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
  • C22C38/60—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
  • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • 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 by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
  • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
• • 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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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 graphite steel that can be applied in various industrial fields such as machine parts, and more particularly, to a wire rod and graphite steel for graphitization heat treatment having a short graphitization heat treatment time and excellent machinability.
• free-cutting steel As a material for small machine parts requiring machinability, free-cutting steel to which machinability imparting elements such as Pb, Bi, and S are added has been used.
• Pb-added free-cutting steel which is the most representative free-cutting steel, emits harmful substances such as toxic fumes during cutting, so it is very harmful to the human body, and there is a very disadvantageous problem in the recycling of steel materials.
• graphite steel is a steel in which graphite grains exist inside a ferrite matrix or ferrite and pearlite matrix.
• Graphite steel has excellent machinability by reducing friction with tools while serving as a chip breaker as the graphite grains inside the matrix act as a crack source during cutting.
• graphite steel has a disadvantage in that a separate graphitization heat treatment is required for a long time for precipitating graphite grains by decomposing cementite, which is a metastable phase. This not only lowers productivity and increases cost, but also causes decarburization during a long period of graphitization heat treatment to adversely affect the performance of the final product.
• Patent Document 1 Korean Patent Publication No. 1995-0006006 (published date: March 20, 1995)
• the present invention is to provide a graphitized heat treatment wire and graphite steel in which fine graphite grains are uniformly distributed in a matrix during graphitization heat treatment while significantly shortening the graphitization heat treatment time.
• the wire rod for graphitization heat treatment is, by weight, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P 0.05% or less, S: 0.05% or less, the balance Fe and unavoidable impurities, and the value of the following formula (1) is -1 or more and 1 or less Satisfied, the microstructure may include 40% or less of ferrite, 5% or less of the total of bainite and martensite, and the remainder of pearlite as an area fraction.
• Equation (1) 100*([Mn]-0.25) 2 - (100*[N]) 2
• [Mn] and [N] mean the weight % of each alloy element.
• the value of the following formula (2) may satisfy 6 or less.
• Equation (2) 100*[Ti] + 10000*[B]
• the graphite steel according to an embodiment of the present invention in weight %, C: 0.65 to 0.85%, Si: 2.0 to 3.0%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less, B: 0.0005% or less, P 0.05% or less, S: 0.05% or less, the balance Fe and unavoidable impurities, and the value of the following formula (1) is -1 or more and 1 or less Satisfied, the microstructure may include 80% or more of ferrite and the remainder graphite grains as an area fraction.
• Equation (1) 100*([Mn]-0.25) 2 - (100*[N]) 2
• [Mn] and [N] mean the weight % of each alloy element.
• the value of the following formula (2) may satisfy 6 or less.
• Equation (2) 100*[Ti] + 10000*[B]
• the tensile strength may be 550 MPa or less.
• the graphite steel according to the present invention can be applied as a material for precision mechanical parts such as automobiles, home appliances/electronic devices, and industrial devices.
• the present invention can significantly reduce the graphitization heat treatment time through alloy composition control to dramatically reduce the manufacturing cost of graphite steel, while ensuring excellent machinability by uniformly distributing fine graphite grains in the matrix structure.
• the wire rod for graphitization heat treatment is, by weight, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less , B: 0.0005% or less, P 0.05% or less, S: 0.05% or less, the remainder Fe and unavoidable impurities, and the value of the following formula (1) satisfies -1 or more and 1 or less, and the microstructure is an area fraction , 40% or less of ferrite, 5% or less of the total of bainite and martensite, and the balance may include pearlite.
• Equation (1) 100*([Mn]-0.25) 2 - (100*[N]) 2
• [Mn] and [N] mean the weight % of each alloy element.
• Graphite grains deposited in the steel matrix improve machinability. Specifically, during cutting, the graphite grain acts as a solid lubricant to suppress wear of the cutting tool, acts as a crack initiation point due to stress concentration, lowers the cutting friction, and improves machinability by making the cutting material segment shorter.
• C and Si are added in large amounts in order to shorten the graphitization heat treatment for a long time while improving the machinability.
• C increases, a large amount of graphite grains are formed after the graphitization heat treatment, which results in better machinability.
• Si destabilizes cementite to promote cementite decomposition, and as a result, graphitization heat treatment can be shortened.
• Si when Si is added excessively, there are problems in cutting tool wear and difficulty in steelmaking.
