximum and minimum values of the surface roughness Ra is preferably 0.40 ⁇ m or less, more preferably 0.30 ⁇ m or less.
  • the lower limit of the surface roughness Ra and the difference between its maximum and minimum values is not particularly limited, but for example, the surface roughness Ra may be 0.20 ⁇ m or more, 0.30 ⁇ m or more, 0.40 ⁇ m or more, or 0.50 ⁇ m or more.
  • the difference between the maximum and minimum values of the surface roughness Ra may be 0 ⁇ m or more, 0.02 ⁇ m or more, 0.03 ⁇ m or more, 0.05 ⁇ m or more, or 0.07 ⁇ m or more.
  • the surface roughness Ra and the difference between the maximum and minimum values of the surface roughness Ra of a hot-rolled steel sheet are measured as follows. More specifically, first, when the hot-rolled steel sheet has a scale on the surface, the sample is pickled and then subjected to roughness measurement. The pickling is performed under the condition that the sample is immersed in hydrochloric acid having a hydrochloric acid concentration of 3 to 10% by volume at a temperature of 85 to 98 ° C for 20 to 300 seconds. The pickling may be performed once, or may be performed several times as necessary.
  • the above pickling time (20 to 300 seconds) means the time of the pickling when pickling is performed only once, and means the total time of these pickling when pickling is performed several times.
  • the pickling temperature 85 ° C or higher, the oxides in the surface layer can be sufficiently removed.
  • the surface of the hot-rolled steel sheet after pickling is measured at 50 mm intervals along the width direction, and the surface roughness is measured in the rolling direction at each measurement point. It is desirable to measure at 10 or more points, but if the plate width is insufficient, similar measurements are performed at positions 50 mm away in the rolling direction, and 10 or more points are measured.
  • the measurement length at each measurement point is 5 mm.
  • a roughness curve is obtained by sequentially applying a profile filter with cutoff values ⁇ c and ⁇ s to the measured cross-sectional curve obtained by measurement. Specifically, a roughness curve is obtained by removing components with a wavelength ⁇ c of 0.8 mm or less and components with a wavelength ⁇ s of 2.5 ⁇ m or more from the obtained measurement results. Based on the obtained roughness curve, the arithmetic mean roughness of each measurement point is calculated in accordance with JIS B 0601:2013. The average value of all the obtained arithmetic mean roughnesses is determined as the surface roughness Ra, and similarly, the difference between the maximum and minimum values of the surface roughness Ra is determined from the maximum and minimum values of all the obtained arithmetic mean roughnesses.
  • the hot-rolled steel sheet has a surface treatment film such as a plating layer and paint on the surface
  • the measurement is performed on the surface of the steel base obtained after removing the surface treatment film.
  • the method for removing the surface treatment film can be selected appropriately according to the type of surface treatment film, as long as it does not affect the surface roughness of the steel base.
  • the surface treatment film is a zinc plating layer such as electrolytic zinc plating, electrolytic Zn-Ni alloy plating, hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, or hot-dip Zn-Al-Mg-Si alloy plating
  • the zinc plating layer can be dissolved using dilute hydrochloric acid with an inhibitor added. This allows only the zinc plating layer to be peeled off from the steel sheet.
  • An inhibitor is an additive used to suppress changes in roughness due to excessive dissolution of the steel base. For example, a solution of adding "Ibit No.
  • the surface treatment film is an aluminum plating layer such as hot-dip aluminum plating
  • the Al plating is dissolved by successively immersing the material in an aqueous sodium hydroxide solution and a dilute hydrochloric acid solution containing added hexamethylenetetramine in accordance with the description of JIS G 3314:2019 until the foaming caused by the dissolution of the plating subsides.
  • the surface treatment film is an electrodeposition coating
  • the electrodeposition coating is peeled off using a stripping agent (Neo River SP-751: manufactured by Sansai Kako Co., Ltd.).
