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
  • C22C38/18—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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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

Definitions

• the present invention relates to structural hardening steel usable, in particular, for the manufacture of molds for material injection plastic.
• the molds for plastic injection are made up assemblies of parts machined from steel blocks so as to form a imprint having the form of the objects to be produced by molding.
• the objects are mass-molded and successive moldings cause surface wear of the imprint.
• the molds are out of order and must be replaced or repaired.
• Repair when it is possible, consists of reloading by welding followed by machining and polishing or chemical blasting of the surface of the impression. So that the repair by welding is possible, it is necessary, in particular, that the metal brought by welding and that the areas affected by the welding heat in the metal base have satisfactory properties.
• the ability to repair by welding is obtained, in particular, by using a steel with structural hardening treated by quenching and tempering.
• Structural hardening is obtained by adding to steel from 2% to 5% nickel and at least one element selected from aluminum and copper, in contents between 0.5% and 3%.
• the combined presence of nickel and copper or aluminum provides quenching and tempering bainitic or martensitic structure, the tensile strength of which is around 1400 MPa and the hardness of around 400 HB.
• the hardening resulting from precipitation during the income of intermetallic compounds, the content of carbon may be limited. This limited carbon content makes it possible to repair the welded parts without the hardness of heat affected areas significantly exceeds 400 HB.
• the chemical composition of steel includes, by weight, less than 0.25% carbon, less than 1% silicon, 0.9% to 2% manganese, 2% to 5% nickel, 0% to 18% chromium, 0.05% to 1% molybdenum, 0% to 0.2% sulfur, possibly titanium, niobium or vanadium in contents of less than 0.1%, possibly boron in contents of less than 0.005%, the remainder being iron and impurities resulting from processing.
• the chromium content is chosen to be greater than 8%.
• corrosion resistance is of no particular interest, and the chromium content remains below 2%.
• the object of the present invention is to remedy these drawbacks by proposing a steel, usable for the manufacture of molds for injection of plastic material, having a tensile strength Rm of the order of 1400 MPa, a hardness greater than 350 HB, and preferably greater than 380 HB, a good weldability, satisfactory machinability even for very thick materials important and making it possible to increase the productivity of the installations of injection molding by shortening the cooling times after injection.
• composition of the steel can advantageously be chosen in such a way that: 3.8 x C + 3.3 x Mn + 2.4 x Ni + ⁇ x Cr + 1.4 x (Mo + W / 2) ⁇ 8
• the chemical composition of the steel must be such that the manganese content is less than or equal to 0.7%, and, better still, less than or equal to 0.5%; Similarly, it is preferable that the silicon content either less than or equal to 0.1%.
• the chromium content should preferably be greater than or equal to 8%.
• the content of chromium should preferably be less than or equal to 5%, and more preferably less than or equal to 2%, and it is preferable that the steel contains boron.
• the invention also relates to a steel block according to the invention of characteristic dimension d greater than or equal to 20 mm, which has, at all points, either a martensitic, bainitic or martensito-bainitic structure, tempered, hardness greater than 350 HB.
• log (d) represents the decimal logarithm of the dimension characteristic d expressed in mm.
• the nitrogen content it is not always possible or desirable to limit the nitrogen content to less than 0.003%, in particular because it is expensive to remove the nitrogen supplied by the preparation.
• the nitrogen content cannot be limited to less than 0.003%, it is preferable to fix the nitrogen in the form of fine titanium or zirconium nitrides.
• the contents of titanium, zirconium and nitrogen are such that: 0.00003 ⁇ (N) x (Ti + Zr / 2) ⁇ 0.0016 and that the titanium or zirconium are introduced into the steel by progressive dissolution of an oxidized phase of titanium or zirconium, for example by effecting the addition of titanium or zirconium in non-deoxidized steel, then by adding a strong deoxidizer such as aluminum.
• the number of titanium or zirconium nitrides of size greater than 0.1 ⁇ m, counted over an area of 1 mm 2 of a micrographic section of solid steel is lower 4 times the sum of the total content of titanium precipitated in the form of nitrides and half the total content of zirconium precipitated in the form of nitrides, expressed in thousandths of%.
• the chemical composition of steel must, moreover, satisfy two conditions relating on the one hand to the quenchability and on the other hand to the conductivity thermal.
• the parts constituting the material injection molds plastic must be machined in blocks first tempered for their give a structure either entirely martensitic or entirely bainitic, either mixed martensito-bainitic, but, in any event, free of ferrite and perlite, then returned to harden them by precipitation of intermetallic compounds. Quenching can be done, for example, by water, oil or air cooling after austenitization, preferably, between 850 ° C and 1050 ° C, or directly in the hot forging or rolling. Tempering generally takes place between 500 ° C and 550 ° C.
• the blocks are, for example, rolled sheets or large forged plates whose thickness is greater than 20 mm and can range up to 800 mm, or even 1000 mm. Under these conditions, for the structure to be fully hardened, including the core of the blocks, the hardenability of the steel must be sufficient.
• the constant Bt which represents the minimum quenchability to be obtained must minus be equal to 3.1 and, for large thicknesses, at least equal to 4.1.
• each block has a characteristic dimension d which determines the cooling rate at the core for a determined cooling mode.
• log (d) represents the decimal logarithm of the dimension characteristic d expressed in mm. This characteristic dimension is, by for example, the thickness of a sheet or the diameter of a round bar.
• the inventors have found that it is possible to minimize the thermal conductivity of steel by choosing its chemical composition appropriately. This has the advantage of making it possible to increase the productivity of the plastic injection operations by shortening the cooling phase which follows the injection phase.
• the composition must be such that: 3.8 x C + 3.3 x Mn + 2.4 x Ni + ⁇ x Cr + 1.4 x (Mo + W / 2) ⁇ 8
• ⁇ 1.4 if the chromium content is less than 8%
• ⁇ 0 if the chromium content is greater than or equal to 8%.
• Kth is a dimensionless index varying in the same direction as the thermal resistivity of steel, i.e. inversely proportional to the thermal conductivity.
• the main difficulty consists in reconciling a sufficient hardenability to obtain the core of thick pieces desired mechanical characteristics, low manganese content for limit, or even avoid, the presence of segregated bands, and a resistivity the lowest possible thermal or, which is equivalent, a conductivity as high as possible (for steels which must resist corrosion, because of the high chromium content, the quenchability problem does not arise not).
• Kth / Tr is less than or equal to 3, preferably less than or equal to 2.8, and better still less than or equal to 2.5.
• mold parts were made for plastic injection, by machining sheets of thickness from 80 to 500 mm marked A, B, C, D, E, F, F1, G, H, I, J and J1.
• the sheets marked A to F1 were in accordance with the invention, and, for comparison, the sheets marked G to J1 were according to the prior art.
• the chemical compositions, in thousandths of% by weight are indicated in Table 1.
• the thicknesses d (in mm), the heat treatments, the thermal resistivity indices Kth, the thermal conductivity values Cth (in W / m / ° K) and the hardenability indices Tr (K and Tr are dimensionless indices) are shown in Table 2.
• the results reported in Table 2 show that the steels according to the invention have thermal conductivities of 10% (E compared to H) to 60% (F compared to J) stronger than those of steels according to the prior art. These higher thermal conductivities make it possible to significantly increase the productivity of the molds by reducing the duration of the cooling phases during the molding cycles.
• the steel F1 according to the invention has a thermal conductivity 30% higher than that of the steels J and J1 according to the prior art.
• the manganese content of the steel F1 is very significantly lower than that of these steels, which is very favorable for the reduction of segregation.
• the steel according to the invention is, in general, produced in the form of sheets rolled or in the form of bars or wide forged plates but it can, also, be manufactured in any other form, and, in particular, in the form of thread.
• the repair by welding must, preferably, be made with welding wires of composition close to the composition of the mold mass.
• the steel according to the invention is also manufactured in the form of welding wire.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Moulds For Moulding Plastics Or The Like (AREA)
• Heat Treatment Of Steel (AREA)
• Injection Moulding Of Plastics Or The Like (AREA)
• Heat Treatment Of Articles (AREA)
• Nonmetallic Welding Materials (AREA)

