US3788903A - Method of processing steel material having high austenitic grain-coarsening temperature - Google Patents

Method of processing steel material having high austenitic grain-coarsening temperature Download PDF

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US3788903A
US3788903A US00134332A US3788903DA US3788903A US 3788903 A US3788903 A US 3788903A US 00134332 A US00134332 A US 00134332A US 3788903D A US3788903D A US 3788903DA US 3788903 A US3788903 A US 3788903A
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temperature
steel material
grain
working
treatment
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A Suzuki
S Kinoshita
T Ueda
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Kobe Steel Ltd
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Kobe Steel Ltd
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    • 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

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  • FIG. 1 A 8 ww n1 8 O ⁇ 3 Mw v mHM lil AST AKIRA SUZUKI ETAL METHOD OF PROCESSING STEEL MATERIAL HAVING HIGH AUSTENI'IIG CRAIN-COARSENING TEMPERATURE Filed April 15, 1971 Jan. 29, 1974 FIG. 1
  • ABSTRACT OF THE DISCLOSURE A method of processing steel material having a high austenitic grain-coarsening temperature and an abnormal grain growth curve of cold worked austenitic grain, which comprises heating said steel material at a temperature of from the ferrite recrystallization temperature to the A transformation point for more than five minutes to cause recrystallization. This technique prevents the formation of mixed grains upon austenitizing and thereby preserves the mechanical properties of the steel material.
  • This invention relates to a method of processing steel material having a high austenitic grain-coarsening temperature, and more particularly to a method of processing steel material without the danger of producing a mixed grain structure upon heat treatment in the austenitic region following cold working.
  • the mixed grained steels are usually considered to be of inferior quality, and, when such steels are heat treated under severe austenitizing conditions, such as carburizing in the austenitic region following cold working, difficulties with toughness, strain and distortion can result.
  • Another object of the present invention is to provide a method of processing a steel material without losing the desirable attributes of cold working, including its excellent dimensional accuracy, smooth surface skin, etc.
  • a still further object of the present invention is to provide a method of processing a steel material which does not create mixed grains during carburizing treatment following cold Work in the preparation of machine elements.
  • FIGS. 1(a) and 2(a) are photographs (x) showing the preferable fine grain structure obtained after austenitizing the steel material at 950 C. by the method of this invention.
  • FIGS. 1(b) and 2(b) are photographs (XI-00) showing mixed grain structure appearing after austenitizing the same steel material at 950 C. by the conventional method.
  • Any steel material which is characterized by abnormal austenitic grain growth i.e., any steel material in which the austenitic grains start to grow suddenly at a temperature in the austenitic region, can be treated according to the present invention.
  • this technique can be successfully applied to carbon steels and alloy steels containing one or more grain refining elements, such as aluminum, titanium, niobium, zirconium, vanadium, tantalum, rare earth elements, etc. which precipitate as nitrides, carbides and oxides.
  • the steel material is prepared for cold working, after hot working and/or heat treatment, etc., and is formed to a desired shape by rolling, forging, extruding, etc. at a temperature below the ferritic recrystallization temperature region.
  • Mechanical working such as machining, may be used as the cold working.
  • the cold worked steel material has a lower austenitic grain-coarsening temperature than non-cold worked steel material as aforementioned.
  • This steel material is then recrystallized at a temperature within the ferritic recrystallization region so as to raise the austenitic graincoarsening temperature and to thereby cause the cold worked ferrite structure of the steel material to substantially recrystallize in the ferritic region.
  • the steel material must be heated to at least the temperature at, or above which, the deformed ferritic structure is able to recrystallize.
  • the upper temperature limit A transformation temperature is below that at whichferritic structure exists.
  • the specific temperature range will, of course, depend upon the type of steel and the cold working conditions, but the lower limit is usually about 400500 C., while the upper limit is about 700- 800" C.
  • the deformed ferritic structure will not substantially reerystallize, while if the temperature of the heat treatment is higher than the upper limit, the deformed ferritic structure will not be recrystallized and it will result in lower austenitic grain-coarsening temperature with its consequent risk of mixed grain structure formation on subsequent austenitizing.
  • the lower the heating temperature the longer the period of heating time required.
  • the shortest time period is 5 minutes, but periods of longer than 30 minutes are preferred. When the heat time period is less than 5 minutes, sufficient recrystallization will not occur.
  • One good method for heat treating is to hold the steel material at a constant temperature within the ferritic recrystallization region, and thereafter to cool it. However, the same effect may be expected by cyclically heating and cooling within the ferritic recrystallization region at a sufiicient cyclic rate.
  • the upper limit of the heating temperature be the A; transformation temperature.
  • the upper temperature limit of the recrystallization treatment may be the A transformation temperature.
