MX2011010440A - Bainitic steel for moulds. - Google Patents

Abstract

A bainitic steel for moulds has the following composition of alloying elements, in percentage by weight: 0.05-1.0 carbon; 0.5-3.0 manganese; phosphorus, boron, titanium and vanadium in a ratio NU = [%Ti + %P + 10%B + (%V - 0.10)], where the values of NU range from 0.02 to 0.30, those of titanium are always higher than 0.005, those of boron always lower than 0.010 and vanadium can be entirely or partially replaced by niobium in a proportion of two parts by weight niobium for one part by weight vanadium; nickel, molybdenum and chromium in a ratio G = [0.13% Ni + 0.60% Mo + 0.26% Cr], where the values of G are higher than 0.10 and lower than 1.0; those of nickel are higher than 0.1; those of molybdenum range from 0.07 to 0.27; those of vanadium from 0.1-0.15; those of sulphur do not exceed 0.10; those of silicon range from 0.05 to 3.0; those of nitrogen are lower than 0.10; the calcium content does not exceed 0.02; the aluminium content is lower than 0.5; the chromium content is lower than 1.5; the silicon content ranges from 0.05 to 3.0; the phosphorus content is lower than 0.075 and the remainder consists substantially of iron and inevitable impurities due to the preparation process. In order to produce the bainitic steel for moulds, the final hardness can be obtained by slow cooling in air directly after hot shaping or previous heating in a furnace, even in blocks having a section of up to 1000 mmm; the Vickers hardness values are defined by the equation HV = (450 Â+- 140)% C + (210 Â+- 45), for values ranging from 280 to 450 HV (30-45 HRC); for uses requiring high strength, the steel according to the present invention can also be produced by accelerated cooling from temperatures above 900°C in water and oil.

Description

BAINITIC STEEL FOR MOLDS Description of the invention The present invention relates to the bainitic steel for various applications in tools, molds, mold carriers, tool holders, which have as their main characteristic the homogeneous hardness obtained through a bainitic transformation, without the need for high contents of expensive elements, such as nickel and molybdenum or the shutdown process. Consequently, such steels provide a considerable gain in cost in the alloy, and heat treatment of large blocks to which they are applied. The careful design of the alloy, based on its microstructural aspects, provides the steel subject of the present invention, with hardness and properties similar to those of the traditional hard alloys used in tools, in molds and in foundations, but with a significant decrease in Its cost.

The tools and the molds in general are used in other processes of conformation of materials, either polymeric thermoplastic materials (commonly known as plastic materials) or metallic materials. Depending on the properties of the material used to make them, the tools are used in processes at room temperature or at high temperatures that in general REF.224200 They reach 700 ° C. The steels of this invention are applied particularly to molds or tools that operate at room temperature, or at temperatures below 500 ° C, as well as in mold holders or tool holders for general use. A typical example of such applications could be the molds for shaping plastics, which generally do not exceed 300 ° C. Also, these are applied to mold holders and tool holders, which normally work at room temperature, but support the tension of the tools used in various conditions.

Therefore, plastic molds and mold carriers can be considered as typical applications for the steels of this invention. In such applications, many characteristics of the materials from which the tools are made are important, some related to the use of the molds and others related to their manufacture. Regarding the characteristics of the use of the mold or of the mold holder, the resistance property is important, being commonly related to the hardness of the material, as well as to the homogeneity along the section of the material. On the other hand, properties such as the response to polishing, texturization and machinability of the material are important for the economical manufacture of the mold or mold carrier.

To achieve such requirements, steel Traditional products undergo thermal treatment by means of quenching and tempering. The quenching treatment is complex for blocks with large dimensions, and they need to be quickly cooled in oil tanks or in aqueous media loaded with polymers. For blocks applied to large molds, tanks with more than 80,000 are used, resulting in significant operating difficulties. In addition to 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 shown in Table 1, these elements are found with significant contents in the steels of the state of the art, being also related to the final required hardness.

In this sense, new developments are being made. The purpose of the patents EP0805220 and US5855846, for example, is the production of bainitic steels with lower contents of alloying elements for application in molds. However, in this invention the hardness is obtained with higher chromium contents (within the same range of DIN 1.2738), decreasing any possible gain in thermal conductivity and also generating a higher cost. The invention US5695576, on the other hand, shows a concept of use with high contents of aluminum and silicon, which can damage the capacity of machining of the alloy, due to the presence of non-metallic inclusions. Also, high silicon contents can damage hardenability, as will be shown in example 2. Patents PI9602054-7 and PI0308832-4 follow the same concept, but try to obtain only higher hardness ranges (between 430 and 530 HB) and thickness less than 200 mm, while the largest volume of applications are the 300 HB molds, without fulfilling this need. None of these patents shows the examples of application in large blocks (with thicknesses above 200 mm) without the need for the off treatment (for example with air cooling). Neither do they discuss the possibilities of avoiding possible embrittlement due to slow cooling, either by adjusting the alloy or by means of heat treatment.

Table 1: Alloys of the state of the art.

Only main alloying elements are shown, in percentages in mass and iron balance.

Designation C Cr Mn Ni Mo Hardness Notes of the typical standard (HRC) DIN Nr 1. 2739 * 0.40 2.0 1.4 1.1 0.20 32 General application 1. 2311 0.40 2.0 1.4 32 Sections up to 500 mm (low hardenability) 1. 2312 0.40 2.0 1.5 32 S = 0.07; for applications with high machining volume 1. 2711 * 0.52 0.75 0.7 1.8 0.3 40 V = 0.10 1. 2344 * 0.36 5.0 1.2 40 to V = 0.10 fifty * Most significant of the class.

