WO2024161785A1 - 熱延鋼材及びその製造方法 - Google Patents

熱延鋼材及びその製造方法 Download PDF

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WO2024161785A1
WO2024161785A1 PCT/JP2023/043690 JP2023043690W WO2024161785A1 WO 2024161785 A1 WO2024161785 A1 WO 2024161785A1 JP 2023043690 W JP2023043690 W JP 2023043690W WO 2024161785 A1 WO2024161785 A1 WO 2024161785A1
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steel material
steel
hot
rolled steel
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French (fr)
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勇希 木村
恭寛 建山
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JFE Steel Corp
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JFE Steel Corp
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Priority to JP2024519955A priority Critical patent/JP7768362B2/ja
Priority to KR1020257023089A priority patent/KR20250123845A/ko
Priority to CN202380091457.5A priority patent/CN120530215A/zh
Publication of WO2024161785A1 publication Critical patent/WO2024161785A1/ja
Priority to MX2025008863A priority patent/MX2025008863A/es
Anticipated expiration legal-status Critical
Priority to JP2025140566A priority patent/JP2025169427A/ja
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • 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
    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to hot-rolled steel and its manufacturing method.
  • Parts used in automobiles and other applications are made by first applying preliminary processing such as wire drawing to the hot-rolled steel material, then forming the part shape using hot or cold processing, and then undergoing cutting or heat treatment as necessary to become the final product.
  • preliminary processing such as wire drawing
  • cold or cold processing for example, cold forging
  • Parts manufactured using cold processing become the final product after being left as cold processed or undergoing heat treatment for strength adjustment (quenching and tempering heat treatment, high-frequency quenching and tempering heat treatment, etc.) depending on the required strength.
  • the surface layer of the hot-rolled steel subjected to the above cold processing has a decarburized layer formed during hot rolling. If the decarburized layer of the hot-rolled steel is excessively thick, a soft part with a low carbon content remains near the surface layer of the steel in the parts after cold processing or in the parts that have been subjected to cold processing and high-frequency quenching and tempering heat treatment, and the strength or fatigue properties of the parts are inferior.
  • the decarburized layer of the hot-rolled steel is excessively thick, in the parts that have been subjected to cold processing and quenching and tempering heat treatment, carbon diffuses from the core of the steel to the decarburized layer during heating and holding during quenching, so the soft part as described above does not remain in the surface layer of the steel.
  • the austenite reverse transformation temperature is different.
  • the timing of austenite reverse transformation is different between the surface layer and the core of the steel, and the reverse transformed austenite grain size is nonuniform between the surface layer and the core of the steel. Such uneven grain size induces grain coarsening, leading to reduced fatigue properties and toughness of the parts.
  • Patent Document 1 describes how adding specific amounts of Te, Se, or S as trace elements and appropriately controlling the thermal history after hot rolling makes it possible to both soften the hot-rolled steel and suppress the decarburization reaction.
  • Patent Document 1 uses Te and Se, which are rare and toxic elements that require special care in handling, and it is difficult to say that this technology is generally applicable.
  • both Patent Document 1 and the technology prior to it require precise control of the temperature history after hot rolling, and there remain issues from the standpoint of productivity. As such, no technology has been reported that combines suppression of the thickness of the decarburized layer with excellent cold workability.
  • the present invention aims to provide a hot-rolled steel material in which the thickness of the decarburized layer is sufficiently suppressed and which has excellent cold workability, and a manufacturing method thereof.
  • the inventors focused on the suppression of decarburization reactions by adding Cu and Ni, which are elements that are less easily oxidized than Fe, and obtained the following findings.
  • Cu and Ni are elements that are less easily oxidized than Fe.
  • Fe is preferentially oxidized to form scale.
  • Cu and Ni are not oxidized and are left behind on the surface layer of the steel material, forming concentrated regions in which at least one of Cu and Ni is concentrated. By forming such concentrated regions, it is possible to effectively suppress the decarburization reactions that occur on the surface layer of the steel material.
  • the inventors have found that in order to make the concentrated regions of the hot-rolled steel material, in which at least one of Cu and Ni is concentrated, suitable for suppressing the decarburization reaction, it is necessary to appropriately control the maximum heating temperature in hot rolling. Also, in order to effectively suppress the decarburization reaction, it is necessary to appropriately control the residence time of the steel material in the heating furnace in hot rolling.
  • the inventors have obtained the following findings: In hot-rolled steel to which Sn has been added in addition to Cu and Ni, Sn is concentrated in the concentrated region in the surface layer of the steel where at least one of Cu and Ni is concentrated. Since Sn has the effect of lowering the melting point of the concentrated region, the concentrated region is more likely to form in the surface layer of the steel compared to when Sn is not added, and the concentrated region also has the effect of suppressing the decarburization reaction more easily.
