STEEL ALLOY, HOLDERS AND HOLDER DETAILS FOR PLASTIC MOULDING TOOLS, AND TOUGH HARDENED BLANKS FOR HOLDERS AND HOLDER DETAILS

Abstract: A steel alloy suitable for holders and holder details for plastic moulding tools contains in weight-%: 0.06 - 0.15 C, 0.07 - 0.22 N, wherein the total amount of C + N shall satisfy the condition, 0.16 ≤ C + N ≤ 0.26, 0.1 - 1.0 Si, 0.1 - 2.0 Mn, 12.5 - 14.5 Cr, 0.8 - 2.5 Ni, 0.1 1.5 Mo, optionally vanadium up to max. 0.7 V, optionally, in order to improve the machinability of the steel, one or more of the elements S, Ca and O in amounts up to max. 0.25 S, max. 0.01 (100 ppm) Ca, max. 0.01 (100 ppm) O, balance iron and unavoidable impurities.

Description: STEEL ALLOY, HOLDERS AND HOLDER DETAILS FOR PLASTIC MOULDING TOOLS, AND TOUGH HARDENED BLANKS FOR HOLDERS AND HOLDER DETAILS TECHNICAL FIELD The invention relates to a steel alloy and particularly to a steel alloy for the manufacturing of holders and holder details for plastic moulding tools. The invention also concerns holders and holder details manufactured of the steel, as well as blanks made of the steel alloy for the manufacturing of such holders and holder details. BACKGROUND OF THE INVENTION Holders and holder details for plastic moulding tools are employed as clamping and/or f aming components for the plastic moulding tool in tool sets, in which tool the plastic product shall be manufactured through some kind of moulding method. Among conceivable holder details there can be mentioned bolster plates and other construction parts as well as heavy blocks with large recesses which can accommodate and hold the actual moulding tool. Said holders and holder details are made of many different steel alloys, including martensitic stainless steels. A steel which is manufactured and marketed by the applicant under the registered trade name RAMAX S® belongs to that group and has the following nominal composition in weight-%: 0.33 C, 0.35 Si, 1.35 Mn, 16.6 Cr, 0.55 Ni, 0.12 N, 0.12 S, balance iron and impurities from the manufacturing of the steel. The closest comparable standardized steel is AISI 420F. Steels of this type have an adequate corrosion resistance, but do not have a martensitic micro-structure which is as homogenous that is desirable, but may contain ferrite and hard spots, which are due to retained, untempered martensite, which in turn can be explained by a certain segregation tendency of the steel. Therefore it exists a demand of improvements as far as holder steels are concerned. It is also desirable that the same steel, possibly with some modification of the composition, also shall be useful for the actual moulding tool. DISCLOSURE OF THE INVENTION It is an object of the invention to provide a steel, which after hardening and tempering has a more even structure than the above mentioned steel, essentially without ferrite and/or spots in the material which have a pronouncedly higher hardness. The invention also aims to achieve one or several of the following effects: - A good machinability. - An adequate corrosion resistance. - An adequate hardenability, considering the steel shall be possible to be used for the manufacturing of holder blocks made of plates which may have a thickness of up to at least 300 mm and in some cases even up to 400 mm thickness. An adequate ductility/toughness. - A hardness of 30 - 42 HRC, preferably 38 - 40 HRC in the tough-hardened condition. - A good polishability, at least according to a preferred embodiment, in order to able to be used also for moulding tools on which high demands are raised as far as polishability is concerned. The above objectives can be achieved if the steel has the chemical composition which is stated in the appending patent claims. As far as the importance of the separate elements and their interaction in the steel are concerned, the following may be considered to apply without binding the claimed patent protection to any specific theory. Carbon and nitrogen are elements which have a great importance for the hardness and ductility of the steel. Carbon is also an important hardenability promoting element. Carbon, however, binds chromium in the form of chromium carbides (M7C3 -carbides) and may therefore impair the corrosion resistance of the steel. The steel therefore may contain max 0.15 % carbon, preferably max 0.13 % carbon (in this text always weight- % is referred to if not otherwise is stated). However, carbon also has some advantageous effects, such as to exist together with nitrogen as a dissolved element in the tempered martensite in order to contribute to the hardness thereof, and also acts as an austenite stabilizer and thence counteract ferrite in the structure. The minimum amount of carbon in the steel therefore shall be 0.06%, preferably at least 0.07 %. Nitrogen