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
  • C22C38/001—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
  • C22C38/06—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
  • C22C38/40—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

Definitions

• the present invention relates to machine structural steels, for example, used for power transmission components such as gears and shafts used in automobiles, industrial machines, and the like, and particularly relates to a machine structural steel with small heat treatment deformation.
• heat treatment deformation Deformation of a steel material occurring due to heat treatment such as quenching (hereinafter referred to as “heat treatment deformation”) has such an adverse effect that the number of production steps is increased in order to correct the deformation, that the percentage of defective components is increased when the deformation is too large to correct, or that noise or vibration is caused by the deformation in the case where a deformed steel material is incorporated as a driving system component. Accordingly, minimizing the heat treatment deformation is a very important problem with regard to the practical use.
• the heat treatment deformation has been considered to be affected by a large number of factors such as a component shape, the effect of a step prior to heat treatment, the physical property value of a refrigerant such as a quenching oil, and the non-uniformity of cooling as well as a steel material type, and thus reduction in heat treatment deformation by adjusting these factors properly has been attempted.
• a method of precipitating a soft ferrite phase in the core of a quenched steel material to reduce heat treatment distortion e.g., see Patent Literature 1).
• the means for promoting a heat transfer rate is considered to be due to a coating material for promoting cooling that is placed in a site where cooling is delayed or due to convection of a coolant formed around the site where cooling is delayed, and the means for decreasing a heat transfer rate is considered to be due to glass wool or a heat insulating coating material which covers a site where cooling is easy to proceed.
• Patent Literature 1 involves introduction of the soft phase with low strength into a component
• Patent Literature 2 requires a modification of a heat treating furnace itself
• Patent Literature 3 requires treatment to an individual component to be heat-treated.
• the inventors have extensively researched a steel of which the heat treatment deformation can be reduced to a low level even when cooling of a component is non-uniform under a common technique such as oil quenching, while securing sufficient strength of the steel material without relying on a soft layer such as ferrite.
• the heat treatment deformation can be reduced to a low level by suitably controlling the chemical constituents of the steel, martensitic transformation start temperature (Ms point), and hardenability as measured by a Jominy end quenching method.
• a machine structural steel material with small heat treatment deformation comprising in mass %:
• the above-described steel material is substantially free of Ni, Mo, Ti, and Nb, or comprises them at unavoidable impurity levels.
• the above-described steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%.
• the above-described steel material comprises, in mass %, one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.
• the above-described steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50% and one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.
• the machine structural steel material with small heat treatment deformation comprises in mass %: C: 0.16 to 0.35%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0 to 0.030%; S: 0 to 0.030%; Cr: 1.30 to 2.50%; Cu: 0 to 0.30%; Al: 0.008 to 0.800%; O: 0 to 0.0030%; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.50%; Ti: 0 to 0.200%; Nb: 0 to 0.20%; and the balance Fe and unavoidable impurities, preferably consists essentially of these elements and unavoidable impurities, and more preferably consists of these elements and unavoidable impurities.
• the steel material according to the present invention comprises C in an amount of 0.16 to 0.35%, preferably 0.20 to 0.30%, more preferably 0.22 to 0.27%.
• C is an element necessary for securing the strength of the steel material after quenching and tempering thereof or the strength of its core after carburizing, quenching and tempering thereof for a machine structural component, and adjustment of C content into a specified range is needed for reducing heat treatment deformation.
• a content of C of less than 0.16% fails to secure the strength, while that of more than 0.35% results in too large heat treatment deformation.
• the steel material according to the present invention comprises Si in an amount of 0.10 to 1.50%, preferably 0.20 to 1.00%.
• Si is an element that is necessary for deoxidation and that is effective for imparting strength and hardenability required for steel.
• a content of Si of less than 0.10% fails to give the effects, while that of more than 1.50% results in deteriorated mechanical workability.
• the steel material according to the present invention comprises Mn in an amount of 0.10 to 1.20%, preferably 0.20 to 0.80%, more preferably 0.20 to 0.55%.
• Mn is an element necessary for securing hardenability.
• a content of Mn of less than 0.10% fails to provide a sufficient effect for hardenability, while that of more than 1.20% results in deteriorated mechanical workability.
• the steel material according to the present invention comprises P in an amount of 0 to 0.030%, typically more than 0 and not more than 0.030%.
• P is an unavoidable element that is incorporated from scrap, but its content of more than 0.030% results in grain boundary segregation to deteriorate characteristics such as impact strength and bending strength.
• the steel material according to the present invention comprises S in an amount of 0 to 0.030%, typically more than 0 and not more than 0.030%.
• S is an element that improves machinability, but its content of more than 0.030% generates MnS, which is a non-metallic inclusion, to deteriorate crosswise toughness and fatigue strength.
• the steel material according to the present invention comprises Ni in an amount of 0 to 3.00%, preferably 0.20 to 3.00%.