  • the Sn concentration in an enriched region where at least one of Cu and Ni is enriched is excessively high, the enriched region will penetrate deeply into the grain boundaries, embrittling the grain boundaries and causing cracks during cold working. For this reason, when manufacturing hot-rolled steel material to which Sn has been added in addition to Cu and Ni, it is necessary to limit the Sn concentration in the enriched region to a specified range so that the grain boundary penetration depth does not become excessively deep.
  • Specific effective methods include adjusting the amount of Sn added depending on the amount of Cu and Ni, and shortening the residence time in the heating furnace during hot rolling depending on the amount of Sn.
  • the gist of the present invention is as follows:
  • the composition further comprises, in mass%, Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less; In the enriched region, Sn is enriched in addition to at least one of Cu and Ni,
  • Sn is enriched in addition to at least one of Cu and Ni.
  • the composition further comprises, in mass%, Cr: 0.01-1.50%, Mo: 0.01-0.50%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%
  • the component composition further comprises, in mass %, P: 0.001-0.100%, S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%
  • the hot-rolled steel material according to any one of the above [1] to [3], containing at least one element selected from the group consisting of:
  • composition further comprises, in mass%, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%
  • Pb 0.01 to 0.50%
  • Bi 0.01 to 0.100%
  • Ca 0.0005 to 0.1000%
  • T is the maximum heating temperature (°C)
  • [Ni] is the amount of Ni in the steel material (mass%)
  • [Cu] is the amount of Cu in the steel material (mass%)
  • [Sn] is the amount of Sn in the steel material (mass%).
  • the composition further comprises, in mass %, The method for producing a hot-rolled steel material according to the above [6], containing Sn: 0.001% or more ([Ni] + [Cu]) / 2 or less.
  • the composition further comprises, in mass %, Cr: 0.01-1.50%, Mo: 0.01-0.50%, Al: 0.001-0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, and B: 0.0005 to 0.0050%
  • composition further comprises, in mass%, P: 0.001-0.100%, S: 0.001 to 0.100%, and Sb: 0.0010 to 0.0300%
  • composition further comprises, in mass%, Pb: 0.01 to 0.50%, Bi: 0.001 to 0.100%, and Ca: 0.0005 to 0.1000%
  • Pb 0.01 to 0.50%
  • Bi 0.01 to 0.100%
  • Ca 0.0005 to 0.1000%
  • the present invention provides a hot-rolled steel material and its manufacturing method that has a sufficiently suppressed thickness of the decarburized layer and excellent cold workability.
  • FIG. 1 is a schematic diagram showing a cross section of a hot-rolled steel material according to an embodiment of the present invention.
  • the hot-rolled steel material 100 has a base steel 10 and a decarburized layer 20 formed on the surface of the base steel 10.
  • the hot-rolled steel material 100 may also have a scale 30 on the decarburized layer 20.
  • the decarburized layer 20 is further characterized in that an enriched region 22 in which at least one of Cu and Ni is enriched is present, the coverage of the enriched region 22 on the surface 24 of the decarburized layer 20 is 50% or more, the maximum depth A of the enriched region 22 in the decarburized layer 20 is 1 ⁇ m or more and 150 ⁇ m or less, the total decarburization depth B (DM-T) of the decarburized layer 20 as defined by JIS G 0558 is 0.80 mm or less, the total area ratio of ferrite and pearlite in the base steel 10 is 90.0% or more, and the average Vickers hardness of the base steel 10 is 250 HV or less.
  • DM-T total decarburization depth B
  • the base steel of the hot-rolled steel material contains, in mass%, C: 0.03 to 0.80%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, Cu: 0.010 to 0.500%, Ni: 0.010 to 1.000%, and N: 0.0020 to 0.0250%, with the ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], being 0.10 to 3.00, and the balance being Fe and unavoidable impurities.
  • “%” representing the content means “% by mass” unless otherwise specified.
  • [Ni] and [Cu] represent the amount of Ni and the amount of Cu contained in the base steel, respectively.
  • C is an element that is added to ensure the strength of hot-rolled steel. If the C content of the base steel is less than 0.03%, the necessary strength cannot be ensured. On the other hand, if the C content of the base steel exceeds 0.80%, the hardenability becomes too high, and the base steel exhibits a microstructure containing hard bainite or martensite. Therefore, the C content of the base steel is set to 0.80% or less, preferably 0.65% or less, and more preferably 0.50% or less.