contributes to the provision of a more even, more homogenous distribution of carbides and carbonitrides by affecting the solidification conditions in the alloy system such that larger aggregates of carbides are avoided or are reduced during the solidify- cation. The proportion of M23C6-carbides also is reduced in favour of M(C,N), i.e. vanadium-carbonitrides, which has a favourable impact on the ductility/toughness. In summary, nitrogen contributes to the provision of a more favourable solidification process implying smaller carbides and nitrides, which can be broken up during the working to a more finely dispersed phase. From these reasons nitrogen shall exist in an amount of at least 0.07 %, preferably at least 0.08 %, but not more than 0.22 %, preferably max 0.15 %, at the same time as the total amount of carbon and nitrogen shall satisfy the condition 0.16 ^C + N ^0.26. Preferably, C + N shall be at least 0.17 % but suitably max 0.23 %. Nominally, the steel contains 0.20 - 0.22 (C + N). In the hardened and tempered steel, nitrogen is substantially dissolved in the martensite in the form of nitrogen-martensite in solid solution and thence contributes to the desired hardness. In summary, as far as the content of nitrogen is concerned, it can be stated that nitrogen shall exist in the said minimum amount in order to contribute to the desired corrosion resistance by increasing the so called PRE- value of the matrix of the steel, to exist as a dissolved element in the tempered martensite which contributes to the hardness of the martensite, and to form carbonitrides, M(C, N), to a desired degree together with carbon, but not exceed said maximum content, maximizing the content of carbon + nitrogen, where carbon is the most important hardness contributor. Silicon increases the carbon activity of the steel and thence the tendency to precipitate more primary carbides. This is a first reason why it is desirable that the steel has a low content of silicon. Further, silicon is a ferrite stabilizing element, which is a disadventageous feature of silicon. As the steel also shall contain the ferrite stabilizing elements chromium and molybdenum in sufficient amounts to provide desirable effects by those elements, at the same time as the steel contains a lower content of carbon than is conventional in steels for the application in question, the content of silicon should be restricted in order not to cause the steel to contain ferrite in its matrix. The steel therefore must not contain more that 1 % Si, preferably max. 0J % Si, suitably max. 0.5 % Si, and most conveniently a still lower content of silicon. Generally the rale shall apply that the ferrite stabilizing elements shall be adapted to the austenite stabilizing ones in order to avoid formation of ferrite in the steel. However, silicon exists as a residue from the desoxidation treatment, wherefore the optimum content of silicon lies in the range 0.05 - 0.5 %, normally in the range 0.1 - 0.4 %, and is nominally about 0.2 - 0.3 %. Manganese is an element which promotes austenite and hardenability, which is a favourable effect of manganese, and can also be employed for sulphur refining by forming harmless manganese sulphides in the steel. Manganese therefore shall exist in a minimum amount of 0.1 %, preferably at least 0.3 %. Manganese, however has a segregation tendency together with phosphorous which can give rise to tempering- embrittlement. Manganese therefore must not exist in an amount exceeding 2 %, preferably max.1.5 %. suitably max.1.3 %. Chromium is the main alloying element of the steel and is essentially responsible for provision of the stainless character of the steel, which is an important feature of holders and holder details for plastic moulding tools, as well as for the plastic moulding tool itself, which often is used in damp environments, which may cause less corrosion resistant steels to rust. Chromium also is the most important hardenability promoting element of the steel. However, no substantial amounts of chromium are bound in the form of carbides, because the steel has a comparatively low carbon content, wherefore the steel can have a chromium content as low as 12.5 % and nevertheless get a desired corrosion resistance. Preferably the steel, however, contains at least 13.0 % chromium. The upper limit is determined in the first place by the ferrite forming tendency of chromium. The steel therefore must not contain more than max. 14.5 % Cr, preferably max. 14.0 % Cr. Nominally, the steel should contain 13.1 - 13.7 % Cr. Nickel should exist in the steel in a minimum amount of 0.8 %, preferably at least 1.0 %, in order to afford the steel a very high hardenability. From cost reasons, however, the content should be limited to max. 2.5 %, preferably to max. 2.0 %. Nominally, the