EP0709481B1 - Low alloy steel for the manufacture of moulds for plastic materials or for rubber articles - Google Patents

Abstract

The low alloy steel comprises (wt.%): 0.24-0.35 C; 1-2.5 Mn; 0.3-2.5 Cr; 0.2-1.6 W; 0.1-0.8 (Mo+W/2); 0-25 Ni; 0-0.3 V; 0-0.5 Si; 0.002-0.005 B; 0.005-0.1 Al; 0-0.1 Ti; 0-0.02 P; and 0-2 Cu. There may also be present less than 0.1% of at least one of the following: Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and rare earth elements. The remainder is Fe plus possible impurities. The compsn. also satisfies the following equations: U = 409(%C) + 19.3(%Cr + %Mo + %W/2) + %V) + 29.4(%Si) + 10(%Mn) + 7.2(%Ni) < 200 and R = 3.82(%C) + 9.79(%Si) + 3.34(%Mn) + 11.94(%P) + 2.39(%Ni) + 1.43(%Cr) + 1.43(%Mo + W/2) < 11.14.

Description

The present invention relates to the use of a low alloy steel for the production of molds for plastics or for rubber.

The molds for plastics or for rubber are manufactured by machining massive metal blocks whose thickness can exceed 500mm. The surface of the impression obtained by machining is the more often either polished or chemically grained to give objects obtained by molding the desired surface appearance. In order to reduce maximum wear of the molds, any point on their surface must have a high hardness between 250HB and 400HB and most often between 270HB and 350HB. They must also have the most elastic limit high possible and good resilience to resist shocks and deformations.

The machining operation is very important, since it commonly represents 70% of the total mold manufacturing cost, the metal must be as machinable as possible and, very often, the ability to machining cannot be obtained by conventional additions too important such as Sulfur or Lead, because these additions deteriorate aptitude for polishing or graining.

The molds are quite often repaired by welding, the metal used must also be as weldable as possible.

Finally, the molding of plastics or rubber being hot, the metal used must have a thermal conductivity as high as possible in order to facilitate heat transfers which limit the productivity of molded object manufacturing.

To make the molds, blocks are generally used low-alloy steel sufficiently quenching to obtain, after quenching and returned a martensitic or martensito-bainitic structure having a sufficient hardness, high yield strength, good toughness.

The most used steel is P20 steel according to the AISI standard or W1.2311 or W1.2738 steels according to the German WERKSTOFF standard.

P20 steel contains, by weight, from 0.28% to 0.4% of Carbon, from 0.2% to 0.8% of Silicon, from 0.6% to 1% of Manganese, from 1.4% to 2% chromium, from 0.3% to 0.55% molybdenum, the rest being iron and impurities linked to processing.

W1.2311 and W1.2738 steels contain, by weight, 0.35% to 0.45% of Carbon, from 0.2% to 0.4% of Silicon, from 1.3% to 1.6% Manganese, 1.8% to 2.10% Chromium and 0.15% to 0.25% Molybdenum; W1.2738 steel also contains from 0.9% to 1.2% of Nickel, the rest being iron and impurities linked to the production.

These steels have good wear resistance, but they have a weldability, workability, toughness and conductivity insufficient heat.

In order to improve the weldability, it has been proposed, in patent application EP 0 431 557, a steel containing, by weight, from 0.1% to 0.3% of carbon, less than 0.25% of Silicon, 0.5% to 3.5% Manganese, less than 2% Nickel, 1% to 3% Chromium, 0.03% to 2% Molybdenum, 0.01% to 1% Vanadium, less than 0.002% Boron, an element considered to be a harmful impurity, the rest being substantially iron; the composition must also satisfy the relationship: BH = 326 + 847.3 (% C) +18.3 (% Si) -8.6 (% Mn) -12.5 (% Cr) ≤460

Given this relationship, the Carbon content must stay below 0.238%.

This steel which certainly has good weldability and machinability acceptable, however has insufficient thermal conductivity.

In fact, the skilled person always chooses an analysis located inside the ranges indicated so as to obtain a hardenability sufficient to be able to produce pieces of thickness which may exceed 400mm; in particular the different elements can never be simultaneously at the bottom of the forks. Therefore all these steels have a thermal conductivity below 35W / m / K and when, in some molds, it is necessary to have certain parts whose conductivity is significantly higher, we realize the parts corresponding Copper / Aluminum / Iron alloy with conductivity thermal is greater than 40W / m / K. But this technique presents the disadvantage of complicating the manufacture of molds since these are so composite objects, moreover the alloys used are very more expensive than steel.