• Ni is an optional element that improves hardenability and toughness, and addition of 0.20% or more thereof is preferred for obtaining the effect.
• the Ni content of more than 3.00% significantly deteriorates workability and increases a cost.
• the steel material according to the present invention comprises Cr in an amount of 1.30 to 2.50%, preferably 1.50 to 2.25%.
• Cr is an element necessary for securing hardenability.
• a content of Cr of less than 1.30% results in an insufficient effect for hardenability, while that of more than 2.50% results in inhibited carburization and also in deteriorated mechanical workability.
• the steel material according to the present invention comprises Mo in an amount of 0 to 0.50%, preferably 0.05 to 0.50%.
• Mo is an optional element that improves hardenability and toughness, and addition of 0.05% or more thereof is necessary for obtaining the effect.
• a Mo content of more than 0.50% deteriorates workability.
• the steel material according to the present invention comprises Cu in an amount of 0 to 0.30%, typically more than 0 and not more than 0.30%.
• Cu is an unavoidable element that is incorporated from scrap, but has an aging property and is effective at increasing strength.
• a content of Cu of more than 0.30% results in deteriorated hot workability.
• the steel material according to the present invention comprises Al in an amount of 0.010 to 0.800%, preferably 0.014 to 0.600%.
• Al is an element that is used as a deoxidation material, and is bound to N to be precipitated as AlN to result in the effect of suppressing coarsening of grain size, as described below. Addition of 0.010% or more of Al is necessary for obtaining this effect. In contrast, addition of more than 0.800% of Al results in the formation of large-sized alumina-based inclusions, and deteriorates fatigue characteristics and workability.
• the steel material according to the present invention comprises O in an amount of 0 to 0.0030%, typically more than 0 and not more than 0.0030%, preferably not more than 0.0020%.
• O is an element that is unavoidably contained in steel.
• an O content of more than 0.0030% results in the deterioration of workability and fatigue strength due to the increase of oxides.
• the steel material according to the present invention comprises N in an amount of 0.0020 to 0.0300%, preferably 0.0020 to 0.0200%.
• N is finely precipitated as AIN and Nb nitrides in steel, and provides the effect of preventing coarsening of grain size, and addition of 0.0020% or more thereof is necessary for obtaining the effect.
• a content of more than 0.0300% results in the increase of the nitrides, and deteriorates fatigue strength and workability.
• 0.0020 to 0.0100% of N is preferred for avoiding the deterioration of fatigue strength due to the excessive generation of TiN.
• the steel material according to the present invention comprises Ti in an amount of 0 to 0.200%, preferably 0.020 to 0.200%.
• Ti is an optional element that is bound to C in steel to finely form a carbide, and provides the effect of preventing coarsening of grain size, and addition of 0.020% or more of Ti is preferred for obtaining the effect. In contrast, addition of more than 0.200% thereof results in deteriorated mechanical workability.
• the steel material according to the present invention comprises Nb in an amount of 0 to 0.20%, preferably 0.02 to 0.20%, more preferably 0.02 to 0.12%.
• Nb forms a carbide or a nitride, and provides the effect of preventing coarsening of grain size.
• NbC or Nb(C, N) with a nanometer-order size which is finely dispersed in steel, suppresses the growth of the grain size.
• Less than 0.02% of Nb fails to provide the effect, while more than 0.20% thereof results in the excessive amount of a precipitate to deteriorate workability.
• its martensitic transformation start temperature (Ms point) is regulated to 460° C. or less, preferably 450° C. or less, in order to reduce the heat treatment deformation of the steel material.
• the reason why the heat treatment deformation can be reduced by regulating the Ms point to 460° C. or less is that occurrence of martensitic transformation during quenching can be avoided in a temperature range in which the cooling performance of a refrigerant is high, even when cooling of a component is non-uniform, and, as a result, a time point at which the martensitic transformation occurs can be inhibited from greatly differing depending on the site of the component.
• the heat treatment deformation in this case refers to bending of a shaft-shaped component or snapping or twisting of a gear tooth.
• a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material, as measured by a Jominy end quenching method is in a range of from 0.68 to 0.97; and a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material is in a range of from 0.63 to 0.94. Within such a range, the heat treatment deformation of the steel material can be suppressed to a low level.
• the heat treatment deformation in this case refers to a variation in dimension (length, diameter, or thickness) before and after quenching a component.
• the limitation of the steel constituents, the limitation of the Ms point, and the limitation of hardenability measured by the Jominy end quenching method, as mentioned above, achieves the smaller heat treatment deformation in the case of processing a steel material into a component and then performing quenching or carburizing and quenching for hardening the component.
• the present invention can provide such a beneficial effect of making it possible to improve the yield of components, to simplify or remove the step of correcting a component, or to omit the grinding of a gear tooth surface for measures against noise and vibration.
• machine structural steel used as components for power transmission such as gears and shafts used in automobiles, industrial machines, and the like
• machine structural steels comprising compositions of present invention examples Nos. 1 to 23 shown in Table 1 and the balance Fe and unavoidable impurities were ingotted in a vacuum induction melting furnace to obtain 100 kg of steel ingots.
