Patent Application
20120288397
The present invention refers to bainitic steel for diverse applications in tools, moulds, mould holders, tool holders, having as its main characteristic the homogeneous hardness obtained through a bainitic transformation, without the need of high contents of expensive elements, such as nickel and molybdenum or quenching process. Consequently, such steels provide a considerable cost gain in alloy and the heat treatment of great blocks to which they are applied. The careful design of the alloy, based on its microstructure aspects, provides the steel subject of present invention with hardness and properties close to those of traditional hard alloys used in tools, moulds and bases, but with a significant decrease in their cost.
The tools and moulds generally are used in other material conformation processes, whether thermoplastic polymeric materials (commonly known as plastic materials) or metallic materials. Depending on the properties of the material used to manufacture them, the tools are used in processes at ambient temperature or at high temperatures that generally reach 700° C. The steels of this invention are applied particularly to moulds or tools that work at ambient temperature, or at temperatures below 500° C., as well as in mould or tool holders for general use. A typical example of such applications would be the moulds for plastic conformation, which generally do not exceed 300° C. Also, they are applied to mould and tool holders, which normally work at ambient temperature, but support the stress of tools used in several conditions.
Therefore, plastic moulds and mould holders may be considered as typical applications for this invention steels. In such applications, many characteristics of materials from which tools are made are important, some related to mould use and others related to their manufacturing. As to the characteristics of mould or mould holder use, the property of strength is important, being commonly related to the hardness of material, as well as to the homogeneity along the material section. On the other hand, properties such as answer to polishing, texturizing and machining capacity of the material are important for the economic manufacturing of mould or mould holder.
To reach such requirements, traditional steels suffer a heat treatment by means of quenching and tempering. The quenching treatment is complex for blocks with large dimensions, and need to be quickly cooled in oil tanks or in aqueous media changed with polymers. For blocks applied to large moulds, tanks with more than 80,000 liters are used, resulting in important operating difficulties. Besides the cooling process, the chemical composition of these materials must be improved, with the use of elements that promote hardenability, such as nickel, manganese and molybdenum. As Table 1 shows, these elements are found with significant contents in state of the art steels, also being related to final required hardness.
In this sense, new developments are being made. The objective of patents EP0805220 and U.S. Pat. No. 5,855,846, for example, is the production of bainitic steels with lower contents of alloy elements for application in moulds. However, in this invention, the hardness is obtained with highest contents of chromium (within the same range of DIN 1.2738), decreasing any possible gain in thermal conductivity and also generating higher cost. The invention U.S. Pat. No. 5,695,576, on the other hand, shows a use concept with high contents of Al and Si, which may damage the machining capacity of alloy, due to the presence of non-metallic inclusions. Also, high contents of Si may damage the hardenability, as will be shown in example 2. The patents PI9602054-7 and PI0308832-4 follow the same concept, but try to obtain only the highest hardness ranges (between 430 and 530 HB) and thickness lower than 200 mm, while the larger volume of applications are the moulds of 300 HB, without fulfilling this need. Neither of these patents show examples of application in great blocks (with thickness above 200 mm) without the need of quenching treatment (i.e., with air cooling). Also, they do not describe the possibilities to avoid possible brittling due to slow cooling, whether by adjustment of alloy or thermal treatment means.
Therefore, the difficulties and costs involved in obtaining the hardness of tool steel blocks are clear, whether through the chemical composition or a special process of thermal treatment. Consequently, it is evident the need of a steel capable of being hardened to produce large blocks (with sections above 500 mm), without using quick cooling, and also without using significant contents of alloy elements. And preferably, such steel should be able to fulfill the entire hardness range applied to moulds, i.e., between 300 and 420 HB.
The matter of this invention is to fulfill all these needs.
The bainitic steel proposed in this invention may be hardened, without the needs of quenching, and also has a lean chemical compositions in terms of high cost elements, such as nickel, molybdenum and chromium.
In order to fulfill the conditions above, the alloys of present invention have alloy element compositions that, in percentage of mass, consist of:
Iron balance and metallic or non-metallic impurities that are common to steel work processes.
