Classifications

• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • 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
  • 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
  • C22C38/02—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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/60—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
  • C21D2211/002—Bainite
• • 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

Definitions

• the present invention relates to a steel material with excellent cold forgeability and a method for manufacturing the same.
• Cold forged parts used in automobiles and other applications are made by cold forging hot rolled steel into the desired shape after preliminary processing such as wire drawing. If necessary, cutting and heat treatment are then performed to produce the final product.
• Parts manufactured by cold forging are either left as cold worked or undergo heat treatment for strength adjustment (quenching and tempering heat treatment, induction hardening and tempering heat treatment, etc.) depending on the required strength, before becoming the final product.
• the machinability of the steel material is also an important characteristic.
• a common method for improving the machinability of steel materials is to utilize MnS inclusions. However, if the MnS inclusions formed during casting are too coarse, cracks are likely to occur during cold forging, originating from such MnS inclusions.
• Patent Document 1 discloses a steel wire material in which the size and number density of sulfides or sulfide-based composite compounds present in a longitudinal section parallel to the central axis of the steel material are specified.
• Patent Document 1 specifies steel wire rod that is cold forged after spheroidizing annealing, and does not take into consideration the omission of heat treatment for the purpose of adjusting strength. Furthermore, Patent Document 1 is poor in describing specific manufacturing methods for controlling the size and number density of sulfides or composite compounds mainly composed of sulfides to predetermined values.
• the present invention has been made in consideration of the above circumstances, and has an object to suppress the occurrence of cracks during cold forging, that is, to provide a steel material with excellent cold forgeability and a manufacturing method thereof. Furthermore, an object of the present invention is to provide a steel material having excellent cold forgeability, which is preferably useful as a non-tempered steel, and a method for producing the same.
• the present inventors focused on controlling the morphology of sulfide-based inclusions by adding Cu.
• pure MnS inclusions which are an example of sulfide-based inclusions, have high thermal stability and are therefore prone to crystallization as coarse MnS inclusions during the casting of steel materials.
• the coarse MnS inclusions thus generated during casting do not form a solid solution in the steel when the material is heated before hot rolling, and therefore remain in the steel material as coarse inclusions even after hot rolling, inducing cracks during cold forging.
• the inventors further investigated the composition of steel materials with excellent cold forgeability and the manufacturing conditions of the steel materials. As a result, they discovered that it is necessary to appropriately control the maximum heating temperature during hot rolling and the residence time of the material in the heating furnace while appropriately controlling the composition of the steel materials.
• the present invention is a steel material developed based on the above findings, and the gist of the present invention is as follows. 1.
• the composition of the alloy is, in mass%, C: 0.05-0.60%, Si: 0.01-1.00%, Mn: 0.01-1.50%, S: 0.001-0.100%, Cu: 0.010-1.000%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, and N: 0.0020-0.0250%, with the balance being Fe and unavoidable impurities;
• the area ratio of martensite is 10.0% or less, and the remainder has a structure containing at least bainite,
• the steel material described in 1 above further comprising, in mass%, one or more elements selected from the group consisting of Ni: 0.01-1.00%, Al: 0.001-0.100%, Ti: 0.001-0.100%, V: 0.001-0.300%, Nb: 0.001-0.100%, B: 0.0005-0.0050%, and Sb: 0.0010-0.0300%.
• the material has a composition, in mass%, of C: 0.05 to 0.60%, Si: 0.01 to 1.00%, Mn: 0.01 to 1.50%, S: 0.001 to 0.100%, Cu: 0.010 to 1.000%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, and N: 0.0020 to 0.0250%,
• the alloy further contains, in mass%, one or more elements selected from the group consisting of Ni: 0.01 to 1.00%, Al: 0.001 to 0.100%, Ti: 0.001 to 0.100%, V: 0.001 to 0.300%, Nb: 0.001 to 0.100%, B: 0.0005 to 0.0050%, and Sb: 0.0010 to 0.0300%, with the balance being Fe and unavoidable impurities; the maximum heating temperature of the material is 1000°C or higher and 1200°C or lower, and the retention time of the material in the heating furnace is t1 (unit: minute
• fine sulfide-based inclusions are present in an appropriate amount to ensure machinability, while the number density of coarse sulfide-based inclusions is reduced to suppress cracking during cold forging. Therefore, according to the present invention, it is possible to suppress cracking during cold forging while ensuring machinability. As a result, it is possible to provide a steel material with excellent cold forgeability, together with a manufacturing method thereof. Furthermore, according to the present invention, a steel material having excellent cold forgeability, which is also useful as a non-tempered steel, can be provided together with a manufacturing method thereof.