  • the hot rolled steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm, although not particularly limited thereto.
  • the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 5.0 mm or less, or 4.0 mm or less.
  • the hot-rolled steel sheet according to the embodiment of the present invention may be a painted steel sheet having a paint layer on at least one surface.
  • the paint layer may be formed on the plating layer and/or the chemical conversion coating.
  • the paint layer is not particularly limited and may be any appropriate paint layer known to those skilled in the art.
  • the thickness of the paint layer is also not particularly limited and may have any appropriate thickness.
  • the paint layer generally includes an electrocoat paint layer, and may further include an intermediate paint layer, a base coat layer, and a clear coat layer thereon.
  • the hot-rolled steel sheet according to the embodiment of the present invention has high strength, yet improves stretch flangeability, ductility, and notch fatigue properties, and can achieve excellent corrosion resistance after painting. Therefore, the hot-rolled steel sheet according to the embodiment of the present invention can reliably achieve the contradictory properties of high strength and excellent workability, and further has excellent corrosion resistance after painting, so it is particularly useful for use in parts in technical fields where these properties are required.
  • an automobile part containing the hot-rolled steel sheet according to the embodiment of the present invention particularly a part selected from the undercarriage, chassis, bumper, etc. of an automobile, is provided.
  • the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the hot-rolled steel sheet may be 1470 MPa or less, 1250 MPa or less, or 1180 MPa or less.
  • the tensile strength is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
  • Total elongation: El According to the hot-rolled steel sheet having the above chemical composition and metal structure, in addition to high tensile strength, the total elongation can be improved, and more specifically, a total elongation of 15.0% or more can be achieved.
  • the total elongation is preferably 18.0% or more, more preferably 20.0% or more, and most preferably 22.0% or more.
  • the upper limit is not particularly limited, but for example, the total elongation may be 40.0% or less or 35.0% or less.
  • the total elongation is measured by taking a JIS No. 5 test piece from a direction (C direction) in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the hot-rolled steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011.
  • the method for producing a hot-rolled steel sheet according to an embodiment of the present invention includes: (A) a hot rolling process including heating a slab having the chemical composition described above in relation to the hot rolled steel plate, followed by rough rolling, high-pressure water descaling, and finish rolling, and satisfying the following conditions (A1) to (A4); and (A1) the heating temperature of the slab is equal to or higher than the solution temperature (SRTmin) °C represented by the following formula 1 and equal to or higher than 1170 °C; (A2) The maximum heating temperature from the completion of rough rolling to the start of high-pressure water descaling before finish rolling is equal to or higher than T ° C.
  • [Ti] and [C] are the contents (mass%) of each element in the steel.
  • [Si] and [sol. Al] are the contents (mass%) of each element in the steel.
  • a cooling step includes primarily cooling the finish-rolled steel sheet to a temperature range of 650 to 750°C at an average cooling rate of 10°C/s or more, holding the steel sheet in the temperature range for 3.0 to 10.0 seconds, and then secondary cooling to 100°C or less at an average cooling rate of 30°C/s or more. Each step will be described in detail below.
  • the heating temperature of the slab is less than the solution temperature (SRTmin) ° C., Ti is not sufficiently dissolved. If Ti is not sufficiently dissolved during slab heating, it becomes difficult to improve the strength of the steel by precipitation strengthening by finely precipitating Ti as carbide (TiC) in the steel in the cooling process after the hot rolling process. In addition, it becomes difficult to fix C by forming carbide (TiC) and suppress the generation of cementite, which is harmful to stretch flangeability.
  • the heating temperature of the slab needs to be 1170 ° C. or more. The scale generated on the surface of the steel sheet during heating needs to be removed by descaling after heating, but if it is less than 1170 ° C., the scale removal by descaling becomes uneven, and the generation of Si scale patterns may not be sufficiently suppressed.
  • T ° C. or higher Maximum heating temperature from the end of rough rolling to the start of high-pressure water descaling before finish rolling: T ° C. or higher.