  • Silicon is a deoxidizing element during refining and also an element that improves the strength and hardenability of hot-rolled steel. If the silicon content of the base steel is less than 0.01%, the above effects cannot be obtained. Therefore, the silicon content of the base steel is set to 0.01% or more. On the other hand, if the silicon content of the base steel exceeds 1.00%, the hardenability becomes too high, and therefore it is necessary to form a structure containing hard bainite or martensite. As a result, the hardness of the rolled material increases and the cold workability decreases. Therefore, the Si content of the base steel is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.
  • Mn is an element that improves the strength and hardenability of hot-rolled steel. If the Mn content of the base steel is less than 0.01%, the above effects cannot be obtained. On the other hand, if the Mn content of the base steel exceeds 1.50%, the hardenability becomes too high, resulting in a structure containing hard bainite or martensite, and the hardness of the rolled material increases. The cold workability deteriorates. Therefore, the Mn content of the base steel is set to 1.50% or less, preferably 1.20% or less, and more preferably 1.00% or less.
  • Cu is an element that is less susceptible to oxidation than Fe, and is an element that suppresses the decarburization reaction by concentrating in the surface layer of the steel material along with the formation of scale during hot rolling, forming a concentrated region. If the Cu content is less than 0.010%, the concentrated region is not sufficiently formed in the surface layer of the steel material, and the decarburization suppression effect is not sufficiently obtained. Therefore, the Cu content of the base steel is set to 0.010% or more. On the other hand, if the Cu content of the base steel exceeds 0.500%, the concentrated region in the surface layer of the steel becomes excessively deep, making it more likely to crack during cold working. The content is set to 0.400% or less, preferably 0.400% or less, and more preferably 0.350% or less.
  • Ni is an element that is less susceptible to oxidation than Fe.
  • Ni is an element that inhibits decarburization by concentrating on the surface layer of the steel material as scale is generated during hot rolling and forming a concentrated region. If the Ni content of the base steel is less than 0.010%, the concentrated region is not sufficiently formed in the surface layer of the steel material, and the decarburization suppression effect is not sufficiently obtained. On the other hand, if the Ni content of the base steel exceeds 1.000%, the enriched region in the surface layer of the steel becomes excessively deep, making it more susceptible to cracking during cold working. is set to 1.000% or less, preferably to 0.800% or less, and more preferably to 0.600% or less.
  • N is an element that combines with nitride-forming elements in the steel to form nitrides and acts as grain boundary pinning particles, thereby preventing grain coarsening. If the N content is less than 0.0020%, the above-mentioned effect cannot be obtained. Therefore, the N content of the base steel is set to 0.0020% or more. On the other hand, if the N content of the base steel exceeds 0.0250%, Not only does this cause blowholes to form in the steel, but the solute N in the steel also causes dynamic strain aging, making it more susceptible to cracks during cold working. Therefore, the N content of the base steel is set to 0.0250% or less, and 0.0150% or less. It is preferably 0.0200% or less, and more preferably 0.0180% or less.
  • the ratio (mass ratio) of the amount of Ni to the amount of Cu, [Ni]/[Cu], is 0.10 or more and 3.00 or less]
  • the ratio (mass ratio) of the Ni amount to the Cu amount [Ni]/[Cu] is less than 0.10, that is, when the Cu amount is excessive relative to the Ni amount, the Cu enriched in the steel surface layer penetrates into the grain boundary, and the enriched region becomes excessively deep, and cracks are likely to occur during cold working.
  • [Ni]/[Cu] is 0.10 or more, preferably 0.15 or more, and more preferably 0.20 or more.
  • [Ni]/[Cu] exceeds 3.00, that is, when the Ni amount is excessive relative to the Cu amount, the enriched region in the steel surface layer becomes excessively deep, and cracks are likely to occur during cold working. Therefore, [Ni]/[Cu] is set to 3.00 or less, preferably 2.50 or less, and more preferably 2.00 or less.
  • the base steel of the hot-rolled steel material according to one embodiment of the present invention may further contain the following elements as necessary.
  • Cr 0.01 to 1.50%
  • Cr is an element that improves the hardenability of steel. If the Cr content of the base steel is less than 0.01%, the above effect cannot be obtained. Therefore, when Cr is added to the base steel, the Cr content should be On the other hand, if the Cr content of the base steel exceeds 1.50%, the effect of the addition will saturate, and the hardenability of the steel will become excessive, increasing the hardness of the steel and decreasing the cooling time. Therefore, when Cr is added to the base steel, the Cr amount is set to 1.50% or less, preferably 1.30% or less, and more preferably 1.15% or less.
  • Mo is an element that greatly improves the hardenability of steel with a small amount of addition. If the Mo content of the base steel is less than 0.01%, the above effect cannot be obtained. Therefore, adding Mo to the base steel On the other hand, if the Mo content of the base steel exceeds 0.50%, the effect of the addition is saturated, and the hardenability of the steel becomes excessive, resulting in a decrease in the hardness of the steel. Therefore, when Mo is added to the base steel, the Mo amount should be 0.50% or less, and preferably 0.30% or less.