• the steel material of the present invention has a predetermined component composition and a predetermined structure, and the number density of relatively coarse sulfide-based inclusions and the number density of relatively fine sulfide-based inclusions are each within a predetermined range.
• the steel material of the present invention can improve cold forgeability without impairing machinability.
• the steel material of the present invention is still useful as a non-tempered steel while exhibiting good cold forgeability.
• the steel material of the present invention can be obtained, for example, according to the manufacturing method of the present invention.
• Component composition 0.05-0.60% C is added to ensure the strength and hardness of steel. If the C content is less than 0.05%, the necessary strength and hardness cannot be ensured. On the other hand, if the C content exceeds 0.60%, the hardenability becomes too high, and a microstructure containing hard martensite is exhibited. As a result, the hardness increases excessively and the cold forgeability decreases. Therefore, the C content is set to a range of 0.05% to 0.60%. The C content is preferably 0.10% or more. The C content is preferably 0.55% or less.
• Si 0.01 ⁇ 1.00% Si is a deoxidizing element during refining, and also improves the strength, hardness, and hardenability of steel. Addition of less than 0.01% Si does not provide these effects. On the other hand, addition of more than 1.00% Si results in too high hardenability, resulting in a structure containing hard martensite. As a result, hardness increases excessively and cold forgeability decreases. Therefore, the Si content is set to a range of 0.01% to 1.00%. The Si content is preferably 0.80% or less, more preferably 0.50% or less.
• Mn 0.01-1.50% Mn is an element that improves the strength, hardness, and hardenability of steel.
• Mn combines with S in steel to form MnS inclusions and/or (Mn, Cu)S inclusions, and also has the effect of improving the machinability of steel. Addition of less than 0.01% Mn does not provide such an effect.
• addition of more than 1.50% Mn results in too high hardenability, resulting in a structure containing hard martensite. As a result, hardness increases excessively and cold forgeability decreases. Furthermore, a large amount of coarse MnS inclusions precipitate during casting of the material, making it easier for cracks to occur during cold forging.
• the Mn amount is set to a range of 0.01% to 1.50%.
• the Mn amount is preferably 1.20% or less, more preferably 1.00% or less.
• the Mn amount is preferably 0.05% or more, more preferably 0.10% or more.
• S 0.001-0.100% S combines with Mn and Cu in the steel to form sulfide-based inclusions such as MnS inclusions and/or (Mn, Cu)S inclusions, and has the effect of improving the machinability of the steel material. If the S content is less than 0.001%, such an effect cannot be obtained. On the other hand, if the S content exceeds 0.100%, coarse MnS inclusions are generated during casting of the material, and cracks are likely to occur during cold forging. Therefore, the S content is set to a range of 0.001% to 0.100%.
• the S content is preferably 0.070% or less, more preferably 0.050% or less.
• the S content is preferably more than 0.025%, more preferably 0.030% or more.
• Cu 0.010-1.000%
• Cu is a useful element that improves the hardenability of steel.
• Mn, Cu composite sulfide
• This composite sulfide has a lower melting point than pure MnS, and is easily dissolved in steel even at the temperature at which the material before hot rolling is heated. Then, since it is reprecipitated as finer inclusions during cooling after hot rolling, it is possible to reduce the number density of coarse sulfide-based inclusions that cause cracks during cold forging while ensuring high machinability.
• the Cu content is set to a range of 0.010% to 1.000%.
