MXPA97003092A - Debilmente alloy steel for the molding of molds for plasti materials - Google Patents

Abstract

The present invention relates to steel for the manufacture of molds by injection of plastic materials, characterized in that its chemical composition contains by weight: 0.35% < - C 5 0.5%; 0% < - Yes < - 0.5%, 0.2% < - Mn < - 2.5%; 0% < - Ni < - 4%; 0% < - Cr < - 4%; 0% S Mo + W / 2 < - 2%; 0% < - Cu < - 1%; 0% < - V + Nb / 2 + Ta / 4 < - 0.5%; 0.005% < - Al < - 0.2%; 0% < - B < - 0.015%; 0% < - Ti + Zr / 2 < - 0.3%; 0% < - S + Se + Te < - 0.2%; 0% < - Pb + Bi < - 0.2%; 0% < - Ca < - 0.1%, the rest being iron and impurities resulting from processing. Satisfying the analysis, simultaneously, the following relationships: Cr + 3x (Mo + W / 2) + 10x (V + Nb / 2 + Ta / 4) - > 1; R = 3.8xC + 10xSi + 3.3xMn + 2.4Ni + 1.4x (Cr + Mo + W / 2) < - 11, Tr = 3.8xC + 1.1xMn + 0.7xNi + 0.6xCr + 1.6x (Mo + W / 2) + KB - > 3, with KB = 0.5 if B - > 0.0005% and KB = 0 otherwise, and: R < - Max (2.33xTr-1; 0.9xTr +