  • Al is a deoxidizing element and also combines with N in the steel to form nitrides, contributing to the refinement of crystal grains.
  • the Al content of the base steel is less than 0.001%, Therefore, when Al is added to the base steel, the Al content is set to 0.001% or more.
  • the Al content of the base steel exceeds 0.100%, the Al content of the steel is increased.
  • the amount of Al oxide increases, which makes the steel more susceptible to cracking during cold working, and also deteriorates the fatigue fracture properties of the steel as a part. Therefore, when adding Al to the base steel, the amount of Al should be 0.100% or less.
  • the content is preferably 0.080% or less, and more preferably 0.050% or less.
  • Ti 0.001 to 0.100%
  • Ti is an element that combines with N in steel to form nitrides and contributes to the refinement of crystal grains. If the Ti content of the base steel is less than 0.001%, the above effect is not achieved. Therefore, when Ti is added to the base steel, the Ti content is set to 0.001% or more. On the other hand, when the Ti content of the base steel exceeds 0.100%, the amount of Ti-based precipitates in the steel is increased. If Ti becomes excessive, the hardness of the steel increases and the cold workability decreases. Therefore, when Ti is added to the base steel, the Ti content should be 0.100% or less, and preferably 0.080% or less. It is more preferable that the content is 0.050% or less.
  • V like Al and Ti, is an element that combines with N in steel to form nitrides and contributes to the refinement of crystal grains. Furthermore, V is an element that contributes to the increase in the strength of steel. If the V content of the iron is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, when V is added to the base steel, the V content should be 0.001% or more. If the amount exceeds 0.300%, the amount of V-based precipitates in the steel becomes excessive, which increases the hardness of the steel material, lowers the cold workability, and also lowers the toughness of the steel material. When V is added to iron, the V content is set to 0.300% or less, preferably 0.200% or less, and more preferably 0.150% or less.
  • Nb is an element that combines with carbon in steel to form carbides and contributes to the refinement of crystal grains. If the Nb content in the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when Nb is added to the base steel, the Nb content should be 0.001% or more. On the other hand, if the Nb content of the base steel exceeds 0.100%, the amount of Nb-based carbides in the steel becomes excessive. Therefore, when Nb is added to the base steel, the Nb content should be 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less. The following is more preferred:
  • B is an element that greatly improves the hardenability of steel with a small amount of addition. If the B content of the base steel is less than 0.0005%, the above effect cannot be obtained. Therefore, adding B to the base steel In this case, the B content is set to 0.0005% or more. On the other hand, if the B content of the base steel exceeds 0.0050%, the effect of improving hardenability becomes saturated. Therefore, when B is added to the base steel, The B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.
  • P is an element that increases the strength of steel. If the P content of the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when P is added to the base steel, the P content should be On the other hand, if the P content of the base steel exceeds 0.100%, P segregates at grain boundaries and reduces the toughness of the steel. Therefore, when P is added to the base steel, The P content is set to 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.
  • S is an element that combines with Mn in steel to form MnS inclusions, improving the machinability of steel. If the S content of the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, if S is added to the base steel, the S content must be 0.001% or more. On the other hand, if the S content of the base steel exceeds 0.100%, the large amount of MnS inclusions It acts as a crack initiation point during cold working, making it easier for cracks to occur during cold working. Therefore, if S is added to the base steel, the S content should be 0.100% or less, and 0.070% or less is recommended. It is preferable that the content of C is 0.050% or less, and more preferable that the content of C is 0.050% or less.
  • Sb 0.0010-0.0300%
  • Sb is an element that easily segregates in the surface layer of the steel material, and has the effect of suppressing the decarburization reaction in the same manner as in the concentrated region where at least one of Cu and Ni is concentrated. If the Sb content of the base steel is less than 0.0010%, the above-mentioned effects cannot be obtained. Therefore, when Sb is added to the base steel, the Sb content should be 0.0010% or more. If the Sb content exceeds 0.0300%, the amount of Sb segregating in the surface layer becomes excessive, which deteriorates the surface properties of the steel material. Therefore, when Sb is added to the base steel, the Sb content should be 0.0300% or less, and 0.0200% or less. It is preferable that the content is 0.0150% or less, and more preferable that the content is 0.0150% or less.
  • Pb 0.01 to 0.50%
  • Pb is an element that improves the machinability of steel. If the Pb content of the base steel is less than 0.01%, the above effect cannot be obtained. Therefore, when Pb is added to the base steel, the Pb content should be On the other hand, if the Pb content of the base steel exceeds 0.50%, the effect of improving machinability saturates, and the amount of inclusions in the steel increases, causing a decrease in toughness. Therefore, when Pb is added to the base steel, the Pb content should be 0.50% or less, and preferably 0.35% or less.