Following are the reasons for specifying the composition of new material, describing the effect of each alloy element. The percentages indicated refer to mass percentage.
C: Carbon is the major responsible for the answer to thermal treatment, for the hardness of martensite or bainite, the latter being the most important micro constituent of present invention steels. Carbon contents, therefore, controls the final hardness obtained for steels of present invention, which may vary depending on application requirements. Consequently, carbon contents must be as higher as the necessary hardness (according to an equation defined below in example 5), according to the following equation: Hardness HV=(450±140) % C+(210±45).
However, contents must be below 1.0%, preferably below 0.7%, typically lower than 0.60%, so that, after quenching, the presence of retained austenite is not very high, and also in order to avoid the promotion of high quantities of secondary precipitated carbides in grain contours. According to above ration, carbon contents must be sufficient to promote needed hardness and mechanic resistance of material, and should be above 0.05%, preferably above 0.1%, typically above 0.15%.
Mn: since the cost is not high and due to its effectiveness to increase the hardenability, the manganese must be used in high contents in the steel of present invention. Therefore, its contents must be higher than 0.5%, preferably higher than 1.0% and typically above 1.5%. However, when excessive, manganese increases retained austenite and the strain hardening of material, causes loss of machining capacity, and also increases the hydrogen solubility, and promotes the formation of flakes. Therefore, manganese contents must be limited to a maximum of 5.0%, preferably a maximum of 3.0%, being typically below 2.5%.
P, B, Ti and V: these four elements have a fundamental role in the steel of present invention, acting jointly to decrease the nucleation of diffusion phases, such as ferrite or perlite phases. Depending on the volumetric fraction, these phases may significantly decrease the hardness, and turn infeasible using the material. The explanation of the decrease of nucleation is based on the concentration of these elements in austenite grain contours; these regions have high free energy and, therefore, they are the initial regions of ferrite and perlite formation. When occupied by phosphorous or boron, or even in the presence of titanium and vanadium carbonitrides, the grain contours are unavailable for the formation of diffusion phases, ferrite or micro constituent perlite phases. Consequently, when these phases are inhibited, the thermodynamic conditions generate a bainite formation, with higher hardness and, in the alloys of present invention, also homogeneous along the bar section.
In this sense, the strongest effect is caused by boron, which was empirically determined as being 10 times higher than that of titanium and phosphorus. In the case of vanadium, part of contents added (about 0.07%) is in sodium solution at 700° C., the temperature at which perlite or ferrite are formed. For this reason, the ratio treats vanadium through the formula (V−0.10). The titanium also promotes the formation of carbonitrides, but since their solubility is low, the titanium is fully considered in the ratio. Therefore, we come to a ration for NU, which correlates the joint effect of these elements:
NU=% Ti+% P+10% B+(% V−0.07%)
When the total of this ratio is very low, this means that the effect of occupying the grain contours is low, and the diffusion phases must be formed more quickly. The results of several compositions indicate that the minimum amount of this relation must be 0.02%, typically 0.06%.
However, extremely high contents of phosphorous, boron, titanium or vanadium promote brittling for several reasons. Titanium and vanadium are strong formers of carbides that, when excessive, may facilitate the propagation of cracking. The excess of carbides is also undesirable for mould applications, since they harm the machining and polishing capacities of material. On the other hand, phosphorous, when segregated excessively in grain contours and other interfaces, promotes brittling by decreasing local cohesion (weakening chemical links between atoms in the interfaces). Excessive boron also may promote similar effects to that of phosphorous, however with the great disadvantage of providing the formation of carbides in grain contours, promoting brittling of these regions and material as a whole. For these reasons, maximum contents of these elements must be controlled, with the definition of a limit for NU ratio. The results shown in the examples indicate that the NU must be below 0.30%, typically below 0.20%.
Ti: although it has been already described above, titanium also has another effect on steel of present invention—to “protect” boron from the reaction with nitrogen (due to higher affinity of titanium with nitrogen than boron with nitrogen). Therefore, it allows boron to have a segregation effect on contours, avoiding its combination with nitrogen. To achieve such effect, titanium must be higher than 0.010%, typically above 0.015%.