Description

ADHERED STEEL FOR THE MANUFACTURE OF MOLDS TO MATUÍ IATIR "PLÁSTICOS DESCRIPTION OF THE INVENTION The present invention relates to a weakly alloyed steel used especially for the manufacture of molds for plastic materials. Molds for plastic or rubber materials are manufactured by machining solid metal blocks whose thickness can reach 1.5 meters. The main purpose of the machining is to form a print that has the shape of the object to be obtained by molding. The surface of the trace obtained by machining is almost always polished or chemically grained, in order to give the objects obtained by molding the desired surface appearance. By making the injection molding operation of hot plastic material, the mold must resist, on the one hand, the stresses generated by the pressure of the plastic material, on the other hand, evacuate the heat of the plastic material as quickly as possible to increase the productivity of the molding operations and, finally, resist the wear generated by the friction of the plastic material on the surface of the footprint. In addition, the characteristics of the steel must remain stable during REF: 24633 its use, that is, to be insensitive to the thermal cycles generated by the molding operations and to remain as homogeneous as possible, even in the thickest sections. To satisfy all these conditions, it would be necessary to have, for the manufacture of molds by injection of plastic materials, a steel that had, mainly and at the same time, a great hardness, a very good machining ability, a very good aptitude to polishing or chemical graining, a good thermal conductivity and a very large homogeneity of all the characteristics, even for the greatest thicknesses. Such ideal steel is not known. To manufacture the molds, they use, in general, blocks of weakly alloyed steel, sufficiently tempered, to obtain, after tempering and tempering, a martensitic or bainic structure, substantially free of ferrite, having a sufficient hardness, a high elastic limit. , and a good t * »pa idad. The most useful steels are P20 steel according to the AISI standard and W1 steels. J1I • Wl.2738 according to the German standard ERKSTOFF. Steel P20 contains, jr. 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% of Chromium, from 0.3% to 0.55% of Molybdenum, the rest being iron and impurities resulting from processing. The steels W1.2311 and W1.2738 contain, by weight, from 0.35% to 0.45% Carbon, from 0.2% to 0.4% Silicon, from 1.3% to 1.6% of Manganese, from 1.8% to 2.10% of Chromium and of 0.15% to 0.25% of Molybdenum; W1.2738 steel also contains 0.9% to 1.2% nickel, the rest being iron and impurities bound in the process. These steels have a good wear behavior. However, the inventors have found, mainly, that the combination of their characteristics of thermal conductivity and hardenability was insufficient. Steel W1.2738 is significantly more hardenable than steel W1.2311, which, in turn, is more hardenable than steel P20. However, steel 1.2378 has a thermal conductivity that is noticeably lower than that of steel 1.2311, which in turn is significantly lower than that of steel P20. Because the steel that has the best thermal conductivity has the worst hardenability, and conversely, it is not possible to manufacture very solid molds that have a good thermal conductivity. If you want to make a large mold that has a good thermal conductivity, you should use steel P20 instead of steel 1.2738, and design the mold in several pieces relatively little massive, instead of conceiving in a single piece very solid. But this technique is never used, nor is it even planned, because