  • Bi is an element that improves the machinability of steel. If the Bi content in the base steel is less than 0.001%, the above effect cannot be obtained. Therefore, when Bi is added to the base steel, On the other hand, if the Bi content of the base steel exceeds 0.100%, the effect of improving machinability saturates, and the amount of inclusions in the steel increases, decreasing toughness. Therefore, when Bi is added to the base steel, the Bi content is set to 0.100% or less, and preferably 0.050% or less.
  • Ca is an element that has the effect of dissolving in the sulfides in steel and spheroidizing the sulfides, and suppresses deterioration of the cold workability and toughness of the steel material. %, the above-mentioned effects cannot be obtained. Therefore, when Ca is added to the base steel, the Ca content is set to 0.0005% or more. On the other hand, when the Ca content of the base steel exceeds 0.1000%, In this case, the amount of Ca-based inclusions in the steel increases, adversely affecting the toughness and fatigue properties of the steel material. Therefore, when Ca is added to the base steel, the Ca amount should be 0.1000% or less, and 0.0500% or less. It is preferable that the content is 0.0300% or less, and more preferable that the content is 0.0300% or less.
  • Total area ratio of ferrite and pearlite is 90.0% or more
  • the structure of the base steel needs to be a mixed structure of relatively soft ferrite and pearlite. Therefore, the total area ratio of ferrite and pearlite in the base steel is set to 90.0% or more, and preferably 95.0% or more.
  • the total area ratio may be 100.0%.
  • the total area ratio of ferrite and pearlite in the present invention can be determined by the following procedure. Three samples including the steel surface layer are prepared by cutting the steel material, and the structure of each sample is observed on the cross section perpendicular to the rolling direction. The microstructure of the surface structure of the sample, excluding the decarburized layer described below, is observed at a magnification of 100 times using an optical microscope. The area of one field of view is set to 600 ⁇ m x 800 ⁇ m, and the total area ratio of ferrite and pearlite in the observed area of each field of view is calculated for five randomly selected fields of view. The average value of the total area ratios obtained in each field of view for each sample is calculated, and this is regarded as the total area ratio of ferrite and pearlite for that steel material. ImageJ, an image analysis software, can be used to calculate the area ratio.
  • the average Vickers hardness of the base steel is set to 250 HV or less, preferably 230 HV or less, and more preferably 220 HV or less.
  • the average Vickers hardness of the base steel is preferably 80 HV or more.
  • the average Vickers hardness in the present invention can be determined by the following procedure. Three samples are prepared by cutting the steel material, and a Vickers hardness tester is used to measure the hardness of the cross section perpendicular to the rolling direction of the sample. If the sample is round steel material, the measurement points are five in total: one point at a position equivalent to 1/2 the diameter of the round steel material from the surface toward the center of the steel material (hereinafter referred to as the "D/2 position”), and four points at positions equivalent to 1/4 the diameter of the round steel material from the surface toward the center of the steel material (hereinafter referred to as the "D/4 positions").
  • the measurement points are five in total: one at a position equivalent to 1/2 of the plate width from one end toward the center and equivalent to 1/2 of the plate thickness in the depth direction from the steel surface (hereinafter referred to as the "W/2-t/2 position"), and four at positions equivalent to 1/4 of the plate width from one end toward the center and equivalent to 1/4 of the plate thickness in the depth direction from the steel surface (hereinafter referred to as the "W/4-t/4 positions").
  • the average value of the hardness obtained at each measurement point of each sample is calculated to be the average Vickers hardness of the steel material. Note that hardness can be measured with a load of 10 kgf.
  • the total decarburization depth (DM-T) of the decarburized layer as specified in JIS G 0558 is 0.80 mm or less. If the decarburized layer is excessively thick, it will lead to a decrease in fatigue properties and toughness as a part, so it is necessary to prevent the decarburized layer from becoming excessively thick.
  • the thickness of the decarburized layer is evaluated by the total decarburization depth (DM-T) (hereinafter simply referred to as "total decarburization depth”) specified in JIS G 0558. If the total decarburization depth of the decarburized layer exceeds 0.80 mm, the deterioration of properties becomes significant.
  • the total decarburization depth of the decarburized layer is set to 0.80 mm or less, preferably 0.50 mm or less, and more preferably 0.30 mm or less.
  • the lower limit of the total decarburization depth of the decarburized layer is not particularly limited, and the total decarburization depth of the decarburized layer may be 0.00 mm.
