AU2007232532B2

AU2007232532B2 - Hot-working steel

Info

Publication number: AU2007232532B2
Application number: AU2007232532A
Authority: AU
Inventors: Bengt Klarenfjord; Odd Sandberg
Original assignee: Uddeholms AB
Current assignee: Uddeholms AB
Priority date: 2006-04-06
Filing date: 2007-03-30
Publication date: 2011-06-02
Anticipated expiration: 2027-03-30
Also published as: CA2659249A1; TW200741017A; BRPI0710268A2; US20090191086A1; NZ570919A; CN101415854A; AU2007232532A1; JP5185923B2; EP2002025A1; MX2008012631A; RU2008136560A; JP2009532585A; NO20084230L; KR20090010187A; SE529809C2; TWI355425B; WO2007114781A1; SE0600797L; RU2430186C2; SG170824A1

Description

WO 2007/114781 PCT/SE2007/050203 HOT-WORKING STEEL TECHNICAL FIELD 5 The invention relates to hot-working steel, i.e. steel intended to be used in the working in hot condition of a working material. Typical examples of the use of the steel are tools for the extrusion pressing of light metals, primarily aluminium. Another form of use is in forging tools. The invention also relates to use of the steel in the manufacturing of hot-working tools and to tools manufactured from the steel. 10 BACKGROUND OF THE INVENTION There are many requirements on high quality hot-working tools, such as a good resistance to hot wear and a beneficial combination of other properties such as tempering resistance, toughness, hardenability and strength. It is important for optimal 15 tool performance that these properties are satisfied. Tools for extrusion pressing of metals such as aluminium are exposed to demanding operating conditions in terms of temperature, pressure and abrasive wear. Extrusion pressing means that material in a plasticized condition is pressed through a restriction/nozzle in which the extruded profile/shape of the extrusion pressed material is formed. Hence, an extrusion pressing 20 nozzle must have high strength in combination with good resistance to hot wear as well as good resistance to tempering at high temperatures, since a material such as aluminium is normally extruded at temperatures of about 500-600 'C. A well known and long used hot-working steel is denoted H13 (or alternatively H 11) according to AISI and has the following nominal composition in % by weight. 0.30 - 0.40 C, 0.20 25 0.40 Mn, 0.80-1.20 Si, 4.75-5.50 Cr, 1.25-1.75 Mo, 0.80-1.20 V, balance iron and unavoidable impurities, and for HI 1, in % by weight: 0.30-0.50 V, the remainder the same as for H13. Over the years many different lines of development have been investigated with the purpose of improving certain properties of the conventional steel H13. Examples are EP 632139, US 4,886,640 and US 4,853,181. 30 A drawback in common for all steels mentioned above is that they have a relatively high content of expensive alloying materials. It is realised that this is an essential drawback that it for long has been a desire to be able to minimize. 35 ACCOUNT OF THE INVENTION 2 Based on many years of experiments we have now succeeded in minimizing the above mentioned problems by a steel, suitable for the production of a die tool for extrusion pressing of aluminium, having the following chemical composition in % by weight: 0.38-0.46% C 5 0.5-1.0% Si 1 < Mn<S 1.8% 1.5-3.0% Cr 0.5-0.7% Mo 0.45-0.7% V 10 up to 0.4% W, and up to 1.0% Al, with the content of (Mo+Cr) less than 3.5% and the remainder being iron and unavoidable impurities. The invention enables provision of a hot-working steel suitable for most applications, which 15 has properties comparable with a traditional H 13 steel but having a alloying cost that is almost the half (at the metal prices of today). Based on the fact that different steels of H13 type have been on the market for a long time, it is realised that it must be considered surprising that we have been able to find an alloy that is so efficient in solving the above mentioned problems, especially when considering that the important properties of resistance to hot wear against 20 aluminium and thermal conductivity are improved as compared to a traditional H13 alloy. It is true that hardenability is somewhat impaired, but as most extrusion tools on the market are used for fairly small dimensions, it is estimated that about 70-80% of the existing production can use of this new alloy, resulting in a considerably reduced cost at a functionality that for the rest is maintained. 25 When nothing else is stated the present description always refers to percent by weight in respect of the chemical composition of the steel and percent by volume in respect of the structural components of the steel. The following is true for individual alloying materials and their mutual relations and for the structure and heat treatment of the steel. 30 Carbon should be present in an adequate amount to give the steel the desired hardness after heat treatment and to contribute to hardenability. Accordingly, there must be at least 0.3% and for the best result about 0.4% of carbon. Too much carbon has a negative effect on 'I6'->, 3 toughness and hence the upper limit should be 0.46, preferably about 0.45%. A suitable carbon content is in the range of 0.38-0.46%, preferably 0.40-0.44%. Silicon is present as a residual element from the manufacturing of the steel, at a content of 5 normally at least 0.2%, most often about 1%. Silicon increases the carbon activity in the steel and hence it contributes to give the steel an adequate hardness. It also contributes to an adequate resistance to oxidation and tempering. Contents that are too high may lead to brittleness problems due to solution hardening and hence the maximum content of silicon in the steel is 1.0%. Silicon may be present in the steel in an amount of 0.5-1.0%. 10 Manganese, chromium and molybdenum should be present in the steel at an amount high enough to give the steel an adequate hardenability. The contents of (Mo + Cr) is to be less than 3.5%. Preferably the %Cr/%Mo is >3. The steel may have a thermal conductivity of more than 26 W/m 0 C, Molybdenum has the property that it, besides contributing to hardenability also contributes to 15 a good tempering resistance. Hence, it has been proven that molybdenum is needed at a content of from 0.5-0.7%. Besides hardenability, chromium also contributes to the alloy's oxidation resistance and it should be present at a content of at least 1.5% but not more than 3.0%, with a suitable content of 2.2 to 2.8%. The nominal content of chromium is preferably 2.6%. 