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US20100051143A1 - Case Hardening Steel Tube Having Improved Workability and a Process for its Manufacture - Google Patents

Case Hardening Steel Tube Having Improved Workability and a Process for its Manufacture Download PDF

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Publication number
US20100051143A1
US20100051143A1 US12/557,353 US55735309A US2010051143A1 US 20100051143 A1 US20100051143 A1 US 20100051143A1 US 55735309 A US55735309 A US 55735309A US 2010051143 A1 US2010051143 A1 US 2010051143A1
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steel
steel tube
tube
case hardening
temperature
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Kenichi Beppu
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Nippon Steel Corp
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Sumitomo Metal Industries Ltd
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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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C21D1/28—Normalising
    • 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
    • C21D1/32—Soft annealing, e.g. spheroidising
    • 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
    • C21D6/02—Hardening by precipitation
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
    • 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
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • 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
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/085—Cooling or quenching
    • 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
    • C22C38/22—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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/44—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/02—Pretreatment of the material to be coated
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/40—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions
    • C23C8/42—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using liquids, e.g. salt baths, liquid suspensions only one element being applied
    • C23C8/44—Carburising
    • C23C8/46—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/80—After-treatment
    • 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
    • C21D2211/003—Cementite
    • 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

Definitions

  • This invention relates to a case hardening steel tube (a steel tube made from case hardening steel) having a high strength and a high toughness and exhibiting a high fracture load after carburizing and quenching, and to a process for its manufacture.
  • this invention relates to a case hardening steel tube having improved workability and a process for its manufacture.
  • case hardening steel has been used for the manufacture of various types of structural parts for use in automobile or industrial machinery and particularly surface-hardened parts typified by shafts, CVJs (constant velocity joints), CVTs (continuously variable transmissions), and gears.
  • Case hardening steel which is a material to be worked is formed into a part having a desired shape by hot or cold forging and machining, for example.
  • the part which has been formed is then subjected to surface hardening treatment such as carburizing or carbo-nitriding in order to increase its wear resistance and fatigue strength.
  • Case hardening steel sometimes has problems such as a decrease in impact fracture strength, a decrease in fatigue properties, and a decrease in dimensional accuracy due to abnormal growth of crystal grains during carburizing and quenching treatment.
  • high temperature carburizing which is carried out in the temperature range of 990-1090° C. is employed in order to shorten the carburizing time from the viewpoint of rationalization of carburizing treatment, the problem occurs that coarse grains develop, thereby making it impossible to obtain the desired fatigue properties such as rolling fatigue properties.
  • JP 2005-240175 A1 proposes suppressing the formation of coarse grains during high temperature carburizing of case hardening steel by controlling the steel composition and the formation of Ti-containing precipitates.
  • JP H09-53150 A1 discloses a high-strength, high-toughness case hardening steel exhibiting a sufficiently high impact fracture load even when a notch is present in the carburized surface and a process of manufacturing a high-strength, high-toughness case hardening steel tube exhibiting improved workability and improved impact fracture strength after carburizing and quenching from this steel.
  • JP H09-53150 A1 one of the causes of the problems of the prior art is the formation of an imperfect hardened structure.
  • the major cause of the formation of this imperfect hardened structure is the precipitation of carbides along austenite grain boundaries which occurs at the time of quenching of a carburized steel. Therefore, a steel composition design is employed in which B is added in order to prevent the above-described precipitation of carbides while N is reduced as much as possible so that B can adequately exhibit its effects.
  • the high-strength, high-toughness case hardening steel tube disclosed in JP H09-53150 A1 has excellent properties particularly in the form of seamless steel tube of case hardening steel.
  • problems sometimes develop with respect to workability, for example, at the time of forging by a user.