  • T°C Maximum heating temperature from the end of rough rolling to the start of high-pressure water descaling before finish rolling
  • the heated slab is subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc., and then the maximum heating temperature from the completion of rough rolling to the start of high-pressure water descaling before finish rolling is controlled to be equal to or higher than T°C represented by the following formula 2.
  • T 1081+63 ⁇ [Si]+27 ⁇ [sol. Al]+10 ⁇ [sol. Al]/[Si]-8/[Si]-1/[sol. Al]...Formula 2
  • [Si] and [sol. Al] are the contents (mass%) of each element in the steel.
  • Si oxides are formed at the interface between the Fe oxides (also called scales) that form on the steel sheet surface during the hot rolling process and the steel.
  • the Si oxides tightly adhere the Fe oxides to the steel, so that the scales may not be sufficiently removed even by subsequent descaling using high-pressure water or the like. In such cases, the scales that have not been sufficiently removed are pushed into the steel sheet surface by the subsequent finish rolling, resulting in unevenness on the steel sheet surface and deterioration of the surface properties.
  • the outlet temperature of rough rolling is not necessarily limited, and an appropriate temperature may be selected as appropriate.
  • the rolled material is subjected to high-pressure water descaling before finish rolling.
  • High-pressure water descaling is performed using high-pressure water with a water pressure of 10 to 50 MPa.
  • Scale growth also occurs in the hot-rolled steel sheet after finish rolling.
  • by sufficiently or completely removing the scale before finish rolling it is possible to suppress the scale from being pushed into the steel sheet surface during finish rolling as described above. Therefore, even if scale grows thereafter, it is merely formed uniformly over the entire steel sheet surface, and the formation of such scale has no effect on the generation of the Si scale pattern.
  • such scale can be relatively easily removed by performing an appropriate pickling treatment after hot rolling.
  • Crystal grains with an intragranular misorientation of 5 to 14° are generated by transformation in a paraequilibrium state at a relatively low temperature. For this reason, it is possible to control the generation of crystal grains with an intragranular misorientation of 5 to 14° by limiting the dislocation density of austenite before transformation to a certain range in the hot rolling process and limiting the cooling rate to a certain range in the subsequent cooling process. That is, by controlling the accumulated strain in the latter three stages of finish rolling and the subsequent cooling, it is possible to control the nucleation frequency and the subsequent growth rate of crystal grains having an intragranular misorientation of 5 to 14°.
  • the dislocation density of austenite introduced by finish rolling is mainly related to the nucleation frequency
  • the cooling rate after finish rolling is mainly related to the growth rate.
  • the cumulative strain in the latter three stages of finish rolling is less than 0.50, the dislocation density of the introduced austenite is insufficient, and the proportion of crystal grains with an intragranular misorientation of 5 to 14° is less than 10%.
  • the cumulative strain in the latter three stages of finish rolling is more than 0.60, recrystallization of austenite occurs during hot rolling, and the accumulated dislocation density during transformation decreases. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° is less than 10%.
  • the cumulative strain ( ⁇ eff.) in the latter three stages of finish rolling is calculated by the following formula 3. ⁇ eff.
  • Q 183200J
  • t indicates the cumulative time (seconds) until just before cooling in the corresponding pass
  • T indicates the rolling temperature (° C.) in the corresponding pass.
  • the end temperature of the finish rolling needs to be Ar3+30°C or higher. If the end temperature of the finish rolling is less than Ar3+30°C, when ferrite is generated in a part of the structure due to the variation of the components in the steel sheet and the rolling temperature, the ferrite may be processed. The processed ferrite may cause a decrease in ductility. In addition, if the end temperature of the finish rolling is less than Ar3+30°C, the ratio of crystal grains having an intragranular misorientation of 5 to 14° may exceed 60% and become excessively high.
  • Ar3 (°C) is calculated based on the chemical composition of the hot-rolled steel sheet by the following formula 4.
