Plastic injection mold tooling and a method of manufacture thereof

Abstract

Uniform hardenability is achieved in plastic injection mold and die block tooling of 20 inches and larger by the use of 0.05-0.20 vanadium in conjunction with low carbon steel in which ingots are hot worked to form mold and die blocks having cross sections of 20 inches and larger followed by water quenching and tempering.

Claims

1. A method of manufacturing plastic injection mold and die block tooling having excellent hardenability in sections of 20 inches and larger, said method comprising the steps of: (1) forming a steel melt in a heating unit having less than all of the alloy ingredients, (2) transferring said melt to a receptacle to thereby form a heat, (3) heating, further alloying and refining said heat with argon purging of the alloy composition into specification, (4) vacuum argon degassing, teeming and casting said heat to form ingots by bottom pouring, (5) hot working said ingots to form said low alloy steel into said mold and die blocks having cross sections of 20 inches and larger, said mold and die blocks having the following composition: C 0.15-0.40 Mn 0.60-1.00 Si 0.60 max Cr 1.00-2.00 Ni 0.15-1.00 Mo 0.20-0.55 V 0.05-0.20 Al 0.040 max P 0.025 max S 0.025 max (6) heat treating by quenching and tempering, and (7) forming plastic injection molding tooling from said quenched and tempered blocks.

2. The method of manufacturing plastic injection mold and die block tooling of claim 1 further characterized in that the mold and die blocks have the following composition: C 0.20-0.35 Mn 0.70-1.10 Si 0.15-0.50 Cr 1.10-2.00 Ni 0.20-90 Mo 0.30-0.55 V 0.07-0.20 Al 0.040 max P 0.020 max S 0.015 max

3. The method of manufacturing the plastic injection mold and die block tooling of claim 2 further characterized in that the molds and die blocks have the following composition: C 0.25-0.33 Mn 0.80-1.10 Si 0.20-0.45 Cr 1.20-2.00 Ni 0.30-0.80 Mo 0.35-0.55 V 0.10-0.20 Al 0.020 max P 0.015 max S 0.005 max

4. A plastic injection mold tooling steel having uniform high hardenability in cross sections of 20 inches and larger and having the following composition: C 0.15-0.40 Mn 0.60-1.10 Si 0.60 max Cr 1.00-2.00 Ni 0.15-1.00 Mo 0.20-0.55 V 0.05-0.20 Al 0.040 max P 0.025 max S 0.025 max

5. The plastic injection mold tooling steel of claim 4 further characterized by having the following composition: C 0.20-0.35 Mn 0.70-1.10 Si 0.15-0.50 Cr 1.10-2.00 Ni 0.20-0.90 Mo 0.30-0.55 V 0.07-0.20 Al 0.040 max P 0.020 max S 0.015 max

6. The plastic injection mold tooling steel of claim 5 further characterized by having the following composition: C 0.25-0.33 Mn 0.80-1.10 Si 0.20-0.45 Cr 1.20-2.00 Ni 0.30-0.80 Mo 0.35-0.55 V 0.10-0.20 Al 0.020 max P 0.015 max S 0.005 max

Description

DESCRIPTION OF THE INVENTION

[0009] Carbon is necessary to provide the required hardness and wear resistance. If carbon is significantly higher than 0.40% the mold block will exhibit low machinability and polishing characteristics. Preferable a maximum of 0.35% carbon is used to ensure good machinability. If substantially less than 0.15% carbon is used wear resistance and mechanical properties will not be suitable for service conditions to which the mold blocks are subjected. Preferably a minimum of 0.20% carbon is used to ensure acceptable wear resistance, hardness and mechanical properties. Most preferably carbon in the range of 0.25% to 0.035% with an aim of 0.30% is used.

[0010] Manganese is essential for hardenability and as a deoxidizer in the steelmaking process. It also acts to control sulphides in forging operations in combination with the other alloying elements. If significantly higher than 1.10% is present there is a risk that retained austenite will be present. If substantially less than 0.60% manganese is present the hardenability of the mold block will be lessened. In addition, to ensure sulphur control the manganese content should be present in an amount of at least 20 times the sulphur content. Manganese also contributes to wear resistance, although to a lesser extent than other carbide formers. Preferably manganese is present in the range of 0.70% to 1.10% and most preferably from 0.80% to 1.10%.

[0011] Silicon is specified for its deoxidizing ability in the steelmaking process. If present in substantially greater quantities than 0.60% there is a predisposition toward embrittlement of the final product.

[0012] Chromium is necessary for carbide formation, for hardenability and for wear resistance. If substantially more than the maximum of 2.00% chromium is present the hardening temperature becomes too high for normal production heat treatment processes. Below the specified minimum of 1.00% the wear resistance will be negatively affected. Preferably, chromium is present in the amount of 1.10% to 2.00% and most preferably from 1.20% to 2.00%.