  • JP H09-53150 A1 discloses (i) a process in which a steel tube obtained by hot tube forming is subjected to cold working followed by stress relief annealing (Example 3), and (ii) a process in which a steel tube obtained by hot tube forming is subjected to initial annealing followed by cold working and subsequent stress relief annealing (secondary annealing) (Examples 4 and 5).
  • the present invention provides a case hardening steel tube which has good workability or more specifically an HRB hardness (Rockwell B scale hardness) of 72-80 and which can form a carburized layer of high strength and good wear resistance as well as sufficiently improved resistance to impact fracture when it is formed into a final product by working for forming followed by carburizing and quenching under relatively mild conditions, along with a process for its manufacture.
  • HRB hardness Rockwell B scale hardness
  • JP H09-53150 A1 gives 870° C. as a specific temperature for initial annealing before cold working.
  • the steel is once heated to a temperature higher than the Ac 3 point, so the steel structure after initial annealing becomes a ferrite+pearlite structure.
  • the initial annealing temperature is low, softening of the steel cannot be expected in secondary annealing after cold working if the annealing is carried out by gradual cooling over a sufficient-length of time.
  • a ferrite+pearlite structure is formed during normalizing, and this structure is subjected to subsequent steps of cold working and annealing.
  • the proportion of pearlite which is spheroidized during annealing can be varied. In this manner it is possible to perform fine adjustment of the steel hardness.
  • the present invention is a process for manufacturing a case hardening steel tube characterized by forming a tube from a steel having a steel composition comprising, in mass percent, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: at most 0.02%, S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: less than 0.006%, and a remainder essentially of Fe, subjecting the resulting steel tube to normalizing by holding at a temperature of 880-980° C. followed by cooling at a cooling rate of at most 70° C. per minute in a temperature range of 880-400° C., performing cold working on the normalized steel tube, and then annealing the cold worked steel tube at a temperature of 700-820° C.
  • the present invention is a cold finished, case hardening steel tube characterized by having a steel composition comprising, in mass percent, C: 0.1-0.25%, Si: 0.2-0.4%, Mn: 0.3-0.9%, P: at most 0.02%, S: 0.001-0.15%, Cr: 0.5-0.9%, Mo: 0.15-1%, Al: 0.01-0.1%, B: 0.0005-0.009%, N: less than 0.006%, and a remainder essentially of Fe, and having a steel structure which is a mixed ferrite+pearlite+spheroidized cementite structure or a mixed ferrite+spheroidized cementite structure.
  • the above-described steel composition may further contain one or more elements selected from the following (1) and (2):
  • the B content is preferably B: 0.0005-0.003%.
  • “Case hardening steel” and “case hardening steel tube” refer to steel and steel tube which undergo working to form a prescribed shape of a product (such as the above-described structural part of machinery) and finally carburizing and quenching to form a harder surface skin layer (carburized layer) before being used as a product.
  • the above-described hardness is the hardness of the case hardening steel, i.e., the hardness before the steel undergoes working to be formed into the shape of a part (of course, the hardness prior to carburizing and quenching). Forming into the prescribed shape of a product and carburizing and quenching are normally carried out by the customer (by the user).
  • a remainder essentially of Fe means that the remainder may contain unavoidable impurities.
  • FIG. 1 is a photograph showing the microstructure of a case hardening steel tube according to the present invention produced in an example.
  • C is a basic element for steel to develop its hardness and strength.
  • a hardness of at least 250 Hv is necessary in order to provide steel with such a strength that it will not deform during use of a carburized and quenched product.
  • the C content is made at least 0.1%. If C is contained in excess of 0.25%, the toughness of the central portion of steel deteriorates. Accordingly, the C content is 0.1-0.25% and preferably 0.12-0.20%.
  • the effect of Si of increasing the hardenability of steel is positively utilized in order to achieve a high resistance to impact fracture by hardening of a carburized layer. If the Si content is less than 0.2%, it is not possible to provide a carburized layer with the desired high hardenability. On the other hand, if Si is contained in excess of 0.4%, embrittlement of grain boundaries due to oxidation of Si in the vicinity of the grain boundaries at the time of carburizing becomes marked. Therefore, the Si content is 0.2-0.4%.