  • Ar3 901-325 ⁇ [C]+33 ⁇ [Si]+287 ⁇ [P]+40 ⁇ [sol. Al]-92 ⁇ ([Mn]+[Mo]+[Cu])-46 ⁇ ([Cr]+[Ni]).
  • [C], [Si], [P], [sol. Al], [Mn], [Mo], [Cu], [Cr] and [Ni] are the contents (mass%) of each element in the steel, and are 0 when the element is not contained.
  • the finish-rolled steel sheet is subjected to two-stage cooling in the next cooling step. Specifically, the finish-rolled steel sheet is first cooled to a temperature range of 650 to 750 ° C. at an average cooling rate of 10 ° C./s or more, held in that temperature range for 3.0 to 10.0 seconds, and then secondarily cooled to 100 ° C. or less at an average cooling rate of 30 ° C./s or more.
  • the cooling end temperature of the primary cooling is less than 650 ° C
  • transformation due to paraequilibrium occurs at a temperature lower than the desired temperature range
  • the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is less than 10%.
  • the holding time at 650 to 750 ° C is less than 3.0 seconds
  • the proportion of crystal grains with an orientation difference of 5 to 14 ° in the grains is also less than 10%.
  • the holding time at 650 to 750 ° C exceeds 10.0 seconds or the average cooling rate of the secondary cooling is less than 30 ° C / s, cementite that is harmful to stretch flangeability is likely to be generated.
  • the cooling end temperature of the secondary cooling is more than 100 ° C, the area ratio of martensite is less than 2%.
  • the average cooling rate of the primary and secondary cooling may be 200 ° C / s or less in consideration of the equipment capacity of the cooling equipment.
  • the hot-rolled steel sheet manufactured by the above manufacturing method contains, by area percentage, at least one of ferrite and bainite: 80-98% in total, and martensite: 2-10%, and when the boundaries with an orientation difference of 15° or more are defined as grain boundaries, and the regions surrounded by the grain boundaries and having a circle equivalent diameter of 0.3 ⁇ m or more are defined as crystal grains, it is possible to obtain a metal structure in which the proportion of crystal grains with an orientation difference of 5-14° within the grains is 10-60% by area percentage. As a result, despite the high strength, it is possible to significantly improve the stretch flangeability, ductility, and notch fatigue properties.
  • the obtained hot-rolled steel sheet has a surface roughness Ra of less than 1.50 ⁇ m and the difference between the maximum and minimum values of the surface roughness Ra is controlled to 0.50 ⁇ m or less, the generation of Si scale patterns can be significantly suppressed, and in connection therewith, excellent post-painting corrosion resistance can be achieved. Therefore, hot-rolled steel sheets manufactured by the above manufacturing method can reliably achieve both the contradictory properties of high strength and excellent workability, and also have excellent corrosion resistance after painting, making them particularly useful in the automotive field where these properties are required.
  • hot-rolled steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength, stretch flangeability, ductility, notch fatigue properties, and post-painting corrosion resistance of the obtained hot-rolled steel sheets were examined.
  • molten steel was cast by continuous casting to form slabs having various chemical compositions shown in Tables 1 and 2, and these slabs were heated under the conditions shown in Table 3, and then hot-rolled.
  • Hot rolling was performed by rough rolling, high-pressure water descaling, and finish rolling.
  • the rough rolling conditions were the same for all of the invention examples and comparative examples.
  • High-pressure water descaling was performed using high-pressure water with a water pressure of 15 MPa, and the maximum heating temperature from the completion of rough rolling to the start of high-pressure water descaling before finish rolling was as shown in Table 3.
  • finish rolling was performed using a tandem rolling mill consisting of five rolling stands, and the accumulated strain ( ⁇ eff.) in the last three stages of finish rolling and the end temperature of finish rolling were as shown in Table 3.
