Classifications

• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere

Definitions

• the invention relates to the field of heat treatments of alloy steels. It relates more particularly to the annealing of carbon steels rich in chromium and manganese.
• Carbon steels rich in chromium and manganese such as grades 38Cr4 (with approximately 1% chromium and 0.6% manganese) or 100 C6 (with 1.7% chromium and 0.3% manganese) are used in various applications.
• grades 38Cr4 with approximately 1% chromium and 0.6% manganese
• 100 C6 with 1.7% chromium and 0.3% manganese
• Annealing was also carried out in the laboratory under a reducing nitrogen-hydrogen atmosphere and under pure hydrogen, in order to eliminate any trace of oxygen which could contribute to forming scale by oxidation of the wire. But if such atmospheres are neutral or reducing for iron, in the case of annealing under scale they are oxidizing for other elements such as silicon, manganese and chromium, which is just as bad for the quality of the pickling. Moreover, the additional cost caused by their use, as well as the safety problems posed by hydrogenated atmospheres, make these solutions difficult to envisage for the industrial manufacture of the products concerned.
• the object of the invention is to propose a method for treating carbon steels rich in chromium and in manganese which guarantees a good progress of the phosphating operation, without requiring penalizing modifications of the annealing and pickling stages such as they are currently practiced.
• the subject of the invention is a process for annealing carbon steel steel products, the chromium content of which is between 0.15 and 1.5% by weight and the manganese content of which is between 0, 15 and 1.5% by weight, according to which said products are left to remain in an annealing furnace under a nitrogen atmosphere to cause globulation of the perlite, characterized in that oxygen is added to said atmosphere. content greater than or equal to 0.5% by volume.
• the invention consists in deliberately increasing the oxygen content of the annealing atmosphere, which goes against the solutions which have hitherto been envisaged.
• the metallurgical justification of this characteristic at first sight surprising, will now be exposed.
• this layer of spinels is extremely adherent to the substrate, and that pickling in a sulfuric medium cannot completely remove it. It constitutes an obstacle to a homogeneous phosphating of the substrate, which explains the problems of wear of the tools encountered during the shaping of the material. These problems of pickling are reduced if pickling is carried out in a hydrochloric medium, but this solution is not necessarily applicable on all existing installations. It is therefore necessary to find operating conditions such that, during the annealing operation, the formation of the spinel layer is not observed.
• the inventors' idea consists in creating, during the annealing, operating conditions such as the competition between the formation of conventional scale based on iron oxides, easily pickled, and the formation of spinels turns to the advantage of the first. of these reactions. This is achieved by deliberately imposing a significant presence of gaseous oxygen in the annealing atmosphere. We then observe at the scale-substrate interface a progression of the front of iron oxides more or less enriched in chromium oxides, by reacting iron from the substrate with gaseous oxygen. This reaction occurs at a higher rate than the solid state reaction between the iron oxides and the iron, chromium and manganese elements of the substrate, and therefore succeeds in preventing it.
• the invention is not limited in its application to the steel grades which have been mentioned. It can be applied to all grades of carbon steel rich in chromium (0.15 to 1.5%) and manganese (0.15 to 1.5%), which can give rise to spinels adhering to the metal-scale interface during annealing carried out in a neutral or reducing atmosphere.
• the conditions of The temperature and duration of the annealing operation may vary depending on the exact grade of the steel, the format of the products treated and the technology of the furnace.
• This annealing is designed as a globulization annealing of the perlite, and is conducted in the prescribed atmosphere (nitrogen containing at least 0.5% by volume of oxygen) on steel products whose shade corresponds to the description which has just been made.
• such globulation annealing includes, industrially, a heating step at a maximum temperature below the steel temperature Acl, followed by very slow cooling, and last between 12 and 30 hours.

Landscapes

• 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 Strip Materials And Filament Materials (AREA)

Applications Claiming Priority (2)

Publications (1)

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Citations (6)

• 1993
  • 1993-12-13 FR FR9315138A patent/FR2713661B1/fr not_active Expired - Fee Related
• 1994
  • 1994-11-18 EP EP94470039A patent/EP0657551A1/de not_active Withdrawn
  • 1994-12-06 US US08/354,095 patent/US5536338A/en not_active Expired - Fee Related

Patent Citations (6)

Non-Patent Citations (2)

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