• the Cu content is preferably 0.500% or less, more preferably 0.350% or less, and further preferably 0.300% or less.
• the Cu content is preferably 0.030% or more, and more preferably 0.050% or more.
• Cr 0.01 ⁇ 2.00% Cr is an element that improves the hardenability of steel. If the Cr content is less than 0.01%, this effect cannot be obtained. On the other hand, if the Cr content exceeds 2.00%, the hardenability of the steel becomes excessive, and the steel exhibits a structure containing hard martensite, resulting in a decrease in cold forgeability. Therefore, the Cr content is set to a range of 0.01% to 2.00%. The Cr content is preferably 1.80% or less, and more preferably 1.50% or less.
• Mo 0.01 ⁇ 1.00% Mo is a useful element that greatly improves the hardenability of steel materials with a small amount of addition. If the Mo content is less than 0.01%, such an effect cannot be obtained. On the other hand, if the Mo content exceeds 1.00%, the hardenability of the steel material becomes excessive, and the steel material exhibits a structure containing hard martensite, resulting in a decrease in cold forgeability. Therefore, the Mo content is set to a range of 0.01% to 1.00%. The Mo content is preferably 0.50% or less, and more preferably 0.30% or less.
• N 0.0020-0.0250%
• N combines with nitride-forming elements in steel to form nitrides, and acts as grain boundary pinning particles, thereby preventing the coarsening of crystal grains. If the N content is less than 0.0020%, this effect cannot be obtained. On the other hand, if the N content exceeds 0.0250%, there is a risk of forming blowholes in the steel. In addition, the solid solution N in the steel causes dynamic strain aging, which makes it easier for cracks to occur during cold forging. Therefore, the N content is set to a range of 0.0020% to 0.0250%.
• the N content is preferably 0.0200% or less, more preferably 0.0180% or less.
• the N content is preferably 0.0025% or more, more preferably 0.0030% or more.
• the steel material of the present invention may further contain the following elements as necessary.
• Ni 0.01 ⁇ 1.00%
• Ni is an element that enhances the hardenability and toughness of steel material, and can be added. Addition of less than 0.01% Ni does not provide such effects.
• addition of more than 1.00% Ni excessively increases the hardenability of the steel material, and the steel material exhibits a structure containing hard martensite, resulting in reduced cold forgeability. Therefore, when Ni is added, the content is set to be in the range of 0.01% to 1.00%.
• the Ni content is preferably 0.80% or less, and more preferably 0.60% or less.
• Al 0.001-0.100%
• Al can be added because it is a deoxidizing element and also has the effect of reducing the amount of solute N in steel by bonding with N in the steel to form nitrides. If the amount of Al is less than 0.001%, such an effect cannot be obtained. On the other hand, if the amount of Al is more than 0.100%, a large amount of oxide-based inclusions is generated, which makes it easier for cracks to occur during cold forging. Therefore, when Al is added, it is set to a range of 0.001% to 0.100%.
• the amount of Al is preferably 0.080% or less, more preferably 0.050% or less.
• Ti 0.001-0.100%
• Ti is an element that combines with N in steel to form nitrides and has the effect of reducing the amount of solute N in steel, and can be added. If the amount of Ti is less than 0.001%, such an effect cannot be obtained. On the other hand, if the amount of Ti is more than 0.100%, a large amount of Ti-based inclusions is generated in the steel, which reduces the cold forgeability. Therefore, when Ti is added, the amount is set to be in the range of 0.001% to 0.100%.
• the amount of Ti is preferably 0.080% or less, and more preferably 0.050% or less.
• V:0.001 ⁇ 0.300% V can be added because it combines with N in steel to form nitrides and has the effect of reducing the amount of solute N in steel. If the amount of V is less than 0.001%, such an effect cannot be obtained. On the other hand, if the amount of V is added in excess of 0.300%, the amount of V-based precipitates in the steel becomes excessive, making it easier for cracks to occur during cold forging. Therefore, when V is added, it is set to a range of 0.001% to 0.300%.
• the amount of V is preferably 0.200% or less, more preferably 0.150% or less.