  • the finish-rolled steel plate was subjected to primary cooling and secondary cooling under the conditions shown in Table 3 to obtain a hot-rolled steel plate having a plate thickness of 2.9 mm.
  • the properties of the resulting hot-rolled steel sheets were measured and evaluated using the following methods.
  • TS tensile strength
  • El total elongation
  • the stretch flangeability was evaluated by a saddle-shaped stretch flange test method using a saddle-shaped molded product. Specifically, a molded product of a saddle-shaped shape simulating a stretch flange shape consisting of a straight part and a circular part as shown in FIG. 1 was pressed, and the stretch flangeability was evaluated by the limit forming height at that time.
  • a saddle-shaped stretch flange test method a saddle-shaped molded product with a corner curvature radius R of 50 to 60 mm and an opening angle ⁇ of 120° is used to measure the limit forming height H (mm) when the clearance when punching the corner part is 11%.
  • the clearance indicates the ratio of the gap between the punching die and the punch to the thickness of the test piece. Since the clearance is actually determined by the combination of the punching tool and the plate thickness, 11% means that the range of 10.5 to 11.5% is satisfied.
  • the judgment of the limit forming height H was made by visually observing the presence or absence of cracks having a length of 1/3 or more of the plate thickness after forming, and the limit forming height at which no cracks existed was determined.
  • the product (TS ⁇ H) of tensile strength TS (MPa) and limit forming height H (mm) was used as an index of stretch flangeability, and the stretch flangeability was evaluated as being improved when TS ⁇ H ⁇ 19,500 MPa ⁇ mm.
  • the chemical conversion treatability was evaluated as follows. Specifically, the hot-rolled steel sheet produced was first pickled, then subjected to a phosphating conversion treatment in which a zinc phosphate film of 2.5 g/ m2 was attached, and at this stage, the presence or absence of voids and the P ratio were measured to evaluate the chemical conversion treatability.
  • the voids refer to the parts to which the chemical conversion film is not attached, and the P ratio refers to the value represented by P/(P+H) using the X-ray diffraction intensity P of the (100) plane of phosphophyllite ( FeZn2 ( PO4 ) 2.4H2O ) and the X-ray diffraction intensity H of the (020) plane of hopite ( Zn3 ( PO4 ) 2.4H2O ) measured using an X - ray diffraction device.
  • Phosphating is a chemical treatment that uses a chemical solution whose main components are phosphoric acid and zinc ions. It is a chemical reaction that produces crystals called phosphophyllite with the Fe ions that dissolve from the steel sheet. In phosphating, it is important to (1) promote the reaction by dissolving the Fe ions, and (2) form dense phosphophyllite crystals on the steel sheet surface. In particular, with regard to (1), if oxides due to the formation of Si scale remain on the steel sheet surface, the dissolution of Fe is hindered and scale appears, or if Fe does not dissolve, an abnormal chemical treatment film that does not contain Fe, such as hopite, is formed, which would not normally be formed, and as a result, corrosion resistance after painting may decrease.
  • the presence or absence of scale was determined by observation with a scanning electron microscope. Specifically, about 20 fields of view were observed at a magnification of 1000 times, and if the scale was uniformly attached over the entire surface and no scale was observed, it was evaluated as no scale and was rated as "A”. Also, if the field of view in which scale was observed was 5% or less, it was evaluated as "B”. Anything over 5% is considered questionable and rated a "C.”
  • the P ratio represents the ratio of phosphatite to phosphophyllite in the film obtained by phosphatization conversion treatment, and the higher the P ratio, the more phosphophyllite is contained, meaning that phosphophyllite crystals are densely formed on the steel sheet surface. Therefore, when the P ratio is 0.80 or more and the above scale is evaluated as A or B, the chemical conversion treatability is evaluated as excellent.