• a small amount of Si is added to secure cold forging properties.
• the graphitization is further accelerated by the upward addition of Si in an amount of 2.0 wt% or more.
• the present invention mainly utilizes TiN nitride among AlN, BN, and TiN nitrides that act as nucleation sites for graphite grains.
• BN and AlN have a low precipitation temperature, so austenite is formed, and then, it is unevenly precipitated by being biased at grain boundaries.
• BN and AlN that are non-uniformly precipitated may cause non-uniform distribution of graphite grains because they act as nuclei of graphite grains during graphitization heat treatment.
• TiN since TiN has a higher precipitation temperature than AlN or BN and is crystallized before austenite formation is completed, it is uniformly distributed at the austenite grain boundary and within the grain.
• TiN is uniformly distributed in the microstructure compared to BN and AlN, and as a result, graphite grains formed by using TiN as a nucleation site are uniformly distributed in the microstructure, and graphitization compared to BN and AlN, where the graphite grains grow unbalanced. can further promote In addition, the uniformly distributed graphite grains can improve machinability such as chip handling properties.
• the graphite steel according to the present invention is prepared by graphitizing a wire rod for graphitization heat treatment.
• the wire rod for graphitization heat treatment according to an embodiment of the present invention is, by weight, C: 0.65 to 0.85%, Si: 2.00 to 3.00%, Mn: 0.15 to 0.35%, Ti: 0.002 to 0.1%, N: 0.01% or less , B: 0.0005% or less, P 0.05% or less, S: 0.05% or less, balance Fe and unavoidable impurities.
• the reason for limiting the composition of the wire rod for graphitization heat treatment will be described in detail.
• the reason for limiting the composition of the graphite steel alloy is the same as the wire rod for graphitization heat treatment, so it is omitted for convenience.
• the content of C is 0.65 to 0.85% by weight.
• C is a component element constituting graphite grains, a cutting factor, and the higher the C content, the more graphite grains are formed.
• C activity increases, and as a result, cementite decomposition is promoted, thereby shortening the graphitization heat treatment.
• the C content is less than 0.65% by weight, there is a problem in that the C activity is lowered and the machinability is lowered.
• the C content exceeds 0.85% by weight, the effect of increasing the C activity is saturated, and the toughness of the steel is lowered due to excessively formed graphite grains, so graphite steel is used to manufacture cold drawn bars (CD-Bars) later. There is a risk of breakage when fresh. Therefore, in the present invention, the C content is controlled to 0.65 to 0.85% by weight.
• the content of Si is 2.00 to 3.00% by weight.
• Si is a necessary component as a deoxidizer in manufacturing molten steel, and is actively added as it is a graphitization promoting element that destabilizes cementite in steel so that carbon can be precipitated as graphite. If the Si content is less than 2.00% by weight, there is a fear that the graphitization rate is rapidly slowed down. On the other hand, when the Si content exceeds 3.00% by weight, the graphitization promoting effect is saturated, and there is a risk of causing brittleness due to the increase of non-metallic inclusions and decarburization symptoms during hot rolling. Therefore, in the present invention, the Si content is controlled to 2.00 to 3.00% by weight.
• the content of Mn is 0.15 to 0.35% by weight.
• Mn improves the strength and impact properties of steel, and is actively added because it combines with S in steel to form MnS inclusions and contributes to improvement of machinability.
• Mn content is too small, the graphitization rate is inhibited by S which does not form MnS, and there is a problem in that the material is brittle.
• Mn is added in an amount of 0.15% by weight or more.
• the Mn content is controlled to 0.35% by weight or less.
• the content of Ti is 0.002 to 0.1% by weight or less.
• Ti forms TiN, which is a nitride, such as B, Al, etc. to lower the solid solution nitrogen content that inhibits graphitization, and the formed TiN acts as a nucleation site of graphite to shorten the graphitization time.
• BN and AlN which act as nucleation sites for graphite, have a low precipitation temperature to form austenite, and then are unevenly precipitated by being biased at grain boundaries.
• TiN has a higher precipitation temperature than AlN or BN, it is crystallized before austenite formation is completed, so that it is uniformly distributed at the austenite grain boundary and within the grain.
• TiN is uniformly distributed in the microstructure compared to BN and AlN, and as a result, graphite grains formed by using TiN as a nucleation site are uniformly distributed in the microstructure, and graphitization compared to BN and AlN, where the graphite grains grow unbalanced. can be further promoted, and the machinability of graphite steel can be improved.