• the steel ingots of the present invention examples and the comparative examples were heated at 1250° C. for 5 hours and then forged to obtain steel bars having a diameter of 32 mm. Then, the steel bars were normalized by heating and maintaining at 900° C. for 1.5 hours, followed by air-cooling. Subsequently, test pieces having a diameter of 20 mm and a length of 200 mm were produced from the steel bars having the diameter of 32 mm, and the sides of the test pieces were subjected to processing to have a groove with a depth of 5 mm, a width of 8 mm, and a length of 200 mm. The groove processing caused a cooling rate to greatly differ depending on a site in each test piece during quenching.
• test pieces were heated at 930° C. for 1 hour, their temperatures were then decreased to 850° C. in the furnace, and were further maintained for 1 hour and then quenched in a quenching oil at 60° C. After the quenching, as for the sufficiently cooled test pieces, the bends and lengths of the test pieces were measured.
• Each bend after the heat treatment was determined by holding both ends of each test piece with V blocks, measuring the maximum and minimum displacements on the circumference of the central portion of the test piece during one revolution of the test piece, with a dial gauge, and dividing the difference between the maximum and minimum displacements by 2. In the case of the measurement, the displacement at the bottom of a groove present on the circumference of the test piece was ignored. Further, the difference between the lengths of the test piece before and after the heat treatment was determined to evaluate the absolute value thereof as the index of dimensional change.
• test pieces having a diameter of 3 mm and a length of 10 mm were extracted from the steel bars having a diameter of 32 mm after subjected to the normalizing described above to measure Ms points as the martensitic transformation start temperatures of the steel materials using a fully-automated transformation record measuring apparatus.
• the Ms points in this context are measured under the conditions simulating the cooling process of a component.
• the Ms points were measured at a cooling rate of 30° C./s during the quenching by simulating the case where the above-described test pieces with grooves having the diameter of 20 mm were oil quenched at an oil temperature of 60° C.
• test pieces were produced from the above-described forged steel bars having the diameter of 32 mm, and were tested and evaluated under the conditions according to “method of hardenability test for steel” (end quenching method) specified in JIS G 0561.
• Table 3 there are shown the measured Ms points of the present invention examples, each value of hardness J1.5 at a distance of 1.5 mm from the quenched ends, hardness J9 at a distance of 9 mm, and hardness J11 at a distance of 11 mm, measured by the Jominy end quenching method, and the determined values of (J9/J1.5) and (J11/J1.5).
• the bends (mm) evaluated after quenching the above-described test pieces and the absolute values (mm) of the differences between the lengths of the test pieces before and after the heat treatment are further shown.
• the martensitic transformation start temperatures i.e., the Ms points were in a range of 372 to 442° C.
• the values of equation (1), shown as follows, of (J9/J1.5) of the steel materials were in a range of 0.70 to 0.95
• the values of equation (2), shown as follows, of (J11/J1.5) were in a range of 0.65 to 0.90
• the bends after the heat treatment were 0.10 to 0.36 mm
• the absolute values of the differences between the lengths of the test pieces before and after the heat treatment were 0.01 to 0.20 mm.
• Table 4 there are shown the measured Ms points of the steels of the comparative examples, Jominy hardenability as hardness J1.5 at a distance of 1.5 mm from the quenched ends, hardness J9 at a distance of 9 mm, or hardness J11 at a distance of 11 mm, measured by the Jominy end quenching method, and the determined values of (J9/J1.5) and (J11/J1.5).
• the bends (mm) after quenching of the above-described test pieces and the absolute values (mm) of the differences between the lengths of the test pieces before and after the heat treatment are further shown.
• the bends of the test pieces after the heat treatment were able to be reduced to the low range of 0.10 to 0.36 mm and the absolute values of the differences between the lengths of the test pieces before and after the heat treatment were also able to be reduced to the small range of 0.01 to 0.20 mm by formulating the composition ranges of the steel materials, except for Fe and unavoidable impurities other than Ni and Mo, to those shown in Table 1, by setting the Ms points at 372 to 442° C., which were lower than or equal to 460° C., and by suitably controlling hardenability measured by the Jominy end quenching method, thereby setting the values of (J9/J1.5) calculated by equation (1) in a range of 0.70 to 0.95 and setting the values of (J11/J1.5) calculated by equation (2) in a range of 0.65 to 0.90.
• the present invention examples Nos. 1 to 23 of which the Ms points satisfy the scope of claims have low bends of the test pieces after the heat treatment, resulting in reduced heat treatment deformation. Further, in comparison with the comparative examples Nos. 1 and 3 to 16 of which the values of (J9/J1.5) and the values of (J11/J1.5) fall outside the scope of claims, the invention examples Nos.
• Such a steel material according to the present invention is a steel material that can be applied to components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like.

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• 2011
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• 2012
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