  • the corrosion resistance after painting was evaluated as follows. Specifically, first, a 25 ⁇ m thick electrocoating was applied to the steel plate after chemical conversion treatment, then a paint baking treatment was performed at 170° C. for 20 minutes, and a 130 mm long cut was made in the electrocoating film with a sharp knife until it reached the base steel (base material). This steel plate was continuously subjected to 5% salt spray at a temperature of 35° C. for 700 hours under the salt spray conditions shown in JIS Z 2371:2015.
  • a 24 mm wide tape (Nichiban 405A-24 JIS Z 1522:2009) was attached to the cut part in a length of 130 mm parallel to the cut part, and the maximum paint peeling width was measured when the tape was peeled off. When the maximum paint peeling width was 4.0 mm or less, it was evaluated as having excellent corrosion resistance after painting.
  • the hot-rolled steel sheet was evaluated as having high strength, but also improved stretch flangeability, ductility, and notch fatigue properties, and excellent corrosion resistance after painting. The results are shown in Tables 4 and 5.
  • Comparative Example 4 the heating temperature of the slab was low, so that the removal of scale by high-pressure water descaling was non-uniform, and the generation of Si scale patterns could not be sufficiently suppressed. As a result, the difference between the maximum and minimum values of the surface roughness Ra exceeded 0.50 ⁇ m, and the chemical conversion treatability and corrosion resistance after painting were reduced. In Comparative Example 5, the maximum heating temperature from the completion of rough rolling to the start of high-pressure water descaling before finish rolling was low, so that the scale could not be sufficiently removed even by the subsequent high-pressure water descaling.
  • the surface roughness Ra was 1.50 ⁇ m or more, and the difference between the maximum and minimum values of the surface roughness Ra exceeded 0.50 ⁇ m, and the chemical conversion treatability and corrosion resistance after painting were reduced.
  • the accumulated strain ( ⁇ eff.) in the last three stages of finish rolling was high, so it is believed that austenite recrystallization occurred during hot rolling, and the accumulated dislocation density during transformation decreased.
  • the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and the stretch flangeability was reduced.
  • Comparative Example 10 it is believed that transformation due to paraequilibrium occurred at a relatively high temperature because the cooling stop temperature of the primary cooling was high. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and the stretch flangeability was reduced. In Comparative Example 11, the cooling stop temperature of the primary cooling was low, so it is believed that para-equilibrium transformation occurred at a temperature lower than the desired temperature range. As a result, the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and the stretch flangeability was reduced.
  • Comparative Example 12 the holding time at 650 to 750°C in the primary cooling was short, so the proportion of crystal grains with an intragranular misorientation of 5 to 14° was less than 10%, and the stretch flangeability was reduced.
  • Comparative Example 13 the cooling stop temperature of the secondary cooling in the cooling process was high, so the area ratio of martensite was less than 2%. As a result, the TS and notch fatigue properties were reduced.
  • Comparative Examples 30 and 32 the C and Mn contents were high, respectively, and therefore the stretch flangeability was reduced.
  • Comparative Examples 31 and 33 the C and Mn contents were low, respectively, and therefore sufficient strength could not be obtained.
  • Comparative Example 34 the sol. Al content was high, and therefore cracks occurred during rolling, and subsequent testing could not be performed.
  • Comparative Example 35 the total content of Si and sol. Al was high, and therefore ferrite formation was promoted, and the area ratio of martensite was less than 2%. As a result, TS was reduced.
  • Comparative Example 36 the Ti content was high, and therefore carbides (TiC) became coarse, and ductility was reduced.
  • Comparative Example 37 the Ti content was low, and therefore it is believed that the formation of cementite could not be sufficiently suppressed, and as a result, the stretch flangeability was reduced.
  • the area ratio of the residual structure was 10% or less, and if a residual structure existed, the residual structure was at least one of retained austenite and pearlite.