steel contains 1.4 - 1.8 % or about 1.6 % Ni. Optionally, the steel of the invention also may contain an active content of vanadium in order to bring about a secondary hardening through precipitation of secondary carbides in connection with the tempering operation, wherein the tempering resistance is increased. Vanadium, when present, also acts as a grain growth inhibitor through the precipitation of MC-carbides. If the content of vanadium is too high, however, there will be formed large primary MC-carbonitrides during the solidification of the steel, and this also occurs if the steel is subjected to ESR-remelting, which primary carbides will not be dissolved during the hardening procedure. For the achievement of the desired secondary hardening and for the provision of a favourable contribution to the grain growth inhibition, but at the same time avoiding formation of large, undissolvable primary carbides in the steel, the optional content of vanadium should lie in the range 0.07 - 0.7 % V. A suitable content is 0.10 - 0.30 % V, nominally about 0.2 % Y. Preferably, the steel also contains an active content of molybdenum, e.g. at least 0.1 %- in order to give a hardenability promoting effect. Molybdenum up to an amount of at least 1.0 % also promotes the corrosion resistance but may have effect also if the content is higher. When tempering, molybdenum also contributes to increasing the tempering resistance of the steel, which is favourable. On the other hand, a too high content of molybdenum may give rise to an unfavourable carbide structure by causing a tendency to precipitation of grain boundary carbides and segregations. Besides, molybdenum is ferrite stabilizing, which is unfavourable. The steel therefore shall contain a balanced content of molybdenum in order to take advantage of its favourable effects but at the same time avoid those ones which are unfavourable. Preferably, the content of molybdenum should not exceed 1J %. An optimal content may lie in the range 0.1 - 0.9 %, probably in the range 0.4 - 0.6 % Mo. Normally, the steel does not contain tungsten in amounts exceeding the impurity level, but may possibly be tolerated in amounts up to 1 %. The steel of the invention shall be possible to be delivered in its tough-hardened condition, which makes it possible to manufacture large sized holders and mould tools through machining operations. The hardening is carried out through austenitizing at a temperature of 850 - 1000 °C, preferably at 900- 975 °C, or at about 950 °C, followed by cooling in oil or in a polymer bath, by cooling in gas in a vacuum furnace, or in air. The high temperature tempering for the achievement of a tough hardened material with a hardness of 30 - 42 HRC, preferably 38 - 41 or about 40 HRC, which is suitable for machining operations, is performed at a temperature of 510 - 650 °C, preferably at 520 - 540 °C, for at least one hour, preferably through double tempering; twice for two hours. The steel may, as an alternative, be low temperature tempered at 200 - 275 °C, e.g. at about 250 °C, in order obtain a hardness of 38 - 42 or about 40 HRC. The steel may, according to a preferred embodiment, also contain an active content of sulphur, possibly in combination with calcium and oxygen, in order to improve the machinability of the steel in its tough hardened condition. In order to obtain best effect in terms of machinability improvement, the steel should contain at least 0.07 % S if the steel does not also contain an intentionally added amount of calcium and oxygen, and at least 0.035 %, respectively, if the steel also contains an active amount of calcium and oxygen. The maximum sulphur content of the steel is 0.25 %, when the steel is intentionally alloyed with a content of sulphur. A suitable sulphur content in this case may be 0.12 %. Also a non-sulphurized variant of the steel, however, can be conceived. In this case the steel does not contain sulphur above impurity level, and nor does that steel contain any active contents of calcium and/or oxygen. It is thus conceivable that the steel may contain 0.035 - 0.25 % S in combination with 3 - 100 weight-ppm Ca, preferably 5 - 75 ppm Ca, suitably max. 40 ppm Ca, and 10 - 100 ppm O. wherein said calcium, which may be supplied as silicon-calcium, CaSi, in order to globulize existing sulphides to form calcium sulphides, counteracts that the sulphides get a non-desired, elongated shape, which might impair the ductility. The steel of the invention can be manufactured conventionally at a production scale by manufacturing a metal melt in the normal way, said melt having a chemical composition according to the invention, and casting the melt into large ingots or casting the melt continuously. It is also possible to cast electrodes of the molten metal and