• Nb 0.001-0.100%
• Nb can be added because it combines with carbon in steel to form carbides and contributes to grain refinement. If the Nb content is less than 0.001%, this effect cannot be obtained. On the other hand, if the Nb content exceeds 0.100%, a large amount of coarse Nb-based carbides are generated, which reduces cold forgeability. Therefore, when Nb is added, the content is set to 0.001% or more and 0.100% or less.
• the Nb content is preferably 0.050% or less, and more preferably 0.030% or less.
• B 0.0005-0.0050%
• B is an element that greatly improves the hardenability of steel material with a small amount of addition, and can be added. If the B content is less than 0.0005%, such an effect cannot be obtained. On the other hand, if the B content exceeds 0.0050%, the effect of improving the hardenability is saturated. Therefore, when B is added, it is set to a range of 0.0005% or more and 0.0050% or less.
• the B content is preferably 0.0040% or less, and more preferably 0.0030% or less.
• Sb 0.0010-0.0300%
• Sb is an element that easily segregates in the surface layer of a steel material and has the effect of suppressing the decarburization reaction on the surface of the steel material, so it can be added. If the amount of Sb is less than 0.0010%, such an effect cannot be obtained. On the other hand, if the amount of Sb added exceeds 0.0300%, the amount of Sb segregated in the surface layer becomes excessive, which deteriorates the surface properties of the steel material. Therefore, when Sn is added, it is set to a range of 0.0010% or more and 0.0300% or less. The amount of Sb is preferably 0.0200% or less, and more preferably 0.0150% or less.
• the steel material of the present invention contains at least bainite as the remainder of the structure.
• bainite the area ratio of bainite is more than 0%
• the area ratio of such bainite is preferably 30% or more, more preferably 33% or more, and even more preferably 35% or more.
• the area ratio of bainite is equal to or more than the above lower limit, the strength and hardness of the steel material can be further increased, and an even more excellent non-tempered steel can be obtained.
• the area ratio of bainite is preferably 90% or less, and more preferably 85% or less. If the area ratio of bainite is equal to or less than the above upper limit, the strength and hardness are not increased too much, and good cold forgeability and machinability can be easily achieved at the same time.
• the area ratio of bainite is preferably in the range of about 30 to 90%.
• the remaining structure other than martensite and bainite may be a structure known in steel materials for cold forging, such as ferrite and pearlite.
• the remaining structure may be 0% in area ratio, preferably 5% or more, preferably 20% or less, more preferably 15% or less, and preferably in the range of about 5 to 20%.
• C can be sufficiently dissolved in bainite to improve hardenability.
• bainite can be sufficiently generated, and both excellent strength and cold forgeability can be achieved.
• the area ratio of martensite, bainite and other remaining structures refers to the area ratio in the cross-sectional area of the steel material, and specifically, can be measured by cross-sectional observation according to the examples described later.
• the number density of sulfide-based inclusions with a circle equivalent radius of 1 to 20 ⁇ m is 10.0 pieces/mm 2 or more, and the number density of sulfide-based inclusions with a circle equivalent radius of 1 mm or more is 0.10 pieces/mm 2 or less.
• Sulfide-based inclusions are effective in improving the machinability of steel materials.
• coarse sulfide-based inclusions become the starting point of cracks during cold forging. Therefore, it is important to disperse fine sulfide-based inclusions in large quantities while reducing the number density of coarse sulfide-based inclusions in steel.
• the number density of fine sulfide-based inclusions with a circle equivalent radius of 1 ⁇ m or more and 20 ⁇ m or less is set to 10.0 pieces/mm 2 or more, preferably 20.0 pieces/mm 2 or more. If the number density of the fine sulfide-based inclusions is equal to or higher than the lower limit, the machinability can be improved without impairing the cold forgeability.
• the upper limit of the number density of the fine sulfide-based inclusions is not particularly limited, but from the viewpoint of production costs, it can be set to 50.0 pieces/ mm2 or less.