  • Mn is also added in order to increase the hardenability of a carburized layer and realize a high resistance to impact fracture. If the Mn content is reduced to less than 0.3%, the hardenability of a carburized layer decreases, and the desired high resistance to impact fracture cannot be achieved. It has been found that embrittlement of grain boundaries due to oxidation of Mn in the vicinity of grain boundaries at the time of carburizing is not a problem from a practical standpoint even if the Mn content exceeds 0.9%. However, if Mn is contained in excess of 0.9%, a deterioration in blankability and grindability becomes marked. Properties such as blankability and grindability are particularly important for efficient processing of CVJs and the like. Accordingly, the Mn content is 0.3-0.9%.
  • the P content is preferably reduced as much as possible.
  • the permissible level of P is determined by balancing the desired properties and cost. Taking into consideration the below-described effect of B, the upper limit on the allowable P content in the present invention is 0.02%.
  • the S content is less than 0.001%, its effect on improving machinability is not significant, while if S is contained in excess of 0.15%, the deterioration in the toughness of steel becomes marked. Therefore, the S content is 0.001-0.15%. In applications where a high level of machinability is not required, it is advantageous to suppress the S content to a low value.
  • Cr is an indispensable element for providing the base metal of steel (the steel in the portion excluding the carburized layer on the surface) with hardenability and for achieving the carbon concentration necessary in the carburized layer in a short length of time.
  • the Cr content it is necessary for the Cr content to be at least 0.5%.
  • Cr markedly promotes embrittlement of grain boundaries due to precipitation of cementite on austenite grain boundaries at the time of carburizing and quenching. Therefore, its content is restricted to at most 0.9%.
  • the Cr content is limited to at most 0.9%, the hardenability of steel and particularly the hardenability of a carburized layer having an increased C content becomes inadequate. Therefore, in the present invention, hardenability is supplemented by adding B, Mo, and Ni which do not lead to embrittlement of grain boundaries.
  • the Cr content is 0.5-0.9% but preferably it is adjusted to 0.5-0.65%.
  • Mo is an essential element for increasing the strength and toughness of the steel base metal and a carburized layer and for achieving a carbon concentration necessary in the carburized layer in a short length of time.
  • the effect of Mo on increasing hardenability is almost unaffected by the C content of the steel base metal. Therefore, its effect of increasing hardenability can be stably exhibited even with a carburized layer having an increased carbon content.
  • the Cr content is limited in order to suppress embrittlement of grain boundaries resulting from carburizing, and hardenability is supplemented by the addition of B.
  • hardenability markedly decreases even if the carbon content reaches a high level, so supplementing hardenability of a carburized layer by addition of Mo is extremely important.
  • the Mo content is less than 0.15%, not only can hardenability not be adequately supplemented, but the amount of carbon which penetrates into the surface layer during carburizing treatment performed in a short length of time also decreases. From the standpoint of imparting the above-described effect, it is preferable for the Mo content to be large.
  • the Mo content is 0.15-1%, preferably 0.2-0.7%, and more preferably 0.2-0.6%.
  • Al is an element which is effective for deoxidation and crystal grain refinement of steel. Its effects are inadequate if its content is less than 0.01%. On the other hand, if Al is contained in excess of 0.1%, the amount of inclusions which are harmful to toughness increases. Accordingly, the Al content is 0.01-0.1%.
  • B suppresses precipitation of carbides (Cr carbides and the like) on austenite grain boundaries which form at the time of hardening of a carburized material, thereby preventing the formation of an imperfectly hardened structure in the resulting carburized layer and also preventing grain boundary embrittlement. Therefore, it is an indispensable element for achieving adequate properties in terms of resistance to impact fracture, wear resistance and rolling fatigue properties in a carburized and quenched steel.