[0013] Nickel is required to strengthen the ferrite and provide toughness to the mold block. If present in a quantity substantially more than 1.00% there is a risk of retained austenite and decrease in machinability. Excess nickel may also promote high temperature hairline cracking which requires scarfing and/or conditioning during the forging process. If nickel is substantially less than the specified minimum of 0.30%, the mold block will have reduced hardenability and deficiency of toughness during service. Nickel should be present preferably in the range of 0.20% to 0.90% and most preferably in the range of 0.30% to 0.80%.

[0014] Molybdenum is a key element contributing to hardenability and wear resistance by the fact that it is a strong carbide former. Its beneficial effects are effective in the range of 0.20% to 0.55% molybdenum but preferably it is maintained in the upper band of the range from 0.30% to 0.55% molybdenum and most preferably in the range of 0.35% to 0.55% molybdenum.

[0015] Vanadium is a key element and is specified for its high effect on hardenability, wear resistance and grain refining properties. It has been discovered that the addition of vanadium in the specified range of 0.05% to 0.20% combined with proper heat treatment can significantly improve hardenability, particularly in large sections of at least 20 inches. Testing of steel samples with statistically constant alloy constituents except for vanadium as shown in Table 1 showed that the addition of vanadium significantly increased hardenability.

TABLE-US-00004 TABLE 1 ID C Si Mn Cr Ni Mo V X0 0.35 0.40 0.85 1.82 0.48 0.53 0 X10 0.35 0.43 0.97 1.87 0.47 0.54 0.10 X15 0.36 0.43 1.01 1.85 0.50 0.53 0.13 X20 0.35 0.41 1.00 1.85 0.49 0.51 0.19

[0016] For steel X0, one type of carbide was mostly present containing molybdenum and manganese. X20 showed the same carbides but with the addition of a second type of carbides containing vanadium. The vanadium carbide family is much more stable to aging when compared to chromium carbides. To have optimal effect on all characteristics, preferably vanadium is present in the range of 0.07% to 0.20%, and most preferably in the range of 0.10% to 0.20% with an aim of 0.15% as shown in the Figure. Vanadium also has a significant impact on wear resistance and machinability.

[0017] Aluminum is desirable for grain refinement but can have a detrimental effect on steel quality by causing the presence of aluminates, an undesirable impurity. It is therefore important to minimize the addition of aluminum to a maximum of 0.040% in the final melt composition. Most preferably an aim of 0.020% aluminum will achieve grain refinement.

[0018] Phosphorus could increase machinability but the detrimental effects of this element in tool steels, such as an increase in the ductile-brittle transition temperature, outweigh any beneficial effects. Accordingly, the phosphorus content should not be more than the specified maximum of 0.025% and most preferably lower than 0.015%.

[0019] Sulfur is a key element for machinability and it is commonly believed that a content up to 0.045% in tool steel would render acceptable machinability. However, sulphur also has several detrimental effects in this type of steel including hot shortness during processing and reduced polishing and texturing characteristics. Since the effect of vanadium on carbide size has a significant impact on machinability, it is desirable to maintain sulphur to a value lower than 0.025%, preferably lower than 0.015% and most preferably lower than 0.005%.

[0020] A comparison of core vs. hardness tests in mold and die block sections of 20 inches and larger has disclosed that the hardenability of the pieces are substantially uniform across the entire cross section. This is a marked improvement over tooling sets made from currently available steels in which the hardenability of such large sections tends to fall off near the center.

[0021] The preferred method of manufacturing mold and die blocks for tooling sets of this invention is as follows.

[0022] A steel melt is formed in a heating unit, preferably an electric arc furnace, the melt containing a majority but less than all of the requisite alloys, aluminum for example being deferred until near the end of the process.

[0023] After the melt is formed it is transferred to a receptacle, such as a bottom pour ladle, to thereby form a heat. Thereafter, heating, further alloying and refining the heat with argon purging until the alloys are uniformly dispersed and the alloy composition of the heat is brought into specification.

[0024] Thereafter the heat is subjected to vacuum argon degassing and then teemed into ingot molds by bottom pouring.

[0025] Following solidification, the ingots are hot worked to form the resultant low alloy steel into mold and die blocks.

[0026] Thereafter the blocks are heat treated by quenching, preferably in water, and tempered.

[0027] Although a specific example of the invention has been disclosed herein, it will be obvious to those are skilled in the art that modifications may be made within the spirit and scope of the invention. Accordingly it is intended that the scope of the invention be limited solely by the scope of the hereafter appended claims when interpreted in light of the relevant prior art.