B: As a strongest brittling effect by boron was identified, this element must also be individually limited, with a maximum of 0.010%, preferably a maximum of 0.007% and typically below 0.004%.
Ni, Mo e Cr: these three elements promote an increase of hardenability, due to its effect on the growth of diffusion phases, whether distributed in pearlite micro constituents or by proeutectoid ferrite. After formation, these phases have balance contents and, to be formed, the diffusion of elements in excess must occur. The time for this diffusion may delay the formation process, being the effect of chromium, molybdenum and nickel related to this. Traditionally, this effect is quantified by hardenability factors, used in following equation:
G=[0.13Ni+0.60Mo+0.26Cr]
This equation shows the combined effect of three elements when inhibiting the growth of formed phase. Associated to previous factor, which inhibits nucleation, it is possible to inhibit the formation of diffusion phases, in the morphology of proeutectoid ferrite or pearlite, thus generating the formation of bainite—with higher hardness and mechanic resistance. Therefore, the value of G must assume a minimum value of 0.1%, preferably above 0.2%, typically above 0.25%. For thinner gauges (for example, lower than 400 mm), lower values of G may be sufficient, such as values between 0.1% or 0.2%. This is interesting to decrease the final cost of alloy, since nickel, molybdenum and chromium had a significant valorization in the last few years. Besides the cost, contents of these elements must be controlled, to inhibit the formation of martensite. If this phase is obtained, the superficial hardness of blocks or bars will be very superior to nucleus hardness. In other words, excessively high contents of ratio G promote loss of homogeneity of target hardness, besides increasing the cost of alloy. The value of G must be lower than 1.0%, preferably lower than 0.5% and typically lower than 0.4%. The three elements may be replaced with copper that, although being a significant contaminant of scrap material, has a similar effect on hardenability. If copper is used, it must replace nickel, molybdenum or chromium in mass equivalent proportions.
Ni: besides those above, minimum contents of nickel may be applied to avoid the precipitation of carbides and increase the toughness. In these cases, minimum contents of nickel must be 0.1%, preferably 0.3%.
Cr: besides being contained in ratio G, maximum contents of chromium may be applied to avoid loss of thermal conductivity. Therefore, chromium contents must be limited to 1.5%, preferably below 1.0% and typically between 0.1% and 0.8%.
S: in the steel of this invention, sulphur forms inclusions of manganese sulphide, which become elongated by hot forming process. Since they are malleable and liquid at temperatures developed in the machining process, these inclusions facilitate the break of the flute and lubricate the cutting tool, improving the machining capacity. Therefore, sulphur contents must be above 0.001%, preferably above 0.005%, typically above 0.010%. However, since not all applications require high machining capacity, the use of a sulphur band is optional. Although helping with machining process, the inclusions of manganese sulphide damage the superficial quality given by polishing and also the mechanical properties. Therefore, sulphur contents must be below 0.20%, preferably below 0.05%, typically below 0.010%.
Ca: calcium also has an effect on inclusions, changing the hard inclusions of aluminum, which harm the machining capacity, decreasing the size (spheroidizing) the inclusions in general. However, the calcium contents control is complex, due to its high reactivity. As such, the use of calcium may also be considered optional, for those cases where high machining and polishing capacities are needed. When used, calcium must be in contents above 5 ppm, preferably above 10 ppm, typically above 20 ppm. Excessive contents of calcium may promote the attack of refractories used in channels and cast devices, excessively increasing the inclusion fraction. Therefore, when added, the final contents of calcium must be below 100 ppm, preferably below 50 ppm, typically below 30 ppm.
Al: since it forms hard aluminum inclusions, the aluminum contents may not be too high, to avoid damages to machining. It must be below 0.5%, typically below 0.1%, preferably below 0.05%.