• Ti is added in an amount of 0.002% by weight or more.
• the Ti content is excessive, the shortening effect of the graphitization heat treatment due to TiN is saturated, and there is a fear that coarse carbonitrides are formed, rather reducing graphitization.
• Ti is controlled to 0.1% by weight or less.
• the content of N is 0.01% by weight or less.
• N combines with Ti, B, and Al to form nitrides such as TiN, BN, and AlN.
• BN and AlN have a low precipitation temperature, so they are mainly concentrated at the austenite grain boundary and are non-uniformly precipitated.
• BN and AlN that are non-uniformly precipitated may cause non-uniform distribution of graphite grains because they act as nuclei of graphite grains during graphitization heat treatment. Therefore, it is necessary to appropriately control the N content in order to suppress the formation of BN and AlN as much as possible while mainly precipitating TiN.
• the N content is controlled to 0.01% by weight or less.
• the content of B is 0.0005% by weight or less.
• B combines with N to produce BN, and the produced BN acts as a nucleus for graphite grain generation during graphitization heat treatment.
• BN is unevenly precipitated by being biased at grain boundaries.
• the content of P is 0.05% by weight or less.
• P is an impurity contained inevitably. P helps to improve machinability by weakening the grain boundaries in steel. However, P increases the hardness of ferrite by a significant solid solution strengthening effect, reduces the toughness and delayed fracture resistance of steel, and promotes the occurrence of surface defects. Therefore, it is desirable to keep the P content as low as possible. In the present invention, the upper limit of P is managed as 0.05% by weight.
• the content of S is 0.05% by weight or less.
• S is an impurity contained inevitably.
• S forms MnS and has the effect of improving machinability.
• MnS has an effect of improving machinability
• mechanical anisotropy may appear due to MnS stretched after rolling. Therefore, it is desirable to manage the S content as low as possible.
• the upper limit of S is managed as 0.05% by weight.
• the remaining component of the present invention is iron (Fe).
• Fe iron
• the wire rod for graphitization heat treatment according to an embodiment of the present invention may satisfy the above-described alloy composition, and the value of the following formula (1) may satisfy -1 or more and 1 or less.
• Equation (1) 100*([Mn]-0.25) 2 - (100*[N]) 2
• [Mn] and [N] mean the weight % of each alloy element.
• wire rod for graphitization heat treatment may satisfy the above-described alloy composition and the value of the following formula (2) is 6 or less.
• Equation (2) 100*[Ti] + 10000*[B]
• Equation (2) When the content of Ti and B is increased and the value of Equation (2) exceeds 6, the size of the crystallized TiN increases, making it difficult to perform a role as graphite grain-forming nuclei, so there is a problem in that the graphitization heat treatment time becomes longer.
• BN generated along the grain boundary acts as a graphite grain generating nucleus, so that the graphite grains are non-uniformly generated. As a result, machinability becomes inferior.
• the microstructure of the wire rod for graphitization heat treatment may include, as an area fraction, ferrite: 40% or less, the sum of bainite and martensite: 5% or less, and the balance pearlite.
• the graphite steel according to the present invention is prepared by graphitizing a wire rod for graphitization heat treatment, and the microstructure of the graphite steel preferably includes ferrite and the remainder graphite.
• the microstructure of the graphite steel may include 80% or more of ferrite and the remainder graphite grains as an area fraction.
• the tensile strength of the graphite steel subjected to graphitization heat treatment may be 550 MPa or less.
• the wire rod for graphitization heat treatment is prepared by graphitizing heat treatment at 730 to 770° C. for 6 hours or less to prepare graphite steel, in which case the graphitization rate of the graphite steel may be 99% or more.
• the graphitization rate means the ratio of the carbon content in the graphite state to the carbon content added to the steel, and is defined by the following formula (3), and a graphitization rate of 99% or more means that the added carbon is almost This means that all was consumed to produce graphite.
• Equation (3) the amount of solid-solution carbon and fine carbides in ferrite are extremely small, so they are not considered.
• the graphitization rate of 99% or more means that undecomposed pearlite does not exist in the steel, and is composed of ferrite and residual graphite grains.