  • the Cr content is restricted in order to prevent the adverse effect of Cr of markedly promoting embrittlement in grain boundaries due to precipitation of carbides on grain boundaries at the time of carburizing and quenching.
  • B also serves to compensate for the decrease in hardenability of the steel base metal caused by decreasing the Cr content and thereby provide the central portion of steel with sufficient hardenability.
  • the B content is less than 0.0005%, the above-described desired effects of B are not achieved. On the other hand, if B is contained in excess of 0.009%, it ends up causing grain boundary embrittlement. Therefore, the B content is 0.0005-0.009%.
  • heat treatment is carried out before cold working at a temperature of at least the Ac 3 point and specifically at a temperature of at least 880° C.
  • This heat treatment is contemplated that B is once solutionized in order to achieve the object of decreasing hardness by annealing which is carried out after cold working. If the amount of B is too large, a long time is required for solutionize B and accordingly a long time is required in heat treatment for normalizing. Therefore, the B content is preferably on the lower side of the above-described range. Specifically, it is particularly desirable for the B content to be at most 0.003% (namely, in the range of 0.0005-0.003%).
  • the amount of N in steel is extremely important in making the effect of B significant. Namely, the effect of preventing carbide precipitation on grain boundaries during quenching of a carburized steel which is achieved by the addition of B becomes marked only when the amount of N in steel is reduced to a range of less than 0.006%. In this range, not only is a sufficient resistance to impact fracture achieved, but rolling fatigue properties are also markedly improved.
  • the N content in steel is preferably as low as possible, but in industrial production in air, it is extremely difficult to make the N content less than 0.001% with current steel manufacturing technology.
  • a case hardening steel tube according to the present invention When a case hardening steel tube according to the present invention is used as an inner race or a ball cage or the like of a common drive shaft joint for automobiles, it has adequate strength and toughness even if addition of Ni or below-described Ti, Nb, V, or Zr does not take place. However, when it is used in applications having more severe conditions, addition of one or more of these elements is effective at increasing the strength and toughness of steel.
  • Ni is an element which is effective at increasing the strength and toughness of the steel base metal. In addition, it acts together with Mo to greatly contribute to an increase in strength and toughness of a carburized layer. If the Ni content is less than 0.3%, the above-described effects are inadequate. On the other hand, if Ni is contained in excess of 4.0%, its effects saturate. Accordingly, when Ni is added, its content is 0.3-4.0%.
  • These elements have the effect of refining the crystal grains of steel and increasing its toughness. Accordingly, preferably one or more of these elements are added when severe conditions of use are expected. If the content of any of these elements which is added is less than 0.01%, the above-described effect is inadequate. On the other hand, if any of these elements is contained in excess of 0.3%, it leads to worsening in the toughness and rolling fatigue properties of steel. Accordingly, the content of any of Ti, Nb, V, and Zr, when added, is 0.01-0.3%.
  • a mother tube is manufactured from steel (case hardening steel) having the above-described steel composition using a suitable tube forming process.
  • the mother tube is preferably a seamless steel tube prepared by hot tube forming.
  • heat treatment is performed at a temperature which is equal to or higher than the Ac 3 point of the steel, so the working history in prior steps has no influence of the tube product.
  • the tube forming process there is no particular limitation on the tube forming process.
  • hot tube forming of the seamless steel tube there is no particular restriction on hot tube forming of the seamless steel tube.
  • a seamless steel tube can be manufactured from steel having the above-described steel composition by preparing a billet from a bloom by means of hot forging, and subjecting the billet sequentially to Mannesmann piercing rolling, elongation rolling with a mandrel mill, and sizing rolling.
  • a steel tube (mother tube) which is manufactured by the above-described process or other process is subjected to normalizing prior to cold working instead of to initial annealing as disclosed in Patent Documents 1 and 2.