N: nitrogen is needed to form titanium and vanadium carbonitrides, which inhibit grain growth, help decreasing the free energy of grain contour and avoid nucleation of diffusion phases. On the other hand, excessive nitrogen may react with boron and inhibit the effect of this element in decreasing the energy of grain contour. Also, excessively high contents of nitrogen promote higher formation of titanium carbonitrides, which is harmful to material machining capacity. So, nitrogen must be below 0.1%, typically below 0.05%, preferably between 0.003% and 0.015%.
Si: besides its use as de-oxidant, which is important in situations where aluminum contents are low, as is the case with the steel of this invention, silicon has an important effect in the formation of carbides. This element inhibits the formation of cementite and, as shown in the examples, of other carbides that precipitate in grain contours and undermine the material. For all these effects, contents of silicon must be between 0.05% and 3.0%, preferably between 0.1% and 2.0%, typically between 0.3% and 1.5%.
The process of material production, more specifically its thermal treatment, is also important. As described, the material was designed to have very high hardenability and capacity of homogeneous hardening along the section. Therefore, the material may be air cooled, for most gauges. Such cooling must be employed from a heating temperature above critical AC3 temperature (approximately 850° C.), from a furnace or even directly after hot conformation of material. To obtain better toughness, quicker cooling may be used, for example, through water, oil or forced convection of air, or even water spray. Therefore, the cooling method during hardening may vary, depending on equipment and the toughness required to specific application. This is possible only due to the high hardenability, provided by the fine adjustment of chemical composition previously defined.
In the following description of experiments performed and compositions studied, reference is made to the attached figures, where:
In order to define the compositions of steel of this invention, several alloys were produced and compared to the state of the art ones. Experimental bars were produced, and chemical compositions that were obtained are shown in Table 2, being hereafter called by their sequence numbers; for comparison, a study of a typical composition of DIN 1.2738 steel (very used in plastic moulds and other applications in tool bases) was conducted. Before discussing the results of hardness, it is interesting to note in Table 3 the significant decrease of alloy elements in this invention compositions, which is converted to lower cost.
Table 2 shows the values of NU and G, from ratios previously described, related to inhibition of nucleation and growth of diffusion phases. For each composition, studies of dilatometry were made, and the hardness obtained for cooling rate of 0.05° C./s is also shown in Table 2, being this cooling equivalent to a block of 400 mm air cooled. The objective of such hardness is to be within the range of 30 to 34 HRC, for typical applications of moulds and plastic mould holders. Therefore, the hardness in this range is called as “OK”, being the strength out of this range denominated high or low.
When these results are in the graphical form, as shown in
The explanation for this result is directly related to the nucleation and growth mechanisms. First, the elements that promote the decrease of energy in grain contour are fundamental and, therefore, they avoid formation of diffusion compounds, which cause lower hardness (whether proeutectoid ferrite or ferrite and cementite in pearlite morphology). Such role is provided by the elements that form the NU factor of the formula. Titanium and vanadium tend to form precipitated compounds in grain contours (carbides or carbonitrides), decreasing the free energy of these regions. At ferrite or pearlite formation temperature (around 700° C.), the solubility of titanium is low, being ignored; however, the solubility of vanadium is high and, therefore, its content is decreased by a factor of 0.07%, equivalent approximately to vanadium in solid solution at 700° C. On the other hand, phosphorous and boron tend to segregate and concentrate in these regions, causing as well the decrease of its energy, consequently avoiding nucleation of diffusion phases. The effect of boron was empirically determined as about 10 times higher the phosphorous effect and, therefore, its factor is multiplied by 10. Besides the intrinsic effect of titanium in the formation of carbonitrides, these compounds remove the free nitrogen from matrix, which tends to react with boron and eliminate the important effect of boron when segregated in the grain contour.