20 Manganese should be present at a content of at least 1.0% in order to contribute in giving the steel a desired hardenability at the limited content of molybdenum and chromium that characterises the steel. The steel should not contain more than 1.8% manganese. The nominal content of manganese is preferably 1.4 %. Vanadium should be present in the steel at a content of at least 0.45% and not more than 25 0.7%. Vanadium also contributes to give the steel good tempering resistance, good resistance to wear and also contributes to good strength by forming vanadium carbides that contribute to the formation of a relatively fine crystal structure. Preferably the steel has a vanadium content of 0.5-0.6%. Conventional, known production techniques can be used for the manufacturing of the steel. <filcasm> 4 BRIEF DESCRIPTION OF THE DRAWINGS In the following description of experiments made, reference will be made to the enclosed drawings, of which 5 Fig. 1 shows a graph over the desired resistance to hot wear, for gas nitrided samples in a 4 h test, and Fig. 2 shows the corresponding test results in an 8 h test. ACCOUNT OF CONDUCTED EXPERIMENTS Three alloys have been made in the form of laboratory scale ingots with a weight of 50 kg, 10 by the following process: Forging at 1270*C to the dimension 60 x 60 mm. Soft-annealing at 850*C/2h, cooling by 10*C/h to 600*C and thereafter free cooling in air. The chemical compositions of the investigated charges are shown in Table 1 below. Table 1. Chemical composition (% by weight) of investigated charges, remainder iron and impurities, 15 Ingot. C Si Mn P S Cr Mo V Al N 0 No. PPM 2 0.37 1.06 0.41 0.019 0.001 5.16 1.47 0.82 0.001 0.04 40 6 0.42 0.93 1.25 0.004 0.007 2.53 0.60 0.57 0.001 0.047 50 7 0.38 0.59 1.34 0.005 0.006 2.29 0.55 0.52 0.53 0.023 32 The alloys are heat treated according to Table 2 below. Table 2. Austenitizing and tempering temperatures, and expected hardness of the various alloys. 20 Alloy TA (*C) 30 min T Temp,(*C) 2x2h HRC 2 1020 580 48 6,7 1020 560 48 Plate samples having the dimension 5 x 10 x 30 mm are produced of each species according to Table 2. One surface of the sample, a side of 5 x 30 mm, is polished fine with an RA of about 0.10-0.15 m. <filCm> 5 The different samples were investigated to compare hot wear against aluminium. Fig. I shows the volume worn off after about 4 h for gas nitrided samples. The graph shows that both samples produced according to the invention, i.e. samples 6 and 7, have improved resistance to wear as compared with the reference steels (sample no. 2). 5 Fig. 2 shows the result from a corresponding 8 h test, showing that an improved resistance to wear could be confirmed for the invention also in such an experiment. In one case (sample no. 6), the improvement is more than 50% in respect of resistance to wear, as compared with a traditional H 13 steel (sample 2). 10 It is hence evident, when comparing a steel according to the invention with a classic H13 steel (sample 2) having the composition in % by weight: 0.30 - 0.40 C, 0.20-0.40 Mn, 0.80 1.20 Si, 4.75-5.50 Cr, 1.25-1.75 Mo, 0.80-1.20 V. balance iron and unavoidable impurities, that the steel according to the invention, having the composition in % by weight:0.38 0.46C, 0.5-1.0 Si, 1<MnS1.8, 1.5-3.0 Cr, 0.5-0.7 Mo, 0.45-0.7V, up to 0.4% W and up to 1.0 15 Al, with (Mo + Cr)<3.5, is much improved in respect of the very important property of wear resistance, despite the lower content of alloying materials and lower cost. Considering the conventional understanding in the present technical field, these results are sensational since the prevalent teaching is that a lowering of the content of chromium and/or molybdenum and/or vanadium should result in an impaired nitrification potential. According to the 20 prevalent understanding, the changes made in a steel according to the invention, as compared with a traditional H 13 steel, ought accordingly result in a relatively seen impaired nitrification potential and hence an impaired resistance to wear. Obviously, some type of mechanisms arise in a steel according to the invention, resulting in an excellent nitrification potential and hence the ability to achieve a good resistance to wear, despite the decreased 25 contents of chromium, molybdenum and vanadium. It is realised that the invention is not limited to the preferred, narrow ranges mentioned above, but that many variants are accommodated within the scope of the invention as defined in claim 1, while still retaining the required good properties. Aspects that are specifically preferred according to the invention are set out in the dependent claims. 30 It is clear from the table below (Table 3) that the steel according to the invention fulfils the properties desired for an H13 steel, which table gives a weighted comparison of important properties, wherein 10 is the highest rating corresponding to the best test result (for the 6 comparison between a traditional H 13 steel and the invention) and the rating of the comparing sample being weighted in relation to the best rating. Table 3. 5 Invention H13 Nitrification potential The same The same Alloying cost 10 5 Yield % The same The same Processability The same The same Thermal fatigue strength 7 10 Ductility (20*C, 44 HRC) The same The same Charpy-V (20*C, 48 HRC) The same The same Hardenability 6 10 Resistance to wear against aluminium (550*C) 10 7 Tempering resistance 8 10 Strength (550*C) 9 10 Thermal conductivity 10 9 It is clear from the table above that the ratings of the invention are very good in respect of properties important for an H13 steel, and that it is possible even to achieve improved ratings 10 in respect of the very important properties resistance to wear against aluminium and thermal conductivity, although the alloying cost is lowered by almost 50%. <filenme EDITORIAL NOTE APPLICATION NO. 2007232532 . This specification does not contain pages number 7 and 8.