The object of the invention is to propose a steel for the manufacture of molds for plastics or for rubber which, all having at least the same mechanical properties and ability to the machining of known steels, has a thermal conductivity greater than 40W / m / K to allow in particular to manufacture molds entirely in steel.

To this end, the subject of the invention is the use, for the manufacture of molds for plastics or for rubber, of a low alloy steel whose chemical composition comprises by weight: 0.24% ≤ C ≤ 0.35% 1% ≤ Mn ≤ 2.5% 0.3% ≤ Cr ≤ 2.5% 0.1% ≤ Mo + W / 2 ≤ 0.8% Ni ≤ 2.5% 0% ≤ V ≤ 0.3% If ≤ 0.5% 0.002% ≤ B ≤ 0.005% 0.005% ≤ Al ≤ 0.1% 0% ≤ Ti ≤ 0.1 P ≤ 0.02% Cu ≤ 2% optionally, at least one element chosen from Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and Rare earths, in contents of less than 0.1%, the rest being iron and impurities resulting development,
the chemical composition satisfying, in addition, the following relationships: U = 409 (% C) + 19.3 [% Cr +% Mo +% W / 2 +% V] + 29.4 (% Si) + 10 (% Mn) + 7.2 (% Ni) <200 and, R = 3.82 (% C) + 9.79 (% Si) + 3.34 (% Mn) + 11.94 (% P) + 2.39 (% Ni) + 1.43 (% Cr) + 1.43 (% Mo +% W / 2) <11.14

Preferably the chemical composition of the steel is such that: 0.24% ≤ C ≤ 0.28% 1% ≤ Mn ≤ 1.3% 1% ≤ Cr ≤ 1.5% 0.3% ≤ Mo + W / 2 ≤ 0.4% 0.03% ≤ V ≤ 0.1%

Preferably, the silicon content of the steel is less than 0.1%.

Steel can also add copper to obtain a additional hardening during tempering, the steel must then contain 0.8% to 2.5% Nickel and 0.5% to 2% Copper.

In general, the manufacture of molds for plastics or for rubber is done by machining of quenched hardened steel blocks whose hardness is between 270HB and 350HB.

The invention will now be described with reference to FIG. 1 which represents a measurement diagram of machinability in drilling according to the Taylor method.

• plus de 0,24%C pour obtenir après trempe et revenu à plus de 500°C, une dureté supérieure à 270HB, et moins de 0,35%C pour ne pas trop détériorer la soudabilité et pour limiter l'importance des ségrégations défavorables à l'usinabilité, à la polissabilité et à la grainabilité ; de préférence, la teneur en Carbone doit être comprise entre 0,24% et 0,28%.
• plus de 1 % de Manganèse pour augmenter la trempabilité de l'acier et moins de 2,5% et de préférence moins de 1,3% pour éviter de trop diminuer la conductibilité thermique de l'acier.
• plus de 0,3% de Chrome également pour augmenter la trempabilité et notamment éviter la formation de phases ferrito-perlitiques défavorables à la polissabilité et moins de 2,5% afin de ne pas détériorer la soudabilité et d'éviter la formation d'une quantité trop importante de carbures de Chrome défavorables notamment à l'usinabilité ; de préférence la teneur en Chrome doit être comprise entre 1 % et 1,5% .
• plus de 0,1% et de préférence plus de 0,3% de Molybdène pour augmenter la trempabilité et pour ralentir l'adoucissement au revenu, mais moins de 0,8% et de préférence moins de 0,4% car, en trop grande quantité le Molybdène forme des carbures très durs défavorables à l'usinabilité, et il ségrège fortement en veines ce qui est défavorable à la polissabilité, à la grainabilité et peut également engendrer des ruptures d'outils au cours de l'usinage. Le Molybdène peut être remplacé totalement ou partiellement par du Tungstène à raison de 2% de Tungstène pour 1% de Molybdène, si bien que la teneur à prendre en compte est Mo +W/2.
• entre 0% et 0,3% et de préférence entre 0,03% et 0,1% de Vanadium afin de produire un durcissement secondaire au cours du revenu.
• entre 0,002% et 0,005% de Bore accompagné de 0,005% à 0,1% d'Aluminium et de 0% à 0,1% de Titane de façon à augmenter significativement la trempabilité sans détériorer les autres propriétés. L'aluminium et le Titane servent à éviter que le Bore ne se combine à l'Azote presque toujours en quantité telle qu'il faut protéger le Bore.