Besides avoiding nucleation, the presence of the elements that inhibit the growth of diffusion phases is important. The elements that form this factor in the alloy of this invention are manganese, nickel and chromium. The graphic of
From 17 alloys shown in graphic of Table 1, some represent very well the effect of the alloy elements studied, as is explained below. Alloys 1 and 2 show the effect of phosphorous contents that, when very low, generates low values of NU, and do not reach required hardness. However when this is higher than 0.020% (alloy 2), hardness is very close to the required one. Alloys 5 and 17 have low chromium, nickel or molybdenum contents, thus damaging the ratio G and, consequently, not achieving the required hardness. On the other hand, alloys 7, 8 and 9 show that contents excessively high of Cr and Ni elements result in high values of G, causing excessively high hardness (due to the formation of part of martensite). Alloy 15 shows the importance of vanadium that, at low contents, generates significant decrease of NU value and, consequently, significant decrease of hardness. Therefore, vanadium may be considered absolutely needed for the alloy.
One last and important comment refers to alloy 10. This alloy is the only that is out of proposed list, but the reason is simple. Alloy 10 has low contents of titanium, which would cause the decrease of NU value. However, the decrease of hardness was much more significant than foreseen. This occurs because the lack of titanium generates loss of boron effect, since the lack of titanium leaves more free nitrogen to react with boron and, therefore, to promote loss of its effect (described by some authors as effective boron). This synergetic effect cannot be explained by NU and G equations and, therefore, the alloys of this invention have a special requirement as to titanium.
To give an example,
Once the best composition is defined by pilot studies described in example 1, several industrial lots were produced, with different geometries, as shown in Table 4.
In spite of the homogeneous hardness within the adequate range, the industrial heats, particularly in blocks with sections higher than 400 mm, showed a significantly lower toughness than DIN 1,2738 steel (the reference for this application), with values for the impact test without inserts of about 200 joules (test specimen with 7×10 mm). The comparison between values of toughness to microstructure of material showed that the main cause of such low values is the precipitation of carbides in grain contours, as shows
As to chemical composition, it was observed that the quantity of carbides grows as boron contents increases and decreases when silicon contents increases, with no significant effect for phosphorous contents; Table 5 shows chemical the compositions used in this evaluation. The conclusions may be based on the compositions of Table 6, with results shown in
In the case of boron, although important for the hardenability (example 1), excessive contents help with the formation of these carbides: note that for the quantity of carbides (Table 6) there is twice the increase when boron increases from 20 to 40 ppm.
Probably, this is due to a high condition of metastability, when high contents of boron are concentrated in grain contours, helping with carbide precipitation.
The phenomenon is stronger for large bars, with intense microsegregation effects, generating an increase in the local concentration of boron.
A similar effect, however less intense, occurs with the decrease of silicon contents, as shown in
Therefore, for the production of large bars with high toughness and adequate hardenability, the use of high contents of silicon (between 0.2 and 1.0%), and minimum boron contents is more suitable, being this minimum defined by the factor NU described in Example 1.
The metallurgical reasons for such effects have distinct explanations, which may be discussed in this document. In high contents, boron tends to concentrate in grain contours, forming complex carbides, particularly with Fe and Cr. Through electronic microscopy analysis performed in the steels of present invention, these two elements were found, as well as traits of Mo. Therefore, a decrease in boron contents eliminates carbides in the origin of problem. However, this decrease cannot be excessive, because to avoid formation of ferrite the presence of boron is required in grain contours, as described in example 1. On the other hand, silicon has low solubility in iron carbide (cementite), avoiding its formation in steels. Since these carbides in the steel of this invention also have high contents of iron, the silicon mechanism may be understood as the same that occurs with cementite.
Besides the change in chemical composition, a way to avoid such precipitation is to promote the quick cooling—from high temperature, where carbides are not present yet. These tests were performed as shown in
In order to reduce such intense precipitation, after forging or after a treatment of austenitizing/solubilization, the block may be quickly cooled. Such process was designed based on the results of numerical simulation, and may be applied by cooling in oil or water. In the case of water, to avoid cracks, some steps on air may be introduced, reducing the temperature difference existing between the surface and nucleus. Table 7 shows the results of such experiments, observing a significant increase of strength when the cooling speed is superior. Obviously, this process must be applied to large blocks, where cooling rates are inherently low, or in situations requiring high strength. In opposite cases, air cooling may be applied.