• the number density of coarse sulfide-based inclusions with a circle equivalent radius of 1 mm or more must be limited to 0.10 pieces/ mm2 or less, and is preferably 0.05 pieces/ mm2 or less. If the number density of the coarse sulfide-based inclusions is equal to or less than the upper limit, cracking during cold forging is suppressed, and good cold forgeability is obtained.
• the lower limit of the number density of coarse sulfide-based inclusions with a circle equivalent radius of 1 mm or more is not particularly limited, and may be 0.00 pieces/ mm2 (not contained).
• sulfide-based inclusions refers to sulfides with a single element (e.g., MnS), composite sulfides with multiple elements (e.g., (Mn, Cu)S), or composite compounds mainly composed of sulfides (compounds of sulfides with oxides, carbides, nitrides, etc.), and these may exist alone or in mixtures.
• sulfide-based inclusions also include sulfides in which other elements such as Fe are dissolved.
• MnS and (Mn, Cu)S are preferred, and (Mn, Cu)S is more preferred.
• the number density of sulfide-based inclusions refers to the number per unit area in the cross-sectional area of a steel material, and specifically, can be measured by cross-sectional observation according to the examples described later.
• the Vickers hardness of the steel is preferably 400 Hv or less, and more preferably 350 Hv or less.
• the Vickers hardness of the steel is preferably 150 Hv or more, and more preferably 200 Hv or more. The hardness of the steel can be measured according to the method of the examples described below.
• Step 2 In the method for producing a steel material of the present invention, attention must be paid not only to the composition of the steel material but also to the conditions for hot rolling the material, particularly to the maximum heating temperature and dwell time of the material.
• the method for producing a steel material of the present invention can obtain a steel material with improved cold forgeability without impairing machinability.
• the steel material obtained by the method for producing a steel material of the present invention is still useful as a non-tempered steel while exhibiting good cold forgeability.
• the chemical composition of the steel material is the same as that described above for the steel material.
• the maximum heating temperature of the material is 1000 to 1200°C. If the maximum heating temperature during hot rolling of steel material is too low, the solid solution of the coarse sulfide-based inclusions does not proceed sufficiently, and the coarse inclusions remain in the steel. Therefore, when the steel is cold forged, cracks originating from these coarse inclusions are likely to occur. On the other hand, if the heating temperature is too high, although it is favorable for dissolving the coarse sulfide-based inclusions in the parent phase, the austenite grains become coarse during heating, and the hardenability increases. As a result, martensite is generated after hot rolling, and the cold forgeability decreases. Therefore, when hot rolling a steel material, the maximum heating temperature is set to a range of 1000° C. to 1200° C. The maximum heating temperature is preferably 1030° C. or higher. The maximum heating temperature is preferably 1150° C. or lower.
• Dwell time is t 1 or more
• the inventors have focused on the fact that the more Cu added to the material, the lower the melting point of the Cu-containing composite sulfide, and the earlier the solid solution of the sulfide-based inclusions is completed. In other words, the more Cu is added, the shorter the required heating time is.
• the inventors have intensively studied the influence of the amount of Cu added and the heating time on the precipitation state of the sulfide-based inclusions.
• t 1 (unit: min), which is determined by the following formula (1) according to the Cu concentration (unit: mass%, also written as [Cu]), is the minimum required dwell time for dissolving the coarse sulfide-based inclusions.
• t 1 60-10[Cu]...(1)
• the residence time in the heating furnace is required to be equal to or longer than the above t1 (minutes), with the lower limit being determined according to the Cu concentration ([Cu]).
• the residence time is preferably equal to or longer than 1.1 x t1 , more preferably equal to or longer than 1.3 x t1 , and even more preferably equal to or longer than 1.5 x t1 .
• a longer residence time is advantageous in terms of dissolving coarse sulfide-based inclusions, so there is no need to specify an upper limit.
• the residence time is too long, the amount of scale increases, which can lead to poor yield and reduced productivity. Therefore, a residence time of 6.0 x t1 or less is preferable, and 5.0 x t1 or less is even more preferable.
