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
  • C22C38/08—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
  • C22C38/44—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
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper

Definitions

• the present invention relates to a structural hardening steel which can be used, in particular, for the manufacture of molds for plastic injection.
• Molds for plastic injection consist of assemblies of parts machined from steel blocks so as to form an impression having the shape of objects to be produced by molding.
• the objects are molded in series and successive moldings cause wear on the surface of the imprint. After the manufacture of a certain number of objects, the molds are out of use and must be replaced or repaired.
• Repair when possible, consists of reloading by welding followed by machining and polishing or chemical blasting of the surface of the impression.
• 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 2% to 5% nickel and at least one element from aluminum and copper to the steel, in contents of between 0.5% and 3%.
• the combined presence of nickel and copper or aluminum makes it possible by quenching and tempering a 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 tempering of intermetallic compounds, the carbon content can be limited. This limited carbon content makes it possible to repair the parts by welding without the hardness of the areas affected by heat exceeding substantially 400 HB.
• the chemical composition of steel comprises, by weight, less than 0.25% of carbon, less than 1% of silicon, from 0.9% to 2% of manganese, from 2% to 5% of nickel, from 0% to 18% of chromium, from 0.05% to 1% of molybdenum, from 0% to 0.2% of sulfur, optionally 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 the production.
• the molds need to resist corrosion, and 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 aim of the present invention is to remedy these drawbacks by proposing a steel, usable for the manufacture of molds for plastic injection, having a tensile strength Rm of the order of 1400 MPa, a hardness greater than 350 HB , and preferably greater than 380 HB, good weldability, satisfactory machinability even for very large thicknesses and making it possible to increase the productivity of injection molding installations 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 is less than or equal to 0.1%.
• the chromium content should preferably be greater than or equal to 8%.
• the chromium content 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, a structure either martensitic, bainitic, or martensito-bainitic, tempered, of hardness greater than 350 HB.
• log (d) represents the decimal logarithm of the characteristic dimension 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 carrying out the addition of titanium or zirconium in non-deoxidized steel, then in 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 the steel must, moreover, satisfy two conditions relating on the one hand to the hardenability and on the other hand to the thermal conductivity.
• tensile strength of approximately 1400 MPa and hardness of the order of 400 HB that is to say at least greater than 350 HB, and preferably greater than 380 HB
• the parts constituting the plastic injection molds must be machined in blocks first hardened to give them a structure either entirely martensitic, or entirely bainitic, or mixed martensito-bainitic, but, in any condition cause, free of ferrite and perlite, then returned to harden by precipitation of intermetallic compounds.
• Quenching can be done, for example, by cooling with water, oil or air after austenitization, preferably between 850 ° C and 1050 ° C, or directly in 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 be at least equal to 3.1 and, for large thicknesses, at least equal to 4.1.
• each block has a characteristic dimension d which determines the core cooling rate for a specific cooling mode.
• log (d) represents the decimal logarithm of the characteristic dimension d expressed in mm. This characteristic dimension is, 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
• the chromium content is greater than or equal to 8%, it is adjusted essentially as a function of considerations relating to the resistance to corrosion. Otherwise, this content can be adjusted to maximize the thermal conductivity.
• Kth is a dimensionless index varying in the same direction as the thermal resistivity of steel, ie inversely proportional to the thermal conductivity.
• the essential difficulty consists in reconciling sufficient hardenability to obtain at the heart of thick parts the desired mechanical characteristics, a low manganese content to limit, or even avoid, the presence of segregated bands, and the lowest possible thermal resistivity or, which is equivalent, the highest possible thermal conductivity (for steels having to resist corrosion, because of the high chromium content, the problem of quenchability does not arise).
• the inventors have found that to obtain this optimum, it is desirable and possible to add an additional condition relating to the Kth / Tr ratio, by imposing that 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.
• Chemical compositions, in thousandths of% in weights are shown 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 steel according to the invention is, in general, manufactured in the form of rolled sheets or in the form of bars or large forged plates but it can, also, be manufactured in any other form, and, in particular, in the form of wire.
• the repair by welding should preferably be made with welding wires of composition close to the composition of the mass of the mold.
• 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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Citations (7)

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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 AT AT97400354T patent/ATE219526T1/de not_active IP Right Cessation
  • 1997-02-18 EP EP97400354A patent/EP0792944B1/de not_active Expired - Lifetime
  • 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 PT PT97400354T patent/PT792944E/pt unknown
  • 1997-02-27 KR KR1019970006308A patent/KR100451474B1/ko not_active IP Right Cessation
  • 1997-02-28 MX MX9701554A patent/MX9701554A/es unknown
  • 1997-02-28 JP JP9062279A patent/JPH1036938A/ja not_active Withdrawn
  • 1997-02-28 CN CN97109968A patent/CN1070241C/zh not_active Expired - Fee Related
  • 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

Patent Citations (7)

Non-Patent Citations (5)

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