  • the total decarburization depth of the decarburization layer in the present invention can be found by the following procedure. Three samples including the steel surface layer are prepared by cutting the steel material, and a structural observation is carried out on the cross section perpendicular to the rolling direction of the sample. The surface structure of the sample is observed using an optical microscope at a magnification of 100 times, with an area of one field of view being 600 ⁇ m x 800 ⁇ m. The total decarburization depth of the surface layer of the sample is measured in three randomly selected fields of view using the method described in JIS G 0558, and the total decarburization depth at the deepest position of the decarburization layer is regarded as the total decarburization depth in that field of view. The average value of the total decarburization depth obtained in each field of view of each sample is calculated, and this is regarded as the total decarburization depth of the decarburization layer of that steel material.
  • the decarburized layer has a concentrated region in which at least one of Cu and Ni is concentrated]
  • Sn is preferably concentrated in the concentrated region.
  • the concentrated region is defined as a region in the surface layer of the hot-rolled steel where the concentration of at least one of Cu and Ni is three times or more the reference concentration.
  • the reference concentration is the concentration of Cu and Ni at the D/4 position of the hot-rolled steel when the hot-rolled steel is a round steel, and at the W/4-t/4 position of the hot-rolled steel when the hot-rolled steel is a plate.
  • [Coverage rate of concentrated area on the surface of decarburized layer is 50% or more]
  • a concentrated region in which at least one of Cu and Ni is concentrated is formed on the surface of a hot-rolled steel material, it contributes to suppressing the decarburization reaction. If the coverage of the concentrated region on the surface of the decarburized layer is less than 50%, the decarburization suppression effect becomes insufficient. Therefore, the coverage of the concentrated region on the surface of the decarburized layer is set to 50% or more, preferably 60% or more, and more preferably 70% or more. On the other hand, the upper limit of the coverage is not particularly limited, and the coverage of the concentrated region may be 100%.
  • the coverage rate of the enriched area on the surface of the decarburized layer can be determined by the following procedure. First, the steel is cut to prepare three samples including the steel surface layer, and the reference concentration of Cu and Ni is measured for each sample. In order to eliminate the effects of the enriched area and central segregation in the steel surface layer, three randomly selected points at the above-mentioned positions (D/4 position of hot-rolled steel for round steel, and W/4-t/4 position of hot-rolled steel for plate steel) are analyzed with an EPMA (electron probe microanalyzer), and the average values of the Cu and Ni concentrations at these three points are taken as the reference concentrations of Cu and Ni, respectively.
  • the measurement conditions for EPMA can be a magnification of 100x and an area per field of view of 600 ⁇ m x 800 ⁇ m.
  • the measurement conditions for EPMA can be a magnification of 100x and an area of 600 ⁇ m x 800 ⁇ m per visual field. Regions where at least one of Cu and Ni is present at a concentration three times or more higher than the reference concentrations of Cu and Ni determined above are defined as concentrated regions, and the coverage of the concentrated regions in each visual field is determined. The average of the coverage obtained in each visual field of each sample is calculated and this is regarded as the coverage of the concentrated regions of that steel.
  • Maximum depth of concentrated region in decarburized layer is 1 ⁇ m or more and 150 ⁇ m or less. If the maximum depth of the concentrated region in the decarburized layer is less than 1 ⁇ m, the effect of suppressing the decarburization reaction cannot be obtained. Therefore, the maximum depth of the concentrated region in the decarburized layer is set to 1 ⁇ m or more. On the other hand, if the maximum depth of the concentrated region in the decarburized layer exceeds 150 ⁇ m, it is preferable from the viewpoint of suppressing the decarburization reaction, but the concentrated region is too deep and is likely to crack during cold working. Therefore, the maximum depth of the concentrated region in the decarburized layer is set to 150 ⁇ m or less, preferably 120 ⁇ m or less, and more preferably 100 ⁇ m or less.
  • the maximum depth of the enriched region in the decarburized layer can be found by the following method.
  • Three samples including the steel surface layer are prepared by cutting the steel material, and the enriched region is observed in three fields of view (each field has an area of approximately 600 ⁇ m x 800 ⁇ m) in a cross section perpendicular to the rolling direction of the sample using the method described above, and the maximum depth of each enriched region is measured.
  • the average value of the maximum depths obtained in each field of view of each sample is calculated, and this is regarded as the maximum depth of the enriched region in the decarburized layer of that steel material.
  • the minimum depth of the enriched region can also be found by observing in a similar manner.
  • Sn may be further concentrated in the concentrated area where at least one of Cu and Ni is concentrated. Sn is also considered to be concentrated when its concentration is three times or more the standard concentration.