  • Normalizing is carried out by, for example, heating the steel tube in a suitable heating furnace to hold it at a prescribed temperature for heat treatment (soaking or isothermal heating) followed by cooling.
  • the object of this normalizing treatment is to transform the structure of the steel into a mixed structure of ferrite+pearlite. If the steel structure is once made ferrite+pearlite, it is possible to develop properties desirable for a case hardening steel tube by cold working and subsequent annealing in a specified temperature range.
  • the heat treatment temperature for normalizing is at least 880° C. and at most 980° C. If the heat treatment temperature exceeds 980° C., decarburization may proceed.
  • the lower limit of 880° C. is the temperature necessary for dissolving B in solid solution (i.e., for solutionizing B) into austenite in a short period of time so as to obtain a uniform structure.
  • the hardness of the steel base metal can be decreased by dissolving B in solid solution. If the heat treatment temperature at the time of normalizing is lower than 880° C., adequate dissolving of B in solid solution cannot be achieved, and a decrease in the hardness of the steel base metal is not obtained even if the heat treatment temperature is maintained for a long period.
  • the duration of soaking may be as short as 30 seconds if the entire part of the steel tube can reach the above-described temperature. However, at least one minute is preferable from the standpoint of minimizing variations in properties. If the duration of soaking exceeds 30 minutes, decarburization may proceed, so it is preferably at most 30 minutes.
  • Cooling after heat treatment may be air cooling, but in the range from the heat treatment temperature to 400° C. (and accordingly at least in the range of 800-400° C.), the cooling rate is made at most 70° C. per minute. A cooling rate higher than this causes bainite to form, and the effects of the present invention can no longer be obtained. There is no particular lower limit on the cooling rate as long as it is at least the rate achieved by air cooling in view of the type of heat treatment which is normalizing. Taking into consideration the efficiency such as the treatment time, the cooling rate is preferably at least 20° C. per minute.
  • the steel tube obtained by hot tube forming undergoes cold working after it has been subjected to normalizing.
  • Cold working is generally necessary in order to provide a steel tube with prescribed dimensions and dimensional accuracy.
  • cold working is also effective at causing spheroidizing of the cementite present in pearlite (and accordingly spheroidizing of pearlite) in a secondary heat treatment stage by annealing which is carried out after the cold working.
  • the reduction ratio at the time of cold working is preferably such that the reduction in area is 20-50% and more preferably 25-50%. If the reduction ratio is less than 20%, it becomes difficult to spheroidize a portion of pearlite in the next step. If the reduction ratio exceeds 50%, seizing may occur at the time of working between the tools used and the material being worked. In addition, due to an increase in the accumulation of strains in the steel base metal, abnormal growth of austenite crystal grains occurs at the time of carburizing heat treatment, thereby causing the resulting hardened structure to be coarsened and become a mixed grain structure. Furthermore, if the reduction ratio of cold working exceeds 50%, the steel tube after working has a markedly increased hardness due to work hardening, and during subsequent annealing it is difficult to be softened adequately, leading to worsening in the workability.
  • Annealing after cold working is commonly carried out in order to release strains which accumulated in the steel base metal due to cold working and soften the steel base metal and achieve the workability demanded by the user.
  • it also has the object of spheroidizing at least a portion of the cementite in pearlite.
  • the annealing temperature after cold working is in the range of 700-820° C. If the annealing temperature is less than 700° C. or if it exceeds 820° C., spheroidizing of pearlite does not adequately proceed.
  • the steel structure becomes a mixed structure of ferrite+spheroidized cementite.
  • the steel structure becomes a mixed structure of ferrite+pearlite+spheroidized cementite.
  • a case hardening steel tube according to the present invention can be characterized by this steel structure and the above-described steel composition.
  • Spheroidizing at least a portion of pearlite in this manner results in a decrease in the hardness of the steel tube.
  • a case hardening steel tube having good workability in the form of a hardness of 72-80 HRB can be manufactured according to the present invention.