• the obtained steel material in the temperature range of 800°C to 700°C at an average cooling rate of less than 25°C/s. If the average cooling rate is 25°C/s or more, the transformation to bainite becomes insufficient, the area ratio of martensite increases, and the cold forgeability may decrease.
• the average cooling rate is more preferably less than 20°C/s.
• the average cooling rate is preferably 3° C./s or more.
• any items not described in this specification can use the regulations and common methods for known steel materials.
• a 160 mm square billet (steel material) was used, with the composition shown in Table 1, and the remainder being Fe and unavoidable impurities. This material was heated in a heating furnace under the conditions shown in Table 2 below, and then hot-rolled into a wire rod with a diameter of 15 mm to obtain hot-rolled wire rod as a steel material.
• Steel Nos. 1 to 9 in Table 1 are comparative steels that fall outside the range of the composition of the present invention, and Steel Nos. 10 to 33 are suitable steels that fall within the range of the composition of the present invention.
• the obtained hot-rolled wire rod was cut and microstructure observation was performed.
• the microstructure observation was performed on a cross section (C-section (circular)) perpendicular to the rolling direction and perpendicular to the central axis of the wire rod.
• the microstructure observation was performed on five randomly selected visual fields at a magnification of 100 times, with the observation area per visual field being 600 ⁇ m ⁇ 800 ⁇ m.
• ImageJ which is image analysis software, was used to calculate the area ratios of martensite and bainite.
• the size of the sulfide-based inclusions in the hot-rolled wire was measured.
• the hot-rolled wire was cut to a length of 20 mm, and then cut so that the cross section (longitudinal cross section) parallel to the central axis of the wire was the observation surface.
• three inclusion observation samples with an observation area of 15 mm x 20 mm were taken for each steel type.
• the entire area of the observation surface was observed with a scanning electron microscope (SEM), and SEM images of all inclusions within the field of view were obtained.
• the observation magnification when obtaining the SEM image was appropriately changed in the range of 50 to 2000 times so that the entire inclusions were within the field of view.
• an energy dispersive X-ray spectrometer (SEM-EDX) attached to the SEM was used to identify the inclusion type, and inclusions in which Mn and S, as well as Mn, Cu and S were detected, were considered to be sulfide-based inclusions.
• SEM-EDX energy dispersive X-ray spectrometer
• sulfide-based inclusions in the SEM images were traced and binarized, and the cross-sectional areas of the inclusions were obtained by image analysis and converted into circle-equivalent radii. The cross-sectional areas and circle-equivalent radii of the inclusions were calculated using ImageJ, an image analysis software.
• the critical upsetting ratio of each steel was determined by the method described in the literature "Cold upsetting property test method" (edited by the Materials Research Group of the Cold Forging Subcommittee of the Japan Society for Technology of Plasticity: Plasticity and Processing, 22 (1981), pp. 139-144). That is, the surface scale of the hot-rolled wire rod was completely removed by pickling, and then wire drawing was performed to obtain a wire rod with a diameter of 14 mm.
• This test piece corresponds to the "No. 2 test piece" described in the above literature.
• Ten test pieces for cold forging were taken from each steel material under the conditions shown in Table 2 and used for the following tests. The cold forging test pieces obtained above were subjected to successive compression tests under end face restraint conditions to measure the limit upsetting ratio. The initial compression was 15% in the height direction, and compression tests were performed at 0.5 mm intervals thereafter.
• test pieces were observed at the notch bottom to check for the presence or absence of cracks with a length of 0.5 mm or more. Compression, unloading, and observation of the notch bottom were repeated until cracks with a length of 0.5 mm or more occurred in all test pieces, and the cumulative compression ratio at which cracks with a length of 0.5 mm or more occurred in half of the test pieces (5 pieces) was defined as the limit upsetting ratio for that steel type. Steel types with a limiting upsetting rate of 40% or more were judged to be acceptable (excellent in cold forgeability).