  • the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration, [Sn]/([Cu]+[Ni]), is 0.50 or less in atomic ratio (preferred range)]
  • Sn has the effect of lowering the melting point of the enriched region where at least one of Cu and Ni is enriched, but if the Sn concentration in the enriched region is excessively high, the grain boundary penetration depth of the enriched region becomes deep, which causes cold working cracks. Therefore, the ratio of the Sn concentration to the sum of the Cu concentration and the Ni concentration in the enriched region [Sn]/([Cu]+[Ni]) is set to 0.50 or less in atomic ratio, preferably 0.40 or less, more preferably 0.30 or less.
  • the lower limit of the atomic ratio [Sn]/([Cu]+[Ni]) is not particularly limited, and [Sn]/([Cu]+[Ni]) may be 0.00.
  • the ratio of the Sn concentration to the sum of the Cu concentration and Ni concentration in the enriched region, [Sn]/([Cu]+[Ni]), can be determined by the following procedure. Three samples including the steel surface are prepared by cutting the steel material, and quantitative analysis (mapping) is performed using EPMA for three fields of view on the cross section perpendicular to the rolling direction of the sample to determine the concentrations of Cu, Ni, and Sn.
  • the measurement conditions for EPMA can be a magnification of 100 times and an area of 600 ⁇ m x 800 ⁇ m per field.
  • the enriched region is determined by the above-mentioned method, and the atomic ratio of [Sn]/([Cu]+[Ni]) is calculated from the measured concentration at the point with the highest Sn concentration in the enriched region.
  • the average value of the atomic ratios obtained in each field of view of each sample is calculated to be the [Sn]/([Cu]+[Ni]) in the enriched region of that steel material.
  • the method for producing hot-rolled steel material includes a step of hot-rolling a steel material having the above-mentioned composition under conditions in which the maximum heating temperature T in a heating furnace is 1000° C. or more and 1200° C. or less, and the residence time of the steel material in the heating furnace is within a predetermined range, to obtain a hot-rolled steel material.
  • the steel material used in the method for producing a hot-rolled steel material has a composition containing C, Si, Mn, Cu, Ni, and N, with the balance being Fe and unavoidable impurities.
  • the steel material may contain the above-mentioned elements as necessary. The content of each element is as described above.
  • the steel material is hot-rolled to obtain a hot-rolled steel material.
  • the maximum heating temperature T in the heating furnace during hot rolling is less than 1000°C, the scale on the surface of the steel material is not easily generated and grows, and the formation of the concentrated region is not easily promoted. Therefore, the coverage of the concentrated region on the surface of the decarburized layer is small, and the decarburization reaction cannot be suppressed. Therefore, the maximum heating temperature T in the heating furnace is set to 1000°C or more, and preferably 1030°C or more.
  • the maximum heating temperature T exceeds 1200°C, the scale growth rate of the steel material surface layer is too fast, so the concentrated region generated on the steel material surface layer is discharged to the scale side (scale off).
  • the maximum heating temperature T in the heating furnace is set to 1200°C or less, and preferably 1150°C or less.
  • the longer the residence time of the steel material in the heating furnace the longer the time for which the decarburization reaction occurs in the heating furnace.
  • the inventors have discovered that the decarburized layer and the concentrated area can be made suitable for suppressing the decarburization reaction by setting the upper limit of the residence time of the steel material in the heating furnace to the time t 1 (minutes) determined by the following formula (1).
  • the residence time is preferably 30 minutes or more.
  • the residence time is the time the material stays in the heating furnace from when it is charged into the heating furnace until the heated material leaves the heating furnace.
  • t 1 1150-0.8T-3([Ni]/[Cu])-10[Sn]...(1)
  • T is the maximum heating temperature (°C)
  • [Ni] is the amount of Ni in the steel material (mass%)
  • [Cu] is the amount of Cu in the steel material (mass%)
  • [Sn] is the amount of Sn in the steel material (mass%).
  • [Sn] 0.
  • a 160 mm square billet material having the composition shown in Tables 1 and 2 was heated in a heating furnace under the hot rolling conditions shown in Tables 3 and 4, and then hot rolled to obtain a wire rod with a diameter of 15 mm.
  • Samples for structural observation and hardness measurement were taken from the obtained hot rolled wire rod.
  • the coverage of the enriched area where at least one of Cu and Ni was enriched, the maximum and minimum depth of the enriched area, the total decarburization depth (DM-T) of the decarburized layer, the total area ratio of ferrite and pearlite, and the average Vickers hardness were measured.
  • the results are shown in Tables 3 and 4.
  • the atomic ratio of [Sn]/([Cu] + [Ni]) was measured using the method described above, and the results are shown in Table 4.