The steel according to the invention is a low-alloy steel containing mainly, by weight:

• more than 0.24% C to obtain, after quenching and tempering at more than 500 ° C, a hardness greater than 270HB, and less than 0.35% C so as not to deteriorate the weldability too much and to limit the extent of unfavorable segregations machinability, polishability and grainability; preferably, the carbon content must be between 0.24% and 0.28%.
• more than 1% of manganese to increase the hardenability of the steel and less than 2.5% and preferably less than 1.3% to avoid reducing the thermal conductivity of the steel too much.
• more than 0.3% of chromium also to increase the hardenability and in particular to avoid the formation of ferrito-pearlitic phases unfavorable for polishability and less than 2.5% in order not to deteriorate the weldability and to avoid the formation of a too large a quantity of chromium carbides unfavorable in particular to machinability; preferably the chromium content should be between 1% and 1.5%.
• more than 0.1% and preferably more than 0.3% of molybdenum to increase the hardenability and to slow the softening on tempering, but less than 0.8% and preferably less than 0.4% because, in excess large quantity Molybdenum forms very hard carbides unfavorable to machinability, and it segregates strongly in veins which is unfavorable to polishability, to grainability and can also generate tool ruptures during machining. Molybdenum can be completely or partially replaced by Tungsten at the rate of 2% of Tungsten for 1% of Molybdenum, so that the content to be taken into account is Mo + W / 2.
• between 0% and 0.3% and preferably between 0.03% and 0.1% of Vanadium in order to produce a secondary hardening during tempering.
• between 0.002% and 0.005% of Boron accompanied by 0.005% to 0.1% of Aluminum and from 0% to 0.1% of Titanium so as to significantly increase the quenchability without deteriorating the other properties. Aluminum and Titanium are used to prevent the Boron from combining with the Nitrogen almost always in such a quantity that it is necessary to protect the Boron.

• moins de 0,02% de Phosphore qui est une impureté fragilisante.

For these additions to be effective, when the nitrogen content is greater than 50 ppm the aluminum content must be greater than 0.05% when the titanium content is less than 0.005%; when the titanium content is greater than 0.015%, the aluminum content may be less than 0.03% and preferably be between 0.020% and 0.030%.

• less than 0.02% of Phosphorus which is an embrittling impurity.

Besides these main elements of chemical composition, steel contains or may contain elements such as silicon, Copper, Nickel either as impurities or as alloying elements complementary.

Steel, especially when made from scrap contains a little Copper and Nickel. When the nickel is weak quantity, the copper in too high contents creates defects during hot rolling or hot forging because it weakens the joints of grain. In the absence of specific additions, the nickel and copper contents remain below 0.5% each

Up to 2.5% Nickel can be added to increase the hardenability.

You can also add Copper to produce a structural hardening. In this case, the copper content must be between 0.5% and 2% and be accompanied by a nickel content between 0.8% and 2.5%.

The hardness can also be adjusted by additions of Niobium in contents lower than 0.1%.

When the requirements for suitability for polishing or grinding allow us to improve machinability by adding Sulfur, Tellurium, Selenium, Bismuth, Calcium, Antimony, Lead, Indium, Zirconium or Rare earths in contents lower than 0.1%.

The inventors have found that, in this area of chemical composition, when: U = 409 (% C) + 19.3 [% Cr + (% Mo +% W / 2) +% V] + 29.4 (% Si) + 10 (% Mn) + 7.2 (% Ni) <200 the machinability is very significantly better than for P20 type steels.

Finally, for the thermal conductivity to be sufficient, it is necessary that: R = 3.82 (% C) + 9.79 (% Si) + 3.34 (% Mn) + 11.94 (% P) + 2.39 (% Ni) + 1.43 (% Cr) + 1.43 (% Mo +% W / 2) <11.14 Also, the chemical composition must be chosen so that U <200 and R <11.14. The thermal conductivity is then greater than 40W / m / K

To manufacture a mold, a steel is produced, the composition of which defined in claim 1 is disclosed in document JP-A-5 302 117 possibly by pre-oxidizing with Silicon, then a deoxidation to aluminum, then titanium and boron are added.