Applications Claiming Priority (2)

Publications (2)

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• 1996
  • 1996-03-01 FR FR9602595A patent/FR2745587B1/fr not_active Expired - Fee Related
• 1997
  • 1997-02-13 CA CA002197532A patent/CA2197532A1/fr not_active Abandoned
  • 1997-02-18 EP EP97400354A patent/EP0792944B1/fr not_active Expired - Lifetime
  • 1997-02-18 PT PT97400354T patent/PT792944E/pt unknown
  • 1997-02-18 ES ES97400354T patent/ES2176632T3/es not_active Expired - Lifetime
  • 1997-02-18 DE DE69713415T patent/DE69713415T2/de not_active Expired - Fee Related
  • 1997-02-18 AT AT97400354T patent/ATE219526T1/de not_active IP Right Cessation
  • 1997-02-27 KR KR1019970006308A patent/KR100451474B1/ko not_active IP Right Cessation
  • 1997-02-28 CN CN97109968A patent/CN1070241C/zh not_active Expired - Fee Related
  • 1997-02-28 JP JP9062279A patent/JPH1036938A/ja not_active Withdrawn
  • 1997-02-28 MX MX9701554A patent/MX9701554A/es unknown
  • 1997-03-03 US US08/805,851 patent/US5785924A/en not_active Expired - Fee Related
  • 1997-03-04 TW TW086102546A patent/TW367372B/zh not_active IP Right Cessation

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