it would have the disadvantage of considerably complicating the molds and increasing the cost of their manufacture. Indeed, it would be necessary to ensure a perfect fit of the different pieces to prevent marks from forming on the surface of the objects manufactured by molding, and this is not practically possible. The object of the present invention is to remedy this drawback, proposing a steel that presents, at the same time, the main characteristics desired for a steel intended for the manufacture of molds and, mainly, a very good resistance to wear, and a combination of characteristics of hardenability and thermal conductivity better than what allows to obtain the previous technique. For this purpose, the invention relates to a steel for the manufacture of molds by injection of plastics or rubber, whose chemical composition contains, by weight: 0.35% < C =. 0.5% 0% < Yes =. 0.5% 0.2% < Mn =. 2.5% 0% =. Ni < L 4% 0 i Cr < 4% 0% =. Mo + W / 2 < 2% 0% < Cu =. 1% 0% < V + Nb / 2 + Ta / 4 < 0.5% 0.005% < At 0.2% 0% < L B < L 0.015% optionally, at least one element taken between titanium and zirconium, in contents such that the sum of the titanium content and half of the zirconium content is less than or equal to 0.3%, optionally, at least one element taken in between sulfur, selenium and tellurium, the sum of the contents of these elements being less than or equal to 0.2%, - possibly, at least one element taken between lead and bismuth, the sum of the contents of these being elements less than or equal to 0.2%, optionally calcium in a content less than or equal to 0.1%, the remainder being iron and impurities resulting from processing; In the analysis, simultaneously the following relations are expressed (the contents being expressed in% weight): Cr + 3x (Mo + W / 2) + lOx (V + -Nb / 2 + Ta / 4) > 1 R = 3.8xC + lOxSi + 3.3xMn + 2.4xNi + 1, 4x (Cr + Mo + / 2) =. 11 Tr = 3.8xC + l, lxMn + 0, 7xNi + 0.6xCr + l, 6x (Mo + W / 2) + kB > 3 (with kB = 0.5 if B> 0.0005% and kB = 0 if not) and, R Max (2.33xTr-l; 0.9xTr + 4) Preferably, the chemical composition is such that Mo + W / 2 > 0.4% It is also preferable that Si =. 0.15%, and better yet, that Si < . 0.1%. Finally, it is desirable that the boron content be greater than or equal to 0.0005%. Preferably, the composition of the steel should be chosen in such a way that R 9 and, optionally, Tr 4, 3. The preceding conditions are obtained in a particularly favorable manner when the chemical composition of the steel is such that: 0.37% =. C =. 0.45% 0% < _ If < 0, 15% 0.2% =. Mn =. 1.5% 0% < _ Ni =. 0, 5% 0% < Cr =. 2, 5% 0% =. Mo + W / 2 =. 1% 0% < Cu =. 1% 0% < V + Nb / 2 + Ta / 4 =. 3.2% 0.005% =. Al =. 0.2% 0.0005% =. 3 =. 0, 015 ° s 0% < Ti + Zr / 2 =. 0.3% satisfying the composition, in addition, the relationship: R = 2.33xTr - 2.4 When the steel contains titanium or zirconium, which is preferable, it is desirable that, during processing, the titanium or zirconium be progressively introduced into the liquid steel so that a very fine precipitation of the titanium or zirconium nitrides is obtained. . It is necessary, then, that the contents in titanium, zirconium and nitrogen 'n% by weight) are, preferably, such that: 0.00003 =. (N) x (Ti * Zr / 2) =. 0.0016 Under these conditions, * »n solid state, the number of nitride precipitates«? -: - * child or zirconium larger than 0.1 μm counted ••:! an area of 1 mpv2 of a micrographic cut is less than 4 times the sum of the total titanium content precipitated in the form of nitrides and of half the total content n zirconium precipitated in the form of nitrides, expressed -n -nillions of% in weigh.