• the hardness (Hv) of a steel material is determined as follows. A cross section (cross section perpendicular to the central axis of the wire) of a hot-rolled wire rod having a diameter of 15 mm was mirror-polished, and then measured using a Vickers hardness tester (load: 10 kgf). The measurement positions were one point at the center position of the cross section of the wire rod, and four points at intermediate positions (radial intermediate positions between the center of the cross section and the circumference), for a total of five points, and the average hardness value (Hv) of the steel material was determined.
• Nos. 1 and 2 are examples in which the C and Si contents, respectively, exceeded the specified range. These steel materials had too high hardenability, and therefore exhibited a microstructure containing martensite at an area ratio of more than 10.0%. As a result, the limit upsetting rate was low at less than 40%, and the cold forgeability was poor.
• No. 3 is an example where the Mn content exceeded the specified range.
• This steel had too high hardenability, resulting in a microstructure containing more than 10.0% martensite by area.
• the Mn content was too high, there was a large amount of coarse sulfide-based inclusions with a circle equivalent radius of 1 mm or more. As a result, the limit upsetting rate was low at less than 40%, and the cold forgeability was poor.
• No. 4 is an example where the S content exceeded the specified range. This steel had a large amount of coarse sulfide-based inclusions, so the limit upsetting rate was low at less than 40%, and the cold forgeability was poor.
• No. 5 is an example where the Cu content was below the specified range.
• the Cu content was small, the solid solution of the coarse sulfide-based inclusions did not progress during heating before hot rolling, and Ostwald ripening of the sulfide-based inclusions occurred during heating, reducing the number density of fine sulfide-based inclusions with a circle equivalent radius of 1 to 20 ⁇ m.
• many coarse sulfide-based inclusions remained in the hot-rolled material.
• the limit upsetting rate was low at less than 40%, and cold forgeability was poor.
• machinability was poor.
• Nos. 6 to 8 are examples in which the Cu, Cr, and Mo contents exceeded the specified ranges. These steels had too high hardenability, resulting in a microstructure containing martensite at an area ratio of more than 10.0%. As a result, the limit upsetting rate was low at less than 40%, and cold forgeability was poor.
• No. 9 is an example where the amount of N exceeded the specified range.
• the amount of dissolved N in the steel is high, and due to the effects of dynamic strain aging, the limit upsetting rate is low at less than 40%, and cold forgeability is poor.
• No. 10 is an example where the composition is within the specified range, but the maximum heating temperature during hot rolling exceeded the specified range.
• the limiting upsetting rate was low at less than 40%, and the cold forgeability was poor.
• No. 11 is an example where the composition is within the specified range, but the maximum heating temperature during hot rolling was below the specified range. Because the heating temperature for this steel was too low, the coarse sulfide-based inclusions could not be sufficiently dissolved during heating, and a large amount of coarse inclusions remained after hot rolling. As a result, the limit upsetting rate was low at less than 40%, and the cold forgeability was poor.
• Nos. 12 and 13 are examples in which the composition and maximum heating temperature during hot rolling were within the specified range, but the residence time in the heating furnace was below the specified range. Because the heating time for this steel was short, the coarse sulfide-based inclusions could not be fully dissolved, and a large amount of coarse inclusions remained after hot rolling. As a result, the limiting upsetting rate was low at less than 40%, and the cold forgeability was poor.
• Nos. 14 to 33 are examples in which the composition of the steel material and the heating conditions during hot rolling are both within the specified ranges.
• the area ratio of martensite is low, bainite is present in the steel material, and the number density of coarse sulfide-based inclusions is reduced while fine sulfide-based inclusions are secured.
• all of the examples showed a high limit upsetting ratio of 40% or more, and excellent values for cold forgeability.
• No. 14 is a case where the chemical composition of the steel material and the heating conditions during hot rolling are within the specified range, but the average cooling rate from 800 to 700°C after hot rolling is relatively high. In this steel material, because the average cooling rate was fast, there was little transformation to bainite and the area ratio of martensite increased, resulting in a relatively low critical upsetting rate.
• Nos. 14 to 33 which have a sufficient amount of bainite and have martensite suppressed to a specified area ratio or less, have sufficient hardness and strength that can be used well as non-tempered steel.

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