then remelting the electrodes through Electro-Slag-Remelting (ESR). It is also possible to manufacture ingots powder-metallurgically through gas-atomization of the melt to produce a powder, which then is compacted through a technique which may comprise hot isostatic pressing, so called HIPing, or, as an alternative, manufacture ingots through sprayforming. Further characteristics, aspects and features of the steel according to the invention, and its usefulness for the manufacturing of holders and moulding tools, will be explained more in detail in the following through a description of performed experiments and achieved results. BRIEF DESCRIPTION OF DRAWINGS In the following description of performed experiments and achieved results, reference will be made to the accompanying drawings, in which Fig. 1 shows a holder block of a typical design, which can be manufactured of the steel according to the invention, Fig. 2A is a chart showing the hardness of a first set of steels, produced in the form of so called Q-ingots (50 kg laboratory heats), after hardening but before tempering, versus the austenitizing temperature at a holding time of 30 min, Fig. 2B shows corresponding graphs for another number of tested steels manufactured as Q-ingots, Fig. 3 A shows tempering curves for those steels in the first set which have been hardened from 1030 °C, Fig. 3B shows the tempering temperature range 500 - 550 °C of the tempering curves of Fig. 3 A at a larger scale, Fig. 3C shows tempering curves within the tempering temperature range 500 - 550 °C for those further tested steels, whose hardness versus the austenitizing temperature was shown in Fig. 2B, Fig. 4 is a chart which showing hardenability curves for the steels which were tested as stated above, Fig. 5 is a bar chart illustrating results from impact toughness testing of the above mentioned steels, and Fig. 6A and Fig. 6B are bar charts which illustrate the critical current density, Icr, ■ measured when corrosion testing samples which had been slowly cooled in a vacuum furnace at two different cooling rates from the austenitizing temperature and which thereafter had been high temperature tempered to about 40 HRC. EXAMINATION OF STEELS MANUFACTURED AT A LABORATORY SCALE Fig. 1 shows a holder block 1 of a typical design, which shall be possible to be manufactured of the steel according to the invention. In the blockl there is a cavity 2, which shall accommodate a mould tool, usually a plastic moulding tool. The block 1 has considerable dimensions and the cavity 2 is large and deep. Therefore, a number of different requirements are raised on the material according to the invention, i.a. an adequate hardenability with reference to the considerable thickness of the block, and a good ability to be machined by means of cutting tools, such as mill cutters and borers. Material 17 Q-ingots (50 kg laboratory heats) with compositions according to Table I were manufactured in four rounds. In the first round (Q9043-Q9080), ingots were manufactured having chemical compositions within a wide range; e.g. variants having comparatively high contents of nitrogen were tested. It was revealed that the alloy having the most interesting features was Q9068, i.e. with carbon contents lying in medium range around 0.10 % and with moderate contents of nitrogen. In the second round (Q9129-Q9132) one tried to optimize the features that were obtained by Q9068. The carbon content was slightly varied, vanadium was added in order to obtain a finer grain size, and the nickel content was lowered for one of the variants. In the third round (Q9129-Q9139) variants having increased sulphur contents were tested. In a fourth round only two steels, Q9153 and Q9154, were tested in order to evaluate the relations between carbon and nitrogen. The steels Q9043 and Q9063 are reference materials. Q9043 has a composition according to SIS2314 and AISI 420, while Q9063 corresponds to W.Nr. 1.2316. The Q-ingots were forged to the shape of rods of size 60 x 40 mm, whereupon the rods were cooled in vermiculite. Table I- Test materials; chemical composition in weight-%, balance Fe and unavoidable im urities n.a. = not analyzed Hardness after heat treatment The hardness versus the austenitizing temperature is shown in Fig. 2 A and Fig. 2B. It is evident from the charts of these drawings that the hardness increases with increasing austenitizing temperature for some steels having a higher carbon content, such as for Q9043, Q9063, Q9103, Q9104 and Q9135. 