Finally, the invention relates to a steel block whose chemical composition is in accordance with the invention, the characteristic dimension being comprised between approximately 20 mm and 1,500 mm, the steel structure being, at any point of the block, martensitic or bainitic, essentially free of ferrite, and the hardness being included, at any point of the block, between 250 and 370 Brinell and, preferably, between 270 and 350 Brinell. The steel according to the invention can also be used for the manufacture by casting molds for plastic materials. The invention will now be described in more detail, but in a non-limiting manner, and will be illustrated by the following examples. In order to obtain all the necessary properties of use for the manufacture of molds by injection of plastic materials, the composition of the steel must comprise, by weight: - more than 0.35% C to obtain, at the same time, a sufficient hardness and a very good resistance to wear, which, with equal hardness,. the better the carbon content is, the better; however, the carbon content must not be too high to maintain sufficient mechanability and tenacity; also, the carbon content must remain less than or equal to 0.5%; preferably, the carbon content should be between 0.37% and 0.45%. preferably, more than 0.4% molybdenum, to increase the hardenability and, mainly, to increase the extension of the bainitic field, which allows to obtain a good homogeneity of the characteristics in the blocks of great thickness; molybdenum also has the advantage of increasing the resistance to tempering and of forming carbides with carbon, which, with equal resistance, increase the wear resistance; however, the molybdenum content does not need to be higher than 2% to obtain the full effect of this element and, preferably, the molybdenum content is limited to 1%, mainly because this element is very expensive and because it favors the formation of unfavorable segregations for machinability and for polishing ability; molybdenum can be totally or partially replaced by tungsten at a rate of 2% tungsten to 1% molybdenum, the conditions of chemical composition are thus defined by the magnitude Mo + W / 2; between 0% and 4% of chromium and, preferably, between 1.5% and 2.5%, to improve the hardenability and improve the resistance to softening and wear, without deteriorating too much the thermal conductivity; optionally, vanadium, niobium or tantalum, alone or in combination, the sum being comprised of V + Nb / 2 + Ta / 4 between 0% and 0.5% and, preferably, less than or equal to 0.2%, for increase hardness and improve wear resistance; more than 0.2% Manganese, which has the advantage of fixing the sulfur and increasing the hardenability, but has the disadvantage of appreciably reducing the thermal conductivity of the steel, also its content is limited to 2.5% and, preferably, , at 1.5%; less than 0.5% and, preferably, less than 0.15% and, better still, less than 0.1% silicon, element used for the deoxidation of liquid steel, but which has the drawback of greatly increasing the resistivity of steel and favoring the formation of mesosegregations on large blocks; preferably, the silicon content should be as low as possible; optionally, nickel, in contents comprised between 0% and 4%, to increase the hardenability and, preferably, less than 0.5% in order not to reduce too much the thermal conductivity; this element can also be present as a residual element when the steel is made from scrap; eventually, between 0% and 1% copper, a residual element contributed by the raw materials, but which can have a favorable hardening effect when accompanied by nickel; between 0.005% and 0.2% of aluminum to deoxidize the steel and, if necessary, contribute to the protection of quenching boron; - at least one element taken between titanium and zirconium, the sum of the titanium content and half the zirconium content less than or equal to 0.3%, in order to retain the nitrogen that the steel always contains and which could neutralize the effect of boron; when the steel contains titanium or zirconium, it is preferable that the titanium, zirconium and nitrogen contents (element always present, at least as an impurity, in contents between some ppm and a few hundred ppm) are such that: 0.00003 =. ? N) x (T? * Zr / 2) _ 0.0016 and that titanium and zirconium are introduced into the steel by progressive dissolution of an oxidized phase of titanium or zirconium, for example, by the addition of titanium or of zirconium in the non-deoxidized steel, then adding a strong deoxidant, such as aluminum; conditions that allow obtaining a very fine dispersion of titanium nitrides, favorable for resilience, ecanibility, and polishing; When the titanium or zirconium is introduced in this preferred manner, the number of titanium or zirconium nitrides larger than 0.1 μm, counted in an area of 1 mm2 of a solid steel micrograph cut, is less than 4 times the sum of the total titanium content precipitated in the form of nitrides and half the total content of zirconium precipitated in the form of nitrides, expressed in thousandths of%; optionally, boron, between 0% and 0.015% and, preferably, more than 0.0005%, and better still, more than 0.002%, to increase the hardenability without deteriorating the thermal conductivity; sulfur, well as .mpurity, that is, in contents lower than 0,,. ^ \. or as an addition, possibly accompanied by ísm or tellurium to improve the machinability; being the sum of the sulfur, selenium and tellurium contents;: t- * by or equal to 0.2%; likewise, the machinability -j- »be me; orated by an addition of lead or bismuth - * n ontenues less than or equal to 0.2% or also, by II-.J. calcium content less than or equal to 0.1%, the remainder being iron and impurities resulting from the elaboration. Within this area of composition, the content of each of the elements must be such that, if R is an index representing the thermal resistivity of the steel, and Tr an index representing its hardenability, the following conditions are respected: R < 11 and, preferably, R < . 9 Tr > 3 and, preferably, Tr > 4.3 In these expressions, R and Tr are coefficients without dimensions that are calculated by the formulas: R = 3.8xC + lOxSi + 3.3xMn + 2.4xNi + 1, 4x (Cr + Mo + W / 2) Tr = 3.8xC + l, lxMn + 0.7xNi + 0.6xCr + l, 6x (Mo + / 2) + kB with kB = 0.5 if B 0.0005% and kB = 0 otherwise, These conditions allow obtain a sufficient hardenability so that, after tempering, the structure is martensitic or bainitic (the structure can be mixed martensite-bainitic), essentially free of ferrite (the ferrite is not desired, but a small remainder of it can remain), and to obtain a thermal conductivity of the steel that allows to increase the productivity of the injection molding facilities.