  • the heat treatment in the ferritic recrystallization region according to this invention will be effective regardless of whether the austenitic grain growth inhibitors are nitrides, carbides or oxides and regardless of whether they have already precipitated before or are in the process of precipitating before the ferritic recrystallization treatment. It is also effective, regardless of the state of the precipitation, and regardless of the amount, number, distribution and/or morphology of the particles. However, the positive utilization or progression of the precipitation during the ferritic recrystallization treatment will further raise the austenitic grain-coarsening temperature, since it results in finer austenitic grain growth inhibitor particle size.
  • Another characteristic feature of this invention is that the combination of increased inhibiting force against grain-coarsening and reduced grain growth driving force on subsequent austenization will raise the austenitic grain-coarsening temperature.
  • the increased inhibiting force is derived from the precipitation treatment of the supersaturated steel materials in the ferritic region, and the reduced grain growth driving force is a result of the ferritic recrystallization treatment following cold working.
  • the cooling rate after solution treatment should be not less than 5 C./min. in the temperature range of 900 to 500 C. Cooling rates of less than 5 C./min. give rise to the precipitation of a considerable amount of coarse precipitate, which is less effective in the prevention of austenitic grain-coarsening on subsequent austenitization, and gives rise to a reduced amount of finely precipitated particles in the following precipitation treatment. Finely precipitated particles are more effective in inhibiting the austenitic grain-coarsening.
  • Hot working such as rolling, forging and extruding may follow the solution treatment to impart cold work performance to the steel material. Cooling should then follow the hot work.
  • the steel material is in a supersaturated solid solution with nitrogen, carbon and/or substantial alloy elements to form nitrides and/or carbides after cooling at a definite rate, following solution treatment (and/or hot working).
  • the supersaturated steel material is subjected to precipitation treatment so that the precipitated particles act as the austenitic grain-coarsening inhibitor on the succeeding austenization. This increases the austenitic grain-coarsening temperature.
  • Heating of the supersaturated steel material is most preferable in the temperature range of 500 C. to the A temperature for more than 5 min.
  • the precipitate forms as fine particles in the ferritic structure and the rate of precipitation will depend upon temperature. Below 500 C., the rate of the precipitation is so slow that there is substantially no precipitation progress.
  • Heat treatment of the steel material in the higher ferritic temperature region before cold working reduces the deformation resistance upon cold working, and raises the critical working rate above that at which cold working cracks are generated. If more severe cold working performance is required, this treatment may be combined with a spheroidizing annealing in order to improve cold workability.
  • Austenitizing by intermediate heat treatment such as annealing which may be carried out between cold working processes, should also be avoided, in order to inhibit austenitic grain-coarsening.
  • the austenitic grain-coarsening temperature will be raised, and thus the dangers of mixed grain structure formation upon austenitization with its consequent deterioration of quality is minimized. Since the method of this invention does not necessitate high temperature treatment, compared with conventional methods for achieving similar results, it can preserve the excellent dimensional accuracy and smooth surface skin advantages of cold working. Furthermore, the method of this invention is particularly effective when applied to the steel material for machine elements which are subjected to severe austenitizing conditions, such as carburizing treatment, after cold working.
  • Table 1 shows the chemical composition of test materials adopted for the embodiment of this invention and the comparison data thereof.
  • steel A contains aluminum needed to normal carbon steel.
  • Steel B is Cr-Mo steel containing aluminum.
  • Steels C and G are Cr-Mo steels containing titanium.
  • Steels D and E are Cr-steel, containing aluminum.
  • Steel F is Cr-Mo steel, containing niobium.
  • Table 2 shows the austenitic grain-coarsening temperature measured after holding for 1 hour at the temperatures of ferritic recrystallization region and below it, followed by austenitizing of the steel materials A, B and C, produced by the following process: 1l00 C. 1 hr. solution treatment 5% hot workair cooling (average cooling rate: 20 C./min.)- 50% cold working.
  • Table 3 shows the austenitic grain-coarsening temperatures in relation to the heating time at 700 C. for ferritic recrystallization, for steel B produced in the same process as that for Table 2.
  • the uniform and fine microstructure was obtained by the combined heat treatment of (1) holding at 700 C. for 1 hour followed by (2) air cooling and austenitizing at 950 C. (followed by quenching) as shown in FIG. 1(a) for steel B which was produced by the process indicated in the afiix of Table 2.
  • the mixed grain structure was obtained after austenitizing at 950 2(a), was obtained for steel C by the same treatment as for FIG. 1(a), as shown in 'FIG. 2(a).
  • a mixed grain structure was obtained for steel C treated in the same way as for FIG. 1(b), as shown in FIG. 2(b).
  • Table 4 shows the comparison between the austenitic grain-coarsening temperatures of steel D treated by the process of the invention:
  • Table 6 shows the austenitic grain-coarsening temperature of the steels F and G treated by the respective processes.
  • references Cited UNITED STATES PATENTS after cold working, and having a high austenitic grain- 4 9 g 22:? coarsening temperature wherein said steel material is 93 g 036 1 2 heated to a temperature above 1,000 C. for at least 5 g gg 13 z iggag mmutes to form a solid soluble nitride, carb1de and/or 3:163:565 12/1964 wada 148-143 oxide in the austenite, cooling said steel material to a temperature within the range of 900-500 C. at a rate of over 5 C./min., again heating said steel material to a temperature within the range of 500 C.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
US00134332A 1970-04-15 1971-04-15 Method of processing steel material having high austenitic grain-coarsening temperature Expired - Lifetime US3788903A (en)