1030 °C is an austenitizing temperature which may be appropriate in these cases. For other steels, the hardness decreases or remains constant with increasing austenitizing temperature. In that case it may be more appropriate to choose 950 °C as an austenitizing temperature. The hardness after tempering of those steels which had been hardened from 1030 °C are shown in Fig. 3 A and Fig. 3B, while all the tempering curves for those ones of the Q- ingots 9129-9154 which had been hardened from 950 °C are shown in the diagram in Fig. 3C. The conclusion can be drawn from the tempering curves that all the steels can be tempered down to 40 HRC through tempering in the temperature range 520-600 °C. An appropriate hardness of the steel after tough-hardening is about 40 HRC. In Table II below, the heat treatments are stated which provide the said hardness to the different steels. Table IT - Heat treatment for tough-hardening, measured rest austenite, percent b volume * t8/5=1964s ** t8/5=1885s (t8/5 corresponds to the time for cooling a sample from 800 to 500°C) Hardenability The hardness after hardening from the austenitizing temperatures which are given in Table LI, from which temperatures the samples have been cooled at different rates, is shown in the hardenability curves of Fig. 4. Impact toughness tests Impact toughness testing of un-notched test specimens, mean values for four to six test rods of each steel, was performed at room temperature. The heat treatments and cooling rates, which were employed for the different steels, are given in Table III. The results are disclosed by the bar chart in Fig. 5. From this chart it can be recognized that some variants, such as Q9067, 9068, 9069, 9129, 9131, 9132 and Q9153 have a very high ductility, >350 J, and that the test rods were not raptured, but also that some other steels, including e.g. steel Q9154, have a considerably better ductility than the reference steels, Q9063 and 9043, which lie on the 180-200 J level. Table HE Corrosion tests Polarization curves were established in a first test round for the steels given in Table IN in terms of critical current density, Icr, for the evaluation of the corrosion resistance of the steels. As far as this method of measurement is concerned, the rale is that the lower Ire is, the better is the corrosion resistance. The investigations were performed in two test series, in which the test specimens were subjected to different cooling rates. The heat treatments of the first series are shown in Table IN. Table IV - Heat treatment of polarization test specimens. Cooling in vacuum furnace The results from this first test round are evident from the bar chart in Fig. 6A. From this bar chart it is evident that five steels had a better corrosion resistance than the reference material, Q9063, namely Q9068, Q9070, Q9129,Q9132 and Q9153. Still slower cooling rates t8/5 were employed in a second test round, see Table N and Fig. 6B. Table V Heat treatment of polarization test specimens. Cooling in vacuum furnace Fig. 6B illustrates that best corrosion resistances were notified for samples of Q9063, 9129, 9153 and 9154. Discussion In the introductory disclosure of the invention there were listed a number of purposes of the invention. Besides a good machinability, the steel shall have a good ductility, a good corrosion resistance, and a good hardenability. It can be stated that it is an aim that the steel, besides a good machinability, shall have better ductility, corrosion resistance and hardenability than steel Q9063. Four steels satisfy those criteria, namely Q9068, Q9129, Q9153 and Q9154, which have a rather similar composition; although steel Q9154 has a higher nitrogen content and a lower content of carbon. On the basis of these experiences, it can be assumed that an optimal composition could be the following, namely 0.10 C, 0.075 N, 0.16 Si, 1.1 Mn, 13.1 Cr, 0.13 V, 1.8 Ni, 0.5 Mo, balance Fe and unavoidable impurities. An alternative could be a steel which contains 0.06 C and 0.14 Ni but as for the rest the same composition as the foregoing. Other alternatives - suitably conceivable nominal compositions - could be the following ones: 0.12 C, 0.20 Si, 1.30 Mn, 0.10 S, 13.4 Cr, 1.60 Ni, 0.50 Mo, 0.20 V, 0.10 N, balance iron and unavoidable impurities, and/or 0.14 C, 0.18 Si, 1.30 Mn, 0.10 S, 13.5 Cr, 1.67 Ni, 0.50 Mo, 0.22 V, 0.10 N, balance iron and unavoidable impurities. MANUFACTURING OF STEEL AT A PRODUCTION SCALE A 35 tons heat of molten metal was manufactured in an electric arc furnace. Before tapping, the melt had the following chemical composition: 0.15 C, 0.18 Si, 0.020 P, 0.08 S, 13.60 Cr, 1.60 Ni, 0.48 Mo, 0.20 V, 0.083 N, balance Fe and unavoidable impurities. Of the melt there were manufactured ingots, which were forged to the shape of flat rods of varying dimensions. The forging did not cause any problems. The forged rods were tough-hardened to a hardness of about 380 HB through austenitizing at 950 °C, holding time 2h, fast quenching in air and tempering at 540 °C, 2x2h. The thus tough-hardened rods were machined to final gauges.