Calculating, for each element, the ratio of the coefficients of each of the two preceding formulas, it is found that silicon, nickel and, to a certain extent, magnesium, are unfavorable elements whose contents must be adjusted to the lowest compatible level with the different metallurgical tensions and, on the contrary, that it is necessary to privilege, in this order, boron, molybdenum (or tungsten), carbon and chromium, which are very favorable elements in terms of contribution to hardenability , a given incidence on the thermal resistivity. More precisely, by calculating the ratios of the coefficients involved in the formulas of R and Tr, for the coefficients of molybdenum, carbon and chromium, it was found by the inventors that the chemical composition of the steel should satisfy the ratio: R =. Max (2.33xTr - 1; 0.9xTr + 4) This expression means that R must be less than or equal to the greater of the two values 2.33xTr - 1 and 0.9xTr + 4. Finally, to obtain a hardness greater than 250 HB after tempering at a temperature higher than 500 ° C, necessary to obtain a good stability of the properties of the steel and a satisfactory resilience, the contents in molybdenum, tungsten, chromium, vanadium, niobium and tantalum, must be such that: Cr + 3x (Mo + / 2) + 10x (V + Nb / 2 + Ta / 4) > 1 All these relationships must be satisfied simultaneously. In the composition field thus defined, a preferential scope can be determined, by the following conditions: 0.37% =. C < 0.45% 0% = Yes < 0.15% 0.2% < Mn 1, 5% 0% < Ni < 0, 5% 0% < Cr < 2.5% 0% < ? Mo + W / 2 =. 1% 0% < Cu < 1% 0% =. V + Nb / 2 + Ta / 4 =. , 2% 0.005% < Al =. 0.2% 0.0005% = B =. 0, 015% 0% < Ti + Zr / 2 =. 0.3% Cr + 3x (Mo + W / 2) + lOx (V + Nb / 2 + Ta / 4) 1 R < 2, 33xTr - 2,4 Being able to contain the steel, in addition, the other elements already indicated previously (sulfur, selenium, tellurium, bismuth, lead and calcium). The titanium being introduced, preferably, in the manner indicated above. Although steels whose carbon content is of the order of 0.4% are generally considered as difficult to weld, the molds made with the steel according to the invention can be repaired by welding by preheating and postheating to a temperature greater than or equal to at approximately 300 ° C. From this point of view, the composition field chosen has the advantage of facilitating the use after repair (machining, polishing, graining) so that the difference in hardness? H between the base metal and the welding ZAT is maintained moderate (less than 100 HB) and, with equal hardenability, is significantly lower than the hardness differences between the ZAT and the metal ie base obtained with the steels of the prior art. To make a usable steel block for the manufacture of molds, it is the < The liquid steel is cast in the form of a semiproduc or such as an ingot or a billet, then the mine or the semi-product is forged to obtain a block, which may be, for example, a palastro. The semiproduct is then subjected to a tempering and tempering heat treatment, designed to confer a martensitic or bainitic structure, essentially free of ferrite, an unfavorable component for machinability. The block which, in general, has a parallelepiped shape, has a characteristic dimension d, almost always the smallest dimension, which, in combination with the nature of the quenching medium, determines the speed of cooling in the core. For the structure in the core to be free of ferrite, the hardenability of the steel must be sufficient so that the critical rate at which the ferrite appears is lower than the cooling rate in the core. The tempering must be carried out above 500 ° C and, better yet, above 550 °, but below the Ac point of the steel. The steel according to the invention has a particular interest for the manufacture ie molds by casting. The molds thus obtained are of steel-cast, and not of cast steel drawn as described above, According to this procedure, instead of mechanized the mold footprint in a solid block parallelepiped. In which cooling channels are drilled by water circulation, a blank is made by casting which comprises a blank of the footprint of the mold and external parts having an appropriate shape to ensure sufficient mechanical strength, driving In turn, the walls are much less thick than those obtained by the technique of machining the footprint in a solid block.The mold is obtained, in turn, by a finishing machining of the blank and by a heat treatment The thickness of the walls of the mold being relatively small, the use of the steel according to the invention whose thermal conductivity is very good, makes it possible to reduce or even suppress the cooling by circulation of water per channel. s perforated in the walls of the mold and ensure the cooling of the mold, during its use, by a gas circulation around the external parts of the mold. The machining of the footprint almost always makes appear porosities that are separated by welding. The relatively good solderability of the steel facilitates this operation. The heat treatment is identical to the heat treatment carried out in stretched cast steel molds, however, it may be preceded by one or more austenizations intended to refine the grain. As an example, steels A, B, C, D, according to the invention and the steels E, F, G, according to the prior art. The chemical components of these steels were (in thousandths of% by weight): With these steels, tempered and tempered blocks that can be used for the manufacture of molds have been manufactured. The thicknesses (characteristic dimension d) of the blocks, the hardening molds, the tempering conditions, the R coefficients of thermal resistivity and Tr of hardenability, the thermal conductivity values, the hardness obtained and the hardness differences? H between the ZAT and base metal were: conduct = thermal conductivity expressed in / m / K hardness expressed in Brinell These results show that steels A, B, C and D according to the invention have hardenability rates comparable to those of steels? and F according to the prior art, but much higher thermal conductivity values (about 30%). The steel G according to the prior art has a thermal conductivity comparable to that of steels B, C and D, but a hardenability much lower than that of these steels. Due to this hardenability, the maximum thickness that can be manufactured is less than 200 mm, and the tempering must be done, imperatively, in water, which is an inconvenience for small thicknesses. The steels A to D according to the invention have a hardness difference ΔH between the ZAT and the base metal below 55 HB, while the steels E and F have an H greater than 100 HB. The steel G according to the prior art has a small H (44 HB), but also has a very mediocre hardenability. The steel according to the invention can be used for the manufacture of blocks whose characteristic dimension is comprised between 20 mm and at least 1,500 mm, whose structure is, at any point, martensitic or bainitic, essentially free of ferrite, and whose hardness, in any point, is between 250 Brinell and 370 Brinell. These blocks can be used for the manufacture of molds by injection of plastic or rubber materials, or of any other - material molded at a temperature preferably lower than approximately 500 ° C. When the molds are repaired by welding, it is very desirable that the metal of export has the same composition as the base metal, the steel according to the invention can be manufactured, also, in the form of welding wire or for the manufacture of welding electrodes .