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JP45032078A JPS5015209B1 (fr) 1970-04-15 1970-04-15

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US (1) US3788903A (fr)
JP (1) JPS5015209B1 (fr)
DE (1) DE2118154A1 (fr)
FR (1) FR2089709A5 (fr)
GB (1) GB1350784A (fr)
SE (1) SE375798B (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3928086A (en) * 1974-12-02 1975-12-23 Gen Motors Corp High strength ductile steel
US3930907A (en) * 1974-12-02 1976-01-06 General Motors Corporation High strength ductile hot rolled nitrogenized steel
US4334937A (en) * 1979-07-12 1982-06-15 Doryokuro Kakunenryo Kaihatsu Jigyodan Process for improving decarburization resistance of chrome-molybdenum steel in sodium

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3928086A (en) * 1974-12-02 1975-12-23 Gen Motors Corp High strength ductile steel
US3930907A (en) * 1974-12-02 1976-01-06 General Motors Corporation High strength ductile hot rolled nitrogenized steel
US4334937A (en) * 1979-07-12 1982-06-15 Doryokuro Kakunenryo Kaihatsu Jigyodan Process for improving decarburization resistance of chrome-molybdenum steel in sodium

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Publication number Publication date
DE2118154A1 (de) 1971-11-04
SE375798B (fr) 1975-04-28
JPS5015209B1 (fr) 1975-06-03
FR2089709A5 (fr) 1972-01-07
GB1350784A (en) 1974-04-24

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