  • the hardness can be adjusted to a desired value by varying the proportion of pearlite which is spheroidized during annealing by controlling the reduction ratio at the time of cold working and the annealing conditions.
  • carburizing and quenching are normally carried out by the user after fabrication of the part by working or forming.
  • carburizing and quenching conditions are carburizing by soaking at 920° C. for 2 hours and then quenching from 870° C.
  • One-ton blooms having the steel compositions shown in Table 1 were produced by casting molten steel obtained by vacuum melting. The blooms underwent hot forging to obtain round billets, which were then underwent hot tube forming by piercing rolling, elongation rolling in a mandrel mill, and sizing rolling in a stretch reducer to produce mother tubes (steel tubes) with an outer diameter of 80 mm and a wall thickness of 6.1 mm.
  • the steel tubes were subjected to initial heat treatment (normalizing) under the conditions shown in Table 2 followed by cooling, and they then underwent cold drawing with a reduction in area of 28.4% to provide finished seamless steel tubes having dimensions of an outer diameter of 66.2 mm and a wall thickness of 5.3 mm. These steel tubes were then subjected to annealing under the conditions shown as secondary heat treatment in Table 2. Test pieces were cut from the steel tubes after the completion of the secondary heat treatment (annealing), and the Rockwell B scale hardness (HRB) of tube cross sections was measured for each tube. The results are shown in Table 2.
  • No. 1 and No. 2 in Table 2 had a heat treatment temperature in normalizing before cold working of 700° C. which was lower than the Ac 1 point, and they were finished to a hardness of at least 87 HRB.
  • Nos. 3-7 for which the heat treatment temperature before cold working exceeded the Ac 3 point, when the heat treatment temperature was lower than 880° C., except for No. 3, the hardness was at least 82 HRB, and the object of softening to a value of at most 80 HRB could not be achieved.
  • the cooling rate after heat treatment (soaking) was made a slow value of 10° C. per minutes, the hardness was 77 HRB and the object of softening could be achieved.
  • the heat treatment time including the cooling step became long, and temperature holding equipment in the case of continuous treatment would become elongated, so this cooling rate is clearly not economical.
  • Nos. 8-18 are examples in which heat treatment prior to cold working was soaking at 880° C. or 930° C.
  • the cooling rate after soaking exceeded 70° C. per minute and for Nos. 14 and 16 in which the annealing temperature after cold working was too low or high, the hardness exceeded 80 HRB and sufficient softening could not be achieved.
  • the cooling rate after soaking was at most 70° C. per minute and the annealing temperature after cold working was in the range of 700-820° C.
  • the object of softening to a hardness of at most 80 HRB could be achieved in each case.
  • FIG. 1 shows a photomicrograph of a steel tube obtained by No. 11 of Table 2. It can be seen that carbides (cementite) were spheroidized in the ferrite+pearlite structure.

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EP3085800A4 (fr) * 2013-12-20 2017-07-05 Nippon Steel & Sumitomo Metal Corporation Tuyau en acier soudé à résistance électrique
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AU2013202710B2 (en) * 2012-04-10 2015-12-17 Tenaris Connections B.V. Methods of manufacturing steel tubes for drilling rods with improved mechanical properties, and rods made by the same
EP3085800A4 (fr) * 2013-12-20 2017-07-05 Nippon Steel & Sumitomo Metal Corporation Tuyau en acier soudé à résistance électrique
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US20180030562A1 (en) * 2016-07-28 2018-02-01 The Gleason Works Heat treatment of steel parts, particularly friction-welded steel parts
CN113106334A (zh) * 2021-03-11 2021-07-13 石家庄钢铁有限责任公司 一种rv减速器摆线轮用钢及其制备方法
CN119162420A (zh) * 2024-09-20 2024-12-20 山西太钢不锈钢股份有限公司 时速400公里及以上高铁用大规格车轴钢坯的加工方法

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