The liquid metal thus obtained is poured in the form of a semi-finished product such as an ingot, a slab or a billet.

The semi-finished product is then reheated to a temperature of preferably less than 1300 ° C and either forged or rolled to obtain a bar or sheet metal.

The bar or the sheet is then soaked to obtain in all its mass a martensitic or martensito-bainitic structure.

The quenching can be done either directly in the rolling or forging hot if the end of rolling or end of forging temperature is less than 1000 ° C., or after austenitization at a temperature above point Ac ₃ and preferably below 1000 ° C.

After quenching in air, oil or water depending on the dimensions, bars or sheets are subjected to tempering at temperatures above 500 ° C and preferably above 550 ° C so as to obtain a hardness between 270HB and 350HB, and preferably close to 300HB, in all points of bars or sheets and so that internal stresses generated by quenching are relaxed.

We then cut blocks of desired size which are machined in particular to form the imprint of the object you want obtain by molding.

The surface of the imprint can then be subjected to a surface treatment such as polishing or graining to give it the desired surface appearance and possibly be nitrided or chromed.

C = 0,25%

Si = 0,25%

Mn = 1,1%

Cr = 1,3%

Mo = 0,35%

Ni = 0,25%

V = 0,04%

Cu = 0,3%

B = 0,0027%

Al = 0,025%

Ti = 0,020%

S = 0,001%

P = 0,010%

For example, molds were made with steel A of composition: (% by weight)

C = 0.25%

If = 0.25%

Mn = 1.1%

Cr = 1.3%

Mo = 0.35%

Ni = 0.25%

V = 0.04%

Cu = 0.3%

B = 0.0027%

Al = 0.025%

Ti = 0.020%

S = 0.001%

P = 0.010%

400 mm thick blocks were produced, austenitized at 900 ° C. for 1 hour, soaked in water and then returned to 550 ° C. for 1 hour and cooled in air. There was thus obtained a martensito-bainitic structure of hardness between 300HB and 318HB at all points of the product. The elastic limit Re is 883MPa and the breaking strength Rm is 970MPa, ie a Re / Rm ratio close to 0.91; the KCV resilience at + 20 ° C is around 60J / cm ² .

The equivalent carbon of this steel calculated according to the IIW formula. Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 East : Ceq = 0.808 the BH index is: BH = 508 the machinability index is: U = 151 the thermal conductivity is: λ = 41Wm -1 K -1

C = 0,34%

Si = 0,45%

Mn = 0,95%

Cr = 1,85%

Ni = 0,3%

Mo = 0,38%

après austénitisation à 900°C, trempe à l'eau et revenu à 580°C pendant 1 heure, la dureté était comparable et centrée autour de 300HB. La limite d'élasticité Re était de 825 MPa et la résistance à la rupture Rm de 1010 MPa soit un rapport Re/Rm voisin de 0,82. La résilience KCV à +20°C était de l'ordre de 20J/cm².By way of comparison, a block of the same dimension produced in a P20 type steel of composition,

C = 0.34%

If = 0.45%

Mn = 0.95%

Cr = 1.85%

Ni = 0.3%

Mo = 0.38%

after austenitization at 900 ° C, quenching with water and returning to 580 ° C for 1 hour, the hardness was comparable and centered around 300HB. The elastic limit Re was 825 MPa and the breaking strength Rm of 1010 MPa, ie a Re / Rm ratio close to 0.82. The KCV resilience at + 20 ° C was of the order of 20J / cm ² .

The equivalent Carbon was: Ceq = 0.964

The BH coefficient: BH = 591

The machinability index: U = 207

Thermal conductivity: λ = 35Wm -1 K -1

The difference in machinability index U results in a difference in machinability as shown in fig. 1 which represents Taylor straight lines in drilling for steel A and for steel P20 taken into account example. It can be seen in this figure that at equal cutting speed, the length that can be drilled in steel A is about 10 times longer important than in P20 steel, or, for an equal pierced length, the speed allowable cutting is 25% greater in steel A than in P20 steel.

The weldability being all the better as the Carbon equivalent or the coefficient BH is low we see that the steel according to the invention has better weldability than P20 steel.

It can be seen that steel A has a thermal conductivity of 17% higher than that of P20 steel, moreover it has a yield strength and a much higher resilience than that of P20 steel.