Claims: PATENT CLAIMS 1. A steel alloy, characterized in that it has a chemical composition which contains in weight-%: 0.06-0.15 C 0.16<C+N<0.26 0.1 -1.0 Si 0.1- 2.0 Mn 12.5 - 14.5 Cr 0.8- 2.5 Ni 0.1 -1.5 Mo optionally vanadium up to max.0.7 V optionally one or more of the elements S, C and O in order to improve the machinability of the steel, in amounts up to max.0.25 S, max.0.01 (100 ppm) Ca, max.0.01 (100 ppm) O, balance iron and unavoidable impurities. 2. A steel alloy according to claim 1, characterized in that it contains 0.07 - 0.13 C. 3. A steel alloy according to claim 1, characterized in that it contains 0.08 - 0.15 N. 4. A steel alloy according to any of claims 1-3, characterized in that the total amount of C + N shall satisfy the condition 0.17 < C + N < 0.23. 5. A steel alloy according to claim 1, characterized in that it contains 0.1 - 0.7 Si, preferably max.0.5 Si. 6. A steel alloy according to claim 5, characterized in that it contains 0.1-0.4 Si. 7. A steel alloy according to claim ^characterized in that it contains max.1.5 Mn, preferably max.1.3 Mn. 8. A steel alloy according to claim 1, characterized in that it contains 13.0 - 14.0 Cr. 9. A steel alloy according to claim 8, characterized in that it contains 13.1- 13.7 Cr. 10. A steel alloy according to claim 1, characterized in that it contains 1.0 - 2.0 Ni. 11. A steel alloy according to claim 10, characterized in that it contains 1.4- 1.8 Ni. 12. A steel alloy according to claim 1, characterized in that it contains 0.1 - 0.9 Mo. 13. A steel alloy according to claiml2, characterized in that it contains 0.4 - 0.6 Mo. 14. A steel alloy according to claiml, characterized in that it contains at least 0.07 V. 15. A steel alloy according to claim 14, characterized in that it contains at least 0.10 V. 16. A steel alloy according to claiml 5, characterized in that it contains 0.10- 0.30 V. 17. A steel alloy according to claiml, characterized in that it contains max.0.15 S. 18. A steel alloy according to claiml 7, characterized in that it contains 0.08 - 0.12 S. 19. A steel alloy according to claiml, characterized in that it does not contain S, Ca or O above impurity level. 20. A steel alloy according to any of the preceding claims, characterized in that it contains 0.06-0.13 C 0.08-0.15N 0.1-0.4, preferably 0.2 - 0.3 Si 0.2- 1.3 Mn 12.5- 13.6 Cr 0.1 -0.3 V 0.2 -0.8 Mo 1.4- 1.8 Ni 21. Holders and holder details for plastic moulding tools, characterized in that they consist of a steel alloy according to any of claims 1-20.

Inventors: SANDBERG ODD, TIDESTEN MAGNUS, SANDBERG, ODD, TIDESTEN, MAGNUS
Applicants: SANDBERG ODD, TIDESTEN MAGNUS, UDDEHOLM TOOLING AB, SANDBERG, ODD, TIDESTEN, MAGNUS, UDDEHOLM TOOLING AKTIEBOLAG
Application year: 2001
Main classification: C22C38
IPC codes: C22C33, C22C38