It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention is the conventional one for the manufacture to which it refers. Having described the invention as above, property is claimed as contained in the following:

Claims (11)

1. Steel for the manufacture of molds by injection of plastic or rubber materials, characterized in that its chemical composition comprises, by weight: 0.35% < . C < 0.5% 0% < L Yes < 0, 5% 0.2% < Mn < 2.5% 0% < Ni < 4% 0% =. Cr < _ 4% 0% < Mo + W / 2 < 2% 0% < Cu < 1% 0% < V + Nb / 2 + Ta / 4 < L 0.5% 0.005% < To < 0.2% 0% =. B < _ 0.015% optionally, at least one element taken between titanium and zirconium, in contents such that the sum of the titanium content and half of the zirconium content is less than or equal to 0.3%, optionally, at least one element taken in between sulfur, selenium and tellurium, the sum of the contents of these elements being less than or equal to 0.2%, possibly, at least one element taken between lead and bismuth, the sum of the contents of these elements being less than or equal to 0.2%, possibly calcium in a content less than or equal to 0.1%, the remainder being iron and impurities resulting from processing; satisfying the analysis, simultaneously the following relationships: Cr + 3x (Mo + W / 2) + lOx (V + Nb / 2 + Ta / 4) 1 R = 3,8xC + lOxSi + 3,3xMn + 2,4xNi + 1 , 4x (Cr + Mo + / 2) =. 11 Tr = 3.8xC + l, lxMn + 0, 7xNi + 0.6xCr + l, 6x (Mo + W / 2) + kB 3 (with kB = 0.5 if B> 0.0005% and kB = 0 otherwise) y, R Max (2.33xTr-l; 0.9xTr + 4)

2. Steel according to claim 1, characterized in that: Yes > 0, 15%

3. Steel according to claim 1 or 2, characterized in that: B 0.0005%.

4. Steel according to any of claims 1 to 3, characterized in that-: R < 9.

5. Steel according to any of claims 1 to 4, characterized in that: Tr > 4.3.

6. Steel according to any of claims 1 to 5, characterized in that: Yes =. 0.1%.

7. Steel according to any of claims 1 to 6, characterized in that: 0.37% < L C < 0.45% 0% _ ¿If < 0, 15% 0.2% < Mn ¿1.5% 0% _Nor < L 0, 5% 0% < L Cr < 2, 5% 0% =. Mo + W / 2 =. 1% 0% < Cu =. 1% 0% < V + Nb / 2 + Ta / 4 =. 0.2% 0.005% < , Al =. 0.2% 0.0005% < L B =. 0.015% 0% < Ti + Zr / 2 < 0.3% and R < 2, 33xTr - 2.4

8. Steel according to any of claims 1 to 7, characterized in that the contents of titanium, zirconium and nitrogen are preferably such that: 0.00003 =. (N) x (Ti + Zr / 2) < 0.0016 and because, in a solid state, the number of precipitates of titanium nitride or zirconium larger than 0.1 μm in an area of 1 m * of an icrographic cut is more than 4 times the sum of the total titanium content precipitated in the form of nitrides and ie half the total content of precipitated zirconium ~ > nitride form, expressed in thousandths of% by weight.

9. A steel block according to any of claims 1 to 10, characterized in that its characteristic dimension is between 20 mm and 1,500 mm, the steel structure is, at any point, martensitic or bainitic, the hardness, at any point, it is comprised between 250 -jrinell and 370 Brinell.

10. Use of the steel according to any of claims 1 to 8 for the manufacture by casting a mold for plastic materials.

11. A wire for welding or for the manufacture of welding electrodes, characterized in that it is constituted by a steel according to any one of claims 1 to 8.