• the graphite steel of the present invention described above can be manufactured by various methods, and the manufacturing method is not particularly limited, but the manufacturing method of the graphite steel according to an embodiment of the present invention includes the steps of hot rolling a steel material satisfying the above-described alloy composition. and graphitizing heat treatment at 730 to 770° C. for 6 hours or less.
• the step of hot rolling may include casting an ingot satisfying the alloy composition described above and then performing homogenization heat treatment at 1100 to 1300° C. for 5 to 10 hours and hot rolling at 1000 to 1100° C. After hot rolling, it is possible to manufacture a wire rod for graphitization heat treatment by air cooling to 8° C. or less.
• the wire rod for graphitization heat treatment is subjected to graphitization heat treatment at 730 to 770° C. for 6 hours or less to be prepared as graphite steel.
• the preferred graphitization heat treatment temperature range is 730 to 770° C., and through isothermal heat treatment for 6 hours or less in this temperature range, cementite in all pearlites in the steel can be completely graphitized.
• the graphite steel of the present invention can be applied as a material for precision mechanical parts such as automobiles, home appliances/electronic devices, and industrial devices.
• the present invention can significantly reduce the graphitization heat treatment time through alloy composition control to dramatically reduce the manufacturing cost of graphite steel, while ensuring excellent machinability by uniformly distributing fine graphite grains in the matrix structure.
• the graphite steel according to the present invention may be manufactured into precision mechanical parts such as automobiles, home appliances/electronic devices, and industrial devices by wire drawing, cold forging, cutting, and the like.
• Q/T (quenching and tempering) heat treatment may be performed to secure surface hardness after cutting.
• a wire rod for graphitization heat treatment was prepared by casting an ingot having the composition shown in Table 1 and performing homogenization heat treatment at 1250° C. for 6 hours, followed by hot rolling and air cooling. The finishing temperature during hot rolling was 1000°C.
• 'Equation (1)' and 'Equation (2)' in Table 1 are values derived by substituting the alloy composition content in the above-mentioned Formulas (1) and (2).
• the graphitization heat treatment wire shown in Table 1 was subjected to graphitization heat treatment at 750° C. for 4 hours to prepare graphite steel.
• graphitization heat treatment was performed at the graphitization heat treatment temperature of 711 °C and 803 °C, respectively.
• the structure of the wire rod for graphitization heat treatment in Table 2 means the microstructure of the wire rod for graphitization heat treatment before the graphitization heat treatment.
• the graphite steel structure in Table 2 means the microstructure of the graphite steel after graphitization heat treatment.
• Comparative Examples 1 to 7 were subjected to graphitization heat treatment under the same graphitization heat treatment conditions as in the present invention, but graphitization was not completed due to the remaining pearlite structure, the tensile strength exceeded 550 MPa, and the tool wear depth exceeded 200 mm. The machinability was bad.
• Comparative Example 1 graphitization could not be completed due to low C content and low graphitization driving force.
• Comparative Example 2 had a high Si content, which caused brittleness due to an increase in non-metallic inclusions and decarburization symptoms during hot rolling.
• Comparative Example 3 graphitization could not be completed because Mn inclusions formed and remaining Mn inhibited graphitization.
• Comparative Example 4 the amount of Mn forming MnS was small, and the graphitization could not be completed because the remaining S inhibited graphitization and did not form MnS.
• Comparative Example 5 had a high N content to form nitrides such as TiN, AlN, and BN, and the remaining N inhibited graphitization, so graphitization could not be completed.
• the B content was excessive, and most of the BN was precipitated at the grain boundary, thereby inhibiting graphitization.
• Comparative Examples 6 and 7 did not satisfy the value range of Equation (2) limited by the present invention, and the crystallized TiN size was too large, so graphitization could not be completed.
• BN generated along the grain boundaries acted as graphite grain-forming nuclei, and graphite grains were non-uniformly generated. As a result, the machinability was inferior.
• Comparative Examples 8 and 9 did not satisfy the graphitization heat treatment temperature limited by the present invention. As a result, in Comparative Example 8, in which the graphitization heat treatment temperature was too low, pearlite was not completely graphitized during the graphitization heat treatment. On the other hand, in Comparative Example 9, where the graphitization heat treatment temperature was too high, the phase transformation into austenite was performed to form pearlite again upon cooling.
• the wire rod and graphite steel for graphitization heat treatment according to the present invention greatly shorten the graphitization heat treatment time, and since fine graphite grains are uniformly distributed in the matrix during the graphitization heat treatment, there is industrial applicability.

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