In previous examples, chemical composition and thermal treatment of steel proposed for this invention were defined. Due to the use of titanium in the chemical composition, hard particles of carbonitrides are formed—resulting in higher wear of tools, and damaging material machining capacity. For mould applications, the aspects of machining are essential.
To avoid this, alloys of present invention were studied in terms of highest contents of sulphur and calcium. These two elements influence the formation of inclusions. Sulphur forms manganese sulphides, which have low hardness and help breaking the chip and tool lubrication. On the other hand, calcium changes hard aluminum inclusions, generating complex inclusions with better machining capacity. The addition of calcium also spheroidizes the inclusions, generating better polishing conditions, which is also an important operation for plastic moulds.
Table 8 shows the results of machining for the steel of this invention, with this change in sulphur and calcium contents in alloy 19, and without this change in alloy 18. By comparison, the same test was performed with DIN 1.2738 steel (reference for application in moulds). There is a noticeable increase in the tooled volume, with change performed in calcium and sulphur contents (of alloys 18 and 19).
An alternative to improve machining capacity would be to reduce the volumetric fraction of carbonitrides, thus reducing the cause of accelerated wear of tools. The combination of the effect of decreasing carbonitride volume plus the use of high contents of sulphur and calcium was used in alloy 37. Although similar in composition to alloy 19, there was a significant increase in machining capacity, associated to a drastic decrease of titanium cabonitride volume. In this case, the decrease of carbides was performed by the increase of solidification speed, through the use of a smaller bar. However, the same may occur by reducing contents of nitrogen or titanium. Since titanium is important for NU factor, discussed in example 1, the use of low contents of nitrogen, as controller of carbonitride volume, proves to be most important.
The entire design of previous alloy was based to provide 30 to 34 HRC hardness, since this is the main use range for mould steels. For conventional steels, higher hardness may be obtained using different conditions of annealing treatment. In the steel of present invention, with direct hardening through forging, this may not be performed. Therefore, this invention also tried to provide an alternative to increase hardness, through a change in chemical composition.
From the base composition described in Example 1, compositions with different carbon contents were produced, simulating by dilatometry the cooling of a nucleus in a block with section of about 400 mm (rate of 0.05° C./s). The results in Table 9 and
Hardness HV=450% C+210.
(hardness obtained after cooling at 0.05° C./s, equivalent to air cooling of a block with 400 mm thickness).
Therefore, this example shows that it is possible to assign different hardness to the alloy of this invention, adjusting carbon contents. For example, for hardness 315 HV (approximately 32 HRC), the range obtained in Example 1 is confirmed, being necessary 0.23% of carbon. On the other hand, for hardness of 400 HV (about 40 HRC), 0.42% of carbon contents would be necessary.
As shown in Table 1, the state of the art steels for that same hardness range have carbon contents significantly higher: DIN 1.2738, hardness of 32 HRC and 0.36% of carbon, and DIN 1.2711, hardness of 40 HRC and 0.52% of carbon. This fact has an interesting consequence for the welding processes, which are very used in moulds. Since they work with lowest carbon contents, the hardness of heated region will be much lower in the steel of present invention, as compared to state of the art steels. For carbon contents of 0.23%, the steel of present invention generates an approximate hardness of 45 HRC in the region affected by welding, while this hardness is about 60 HRC for DIN 1.2738 steel and 64 HRC for DIN 1.2711. This fact helps in many machining operations after welding, as well as in the aspect after polishing or texturisation.
Small variations in indexes of previous equations may produce adequate results, within the necessary hardness range for the applications. For a ±20 HB variation, commonly accepted in the industry, the ratio may vary according to
Hardness HV=(450±140)% C+(210±45).
Therefore, depending on the industrial application and the necessary hardness, carbon contents in the steel of present invention must be calculated by above equation.
| Number | Date | Country | Kind |
|---|---|---|---|
| PI 0901378-4 3 | Apr 2009 | BR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/BR2010/000121 | 4/8/2010 | WO | 00 | 12/7/2011 |