  • the cold workability of the obtained hot-rolled wire was evaluated as follows. After the scale on the surface of the hot-rolled wire was completely removed by pickling, the wire was drawn to obtain a wire with a diameter of 14 mm, and then cut to a height of 21 mm to obtain a cylindrical test piece. The shape of this test piece conforms to the No. 1 test piece described in the literature "Cold Upsetting Test Method" (Plasticity and Processing, 22 (1981), 139.). The obtained test pieces were subjected to a cold compression test (under end face restraint conditions) in which the test pieces were compressed 60% in the height direction at a strain rate of 10/s.
  • Nos. 1, 2, and 3 are comparative examples in which the C, Si, and Mn contents exceeded the ranges set forth in the present invention.
  • These steel types contained an excess of either C, Si, or Mn, and had too high hardenability, resulting in a microstructure containing bainite or martensite, and the total area ratio of ferrite and pearlite in the base steel was below 90.0%.
  • the average Vickers hardness of the base steel exceeded 250 HV, which resulted in poor cold workability and cracks occurring after the cold compression test.
  • Nos. 4 and 6 are comparative examples in which the Cu and Ni contents were below the ranges set forth in the present invention. Because the Cu and Ni contents were low in these steel types, the coverage of the enriched region was below the ranges set forth in the present invention, and the total decarburization depth exceeded the ranges set forth in the present invention.
  • Nos. 5 and 7 are comparative examples in which the Cu and Ni contents exceeded the ranges set forth in the present invention. These steels contained excessive Cu or Ni contents, and the maximum depth of the enriched region exceeded the ranges set forth in the present invention, causing cracks to occur during the cold compression test.
  • No. 8 is a comparative example in which the amount of N exceeded the range of the present invention.
  • This steel type had a large amount of dissolved N in the steel, and the cold workability was poor due to the effects of dynamic strain aging, so cracks occurred during the cold compression test.
  • Nos. 9 and 10 are comparative examples in which the ratio of the amount of Ni to the amount of Cu, [Ni]/[Cu], is outside the range of the present invention.
  • the [Ni]/[Cu] ratio was not appropriate, so the maximum depth of the concentrated region exceeded the range of the present invention, and cracks occurred during the cold compression test.
  • No. 11 is a comparative example in which the maximum heating temperature T during hot rolling exceeded the range of the present invention.
  • the heating temperature of the steel material was too high, so the growth rate of the scale was fast, the coverage rate of the concentrated area was low, and the total decarburization depth exceeded the range of the present invention.
  • No. 12 is a comparative example in which the maximum heating temperature T during hot rolling was below the range of the present invention.
  • the heating temperature was too low, so the enriched region was not sufficiently formed on the steel surface, and the coverage rate of the enriched region was significantly below the range of the present invention, resulting in the total decarburization depth exceeding the range of the present invention.
  • No. 13 is a comparative example in which the residence time in the heating furnace during hot rolling exceeded the range of the present invention.
  • the coverage rate and maximum depth of the enriched region were within the range of the present invention, but the total decarburization depth exceeded the range of the present invention because the time for the decarburization reaction to occur was long.
  • No. 35 is a comparative example in which the amount of Sn in a steel containing Sn exceeded the upper limit of ([Cu] + [Ni]) / 2. Because the amount of Sn in this steel was excessive, the atomic ratio of [Sn] / ([Cu] + [Ni]) in the enriched region exceeded 0.5, and the maximum depth of the enriched region also exceeded the range of the present invention, causing cracks to occur during the cold compression test.
  • No. 36 is a comparative example in which the residence time in the heating furnace during hot rolling of a steel containing Sn exceeded the range of the present invention.
  • the atomic ratio of [Sn]/([Cu] + [Ni]) in the enriched region exceeded 0.5, and the maximum depth of the enriched region also exceeded the range of the present invention, causing cracks to occur during the cold compression test.
  • the steel composition and hot rolling conditions of Nos. 14 to 34 and Nos. 37 to 57 are within the range of the present invention, as shown in Tables 1 to 4.
  • the coverage and maximum depth of the enriched region, the total decarburization depth, and the total area ratio of ferrite and pearlite are within the range of the present invention.
  • [Sn]/([Cu]+[Ni]) in the enriched region is within the range of the present invention.
  • the present invention provides a hot-rolled steel material with excellent cold workability and a manufacturing method thereof, in which the thickness of the decarburized layer is sufficiently suppressed.
  • Hot-rolled steel material 10 Base steel 20 Decarburized layer 22 Enriched region 24 Surface of decarburized layer 30 Scale A Maximum depth of enriched region B Total decarburization depth of decarburized layer

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JP2010132943A (ja) * 2008-12-02 2010-06-17 Kobe Steel Ltd 伸線加工性およびメカニカルデスケーリング性に優れた熱間圧延線材およびその製造方法
JP2012072427A (ja) * 2010-09-28 2012-04-12 Kobe Steel Ltd 肌焼鋼およびその製造方法
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