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  • Heat Treatment Of Sheet Steel (AREA)

Abstract

L'invention propose une feuille d'acier laminée à chaud qui : comprend une composition chimique prédéterminée ; contient 80 à 98 % d'au moins un élément parmi la ferrite et la bainite au total et 2 à 10 % de martensite en termes de pourcentage surfacique ; comprend une structure métallique dans laquelle, lorsqu'une limite présentant une différence d'azimut de 15° ou plus est définie comme un joint de grain et une région entourée par le joint de grain et présentant un diamètre équivalent de cercle de 0,3 µm ou plus est définie comme grain cristallin, la proportion du grain cristallin présentant une différence d'azimut dans le grain de 5 à 14° est de 10 à 60 % en termes de pourcentage surfacique ; et comprend une rugosité de la surface Ra inférieure à 1,50 µm, une différence entre la valeur maximale et la valeur minimale de la rugosité de la surface Ra étant de 0,50 µm ou moins.
PCT/JP2023/043668 2022-12-23 2023-12-06 Feuille d'acier laminée à chaud WO2024135365A1 (fr)

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Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009293066A (ja) * 2008-06-03 2009-12-17 Jfe Steel Corp 成形性と耐疲労特性に優れた高張力鋼材およびその製造方法
WO2011062151A1 (fr) * 2009-11-18 2011-05-26 新日本製鐵株式会社 Tôle d'acier à haute résistance mécanique laminée à chaud, dotée d'excellentes qualités de décapage aux acides, d'aptitude au traitement de conversion chimique, de tenue à la fatigue, de bordage par étirage, de résistance à la détérioration superficielle pendant le moulage, et de résistance et de ductilité isotropes, et procédé de production de ladite tôle d'acier à haute résistance mécanique laminée à chaud
WO2015093596A1 (fr) * 2013-12-19 2015-06-25 日新製鋼株式会社 Tôle d'acier revêtue par immersion à chaud par un système à base de zn-al-mg ayant une excellente aptitude au façonnage et son procédé de fabrication
WO2016132542A1 (fr) * 2015-02-20 2016-08-25 新日鐵住金株式会社 Tôle d'acier laminée à chaud
WO2016133222A1 (fr) * 2015-02-20 2016-08-25 新日鐵住金株式会社 Tôle d'acier laminée à chaud
WO2016136672A1 (fr) * 2015-02-25 2016-09-01 新日鐵住金株式会社 Feuille ou tôle d'acier laminé à chaud
WO2021006296A1 (fr) * 2019-07-10 2021-01-14 日本製鉄株式会社 Tôle en acier hautement résistante

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009293066A (ja) * 2008-06-03 2009-12-17 Jfe Steel Corp 成形性と耐疲労特性に優れた高張力鋼材およびその製造方法
WO2011062151A1 (fr) * 2009-11-18 2011-05-26 新日本製鐵株式会社 Tôle d'acier à haute résistance mécanique laminée à chaud, dotée d'excellentes qualités de décapage aux acides, d'aptitude au traitement de conversion chimique, de tenue à la fatigue, de bordage par étirage, de résistance à la détérioration superficielle pendant le moulage, et de résistance et de ductilité isotropes, et procédé de production de ladite tôle d'acier à haute résistance mécanique laminée à chaud
WO2015093596A1 (fr) * 2013-12-19 2015-06-25 日新製鋼株式会社 Tôle d'acier revêtue par immersion à chaud par un système à base de zn-al-mg ayant une excellente aptitude au façonnage et son procédé de fabrication
WO2016132542A1 (fr) * 2015-02-20 2016-08-25 新日鐵住金株式会社 Tôle d'acier laminée à chaud
WO2016133222A1 (fr) * 2015-02-20 2016-08-25 新日鐵住金株式会社 Tôle d'acier laminée à chaud
WO2016136672A1 (fr) * 2015-02-25 2016-09-01 新日鐵住金株式会社 Feuille ou tôle d'acier laminé à chaud
WO2021006296A1 (fr) * 2019-07-10 2021-01-14 日本製鉄株式会社 Tôle en acier hautement résistante

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