Claims

1. A hot-working steel, suitable for the production of a die tool for extrusion pressing of aluminium, wherein the steel has the following chemical composition in % by weight: 5 0.38-0.46% C 0.5-1.0% Si 1%<Mn<1.8% 1.5-3.0% Cr 0.5-0.7% Mo 10 0.45-0.7% V up to 0.4% W, and up to 1.0% Al, with the content of (Mo + Cr) less than 3.5% and the remainder being iron and unavoidable impurities. 15

2. A steel according to claim 1, wherein the content of Cr is 2.2-2.8%.

3. A steel according to claim 1 or claim 2, wherein the content of Cr in relation to the content of Mo is such that %Cr/%Mo>3. 20

4. A steel according to any one of claims I to 3, wherein the content of each of Mo and V is less than 0.7%.

5. A steel according to claim 4, wherein the content of V is 0.5-0.6 % by weight. 25

6. A steel according to any one of claims I to 5, wherein it contains not more than 0.05% by weight of Al.

7. A steel according to any one of claims I to 5, wherein the content of Al is 0.3-1.0%. 30

8. A hot-working steel according to claim 7, wherein the content of Al is about 0.7%. 10

9. A hot-working steel according to claim 3 or claim 4, wherein the steel has a thermal conductivity of more than 26 W/m*C at 200C.

10. Use of the steel according to any one of claims 1-9 for the production of a tool for 5 extrusion pressing.

11. Use of the steel according to any one of claims 1-9, for the production of a tool for extrusion pressing of aluminium. <fiee