C = 0,17%

Si = 0,09%

Mn = 2,15%

Cr = 1,45%

Mo = 1,08%

V = 0,55%

B = 0,0007%

Also for comparison, a block of comparable size was made of steel of composition:

C = 0.17%

If = 0.09%

Mn = 2.15%

Cr = 1.45%

Mo = 1.08%

V = 0.55%

B = 0.0007%

After austenitization at 900 ° C, quench with water, and returned to 570 ° C, the block had a hardness close to 300HB throughout the mass and:

The equivalent Carbon was: Ceq = 1.144

The BH coefficient was: BH = 435

Machinability index U U = 153

Thermal conductivity: λ = 35Wm -1 K -1

This steel has a BH index better than that of steel A but it has a worse equivalent Carbon. Its machinability index is comparable to that of steel A but its thermal conductivity is more 15% low.

We also made 400mm thick blocks in steel B according to the invention austenitized at 920 ° C, quenched with water and tempered at 560 ° C then air-cooled. The hardness at all points was between 300HB and 315HB. The yield strength Re was 878MPa, and the strength at break Rm of 969MPa, i.e. a Re / Rm ratio of 0.91.

C = 0,25%

Si = 0,1%

Mn = 1,3%

Cr = 1,3%

Mo = 0,4%

V = 0,01%

B = 0,0025%

Al = 0,055%

S = 0,002%

P = 0,015%

Ni = 0,8%

Cu = 0,35%

The composition of the steel was:

C = 0.25%

If = 0.1%

Mn = 1.3%

Cr = 1.3%

Mo = 0.4%

V = 0.01%

B = 0.0025%

Al = 0.055%

S = 0.002%

P = 0.015%

Ni = 0.8%

Cu = 0.35%

The equivalent carbon was: Ceq = 0.83

The BH coefficient was: BH = 512

The machinability index was: U = 153

Thermal conductivity: λ = 44Wm -1 K -1

This steel, whose analysis differs from that of steel A mainly by the content of Silicon and Nickel presents the same advantages than steel A and moreover it has a good thermal conductivity better.

Claims (5)

1. Use for the manufacture of a plastics or rubber mould of a steel whose chemical composition consists, by weight, of: 0.24% ≤ C ≤ 0.35% 1% ≤ Mn ≤ 2.5% 0.3% ≤ Cr ≤ 2.5% 0.1% ≤ Mo + W/2 ≤ 0.8% Ni ≤ 2.5% 0% ≤ V ≤ 0.3% Si ≤ 0.5% 0.002% ≤ B ≤ 0.005% 0.005% ≤ Al ≤ 0.1% 0% ≤ Ti ≤ 0.1% P ≤ 0.02% Cu ≤ 2% optionally at least one element taken from Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and rare earths, in amounts of less than 0.1%, the balance being iron and impurities resulting from smelting, the Ni content being between 0.8% and 2.5% when the Cu content is between 0.5% and 2%, the chemical composition furthermore satisfying the following relationships: U = 409(%C) + 19.3[%Cr + (%Mo + %W/2) + %V] + 29.4(%Si) + 10(%Mn) + 7.2(%Ni) < 200 and R = 3.82(%C) + 9.79(%Si) + 3.34(%Mn) + 11.94(%P) + 2.39(%Ni) + 1.43(%Cr) + 1.43(%Mo + %W/2) < 11.14
2. Use for the manufacture of a plastics or rubber mould of a steel according to Claim 1, characterized in that the chemical composition of the steel is such that: 0.24% ≤ C ≤ 0.28% 1% ≤ Mn ≤ 1.3% 1% ≤ Cr ≤ 1.5% 0.3% ≤ Mo + W/2 ≤ 0.4% 0.3% ≤ V ≤ 0.1%
3. Use for the manufacture of a plastics or rubber mould of a steel according to Claim 1 or Claim 2, characterized in that: Si ≤ 0.1%.
4. Use for the manufacture of a plastics or rubber mould of a steel according to any one of Claims 1 to 3, characterized in that it also contains: 0.8% ≤ Ni ≤ 2.5% 0.5% ≤ Cu ≤ 2%
5. Use for the manufacture of a plastics or rubber mould of a steel according to any one of Claims 1 to 4, characterized in that the manufacture is carried out by machining blocks which are quenched and tempered, and of hardness between 270 HB and 350 HB.

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