Therefore, the difficulties and costs involved in obtaining the hardness of steel blocks for tools are clear, either through the chemical composition or a special process of thermal treatment. Consequently, the need for a steel capable of being hardened to produce large blocks (with sections above 500 mm), without using rapid cooling, and also without the use of significant contents of alloying elements is evident. And preferably, such steel must be able to meet the full hardness range applied to the molds, for example between 300 and 420 HB.

The subject matter of this invention is to meet all of these needs.

The bainitic steel proposed in this invention can be hardened, without the need for quenching, and also has a poor chemical composition in terms of high cost element, such as nickel, molybdenum and chromium.

In order to meet the above conditions, the alloys of the present invention have compositions of alloying elements which, in mass percentage, consist of: * Carbon: between 0.05 and 1.0, preferably 0.1 and 0.7, typically 0.15 and 0.6.

* Manganese: between 0.5 and 5.0, preferably 1.0 and 3.0, typically between 1.5 and 2.5. The manganese can be replaced, partially or completely, with nickel or copper, at a ratio of one part by mass of manganese to one part by mass of copper or nickel.

* Phosphorus, boron, titanium and vanadium: these have a similar effect and, therefore, must be dosed according to the following ratio NU = [Ti P + 10B + (V-0.10)]; where NU should have values between 0.02 and 0.30, typically between 0.06 and 0.20. Vanadium can be partially or totally replaced by niobium or tantalum, in a mass proportion where one part of vanadium is equivalent to 2 parts of niobium or tantalum.

* Titanium: regardless of the NU ratio, the minimum titanium content should be 0.005, typically about 0.015 and preferably above 0.020; however, it should never be higher than 0.10, preferably be less than 0.05 and typically less than 0.040.

* Boron: in addition to the above proportion, the maximum boron content must be controlled, being below 0.010, preferably below 0.007, typically below 0.004.

* Nickel, molybdenum and chromium have a similar effect and should be dosed according to the following ratio: G = [0.13 NÍ + 0.60 Mo + 0.26 Cr]; the values of G must be above 0.1 and below 1.0, preferably between 0.2 and 0.5, typically between 0.25 and 0.4. Molybdenum must be partially or totally replaced with tungsten, in a proportion by mass where one part of molybdenum is equivalent to two parts of tungsten. In this proportion, the Ni can be totally or partially replaced with copper, in a proportion where 1 part of nickel is equivalent to one part of copper.

* Nickel: in addition to those above, the minimum contents of 0.1 nickel, preferably 0.3, typically 0.4.

* Chrome: in addition to being contained in the G proportion, the maximum content of chromium can be applied of 1.5, preferably below 1.5, preferably below of 1.0, typically between 0.1 and 0.8.

Sulfur: below 0.10, below 0.05, typically between 0.001 and 0.010.

* Calcium: must be present in contents up to 0.010, preferably up to 0.005, typically between 0.0005 (5 ppm) and 0.003 (30 ppm).

* Aluminum: should be below 0.5, typically below 0.1, preferably below 0.02.

* Nitrogen: must be below 0.1, typically below 0.05, preferably between 0.003 and 0.015.

* Silicon: between 0.05 and 3.0, preferably between 0.1 and 2.0, typically between 0.3 and 1.5.

The rest is iron and metallic or non-metallic impurities that are common to the processes of working with steel.

Below are the reasons for specifying the composition of the new material, which describe the effect of each alloy element. The percentages indicated refer to the percentage by mass.

C: the carbon is the most responsible for the response to the heat treatment, for the hardness of the martensite or bainite, the latter being the most important microconstituent of the steels of the present invention. He Carbon content, therefore, controls the final hardness obtained for the steels of the present invention, which may vary depending on the requirements of the application. Consequently, the carbon content must be as high as the necessary hardness (according to an equation defined later in Example 5), according to the following equation: Hardness HV = (450 ± 140)% C + (210 ± 45).

However, the content should be less than 1.0%, preferably less than 0.7%, typically less than 0.60%, so that after shutdown, the presence of the retained austenite is not very high, and also in order to avoid promotion of high amounts of secondary precipitated carbides in the grain contours. According to the above explanation, the carbon content should be sufficient to promote the necessary hardness and mechanical strength of the material, and should be above 0.05%, preferably above 0.1%, typically above 0.15%.

Manganese: since the cost is not high and due to its effectiveness in increasing hardenability, manganese must be used at high contents in the steel of the present invention. Therefore, its content must be greater than 0.5%, preferably greater than 1.0% and typically above 1.5%. However, when it is excessive, manganese increases the retained austenite and the Stress hardening of the material causes loss of machinability, and also increases the solubility of hydrogen, and promotes the formation of flakes. Therefore, the manganese content should be limited to a maximum of 5.0%, preferably a maximum of 3.0%, typically less than 2.5%.

Phosphorus, boron, titanium and vanadium: these four elements have a fundamental role in the steel of the present invention, acting together to decrease the nucleation of the diffusion phases, such as the ferrite or pearlite phases. Depending on the volumetric fraction, these phases can significantly reduce the hardness, and make the use of the material not feasible. The explanation for the decrease in nucleation is based on the concentration of these elements in the contours of the austenite grain, these regions have high free energy and, therefore, these are the initial regions of ferrite and perlite formation. When they are occupied by phosphorus or boron or even in the presence of titanium and vanadium carbonitrides, the contours of the grain are not available for the formation of diffusion phases1, ferrite or microconstituent perlite phases. Consequently, when these phases are inhibited, the thermodynamic conditions generate a bainite formation, with higher hardness and, in the alloys of the present invention, also homogeneous as length of the bar section.

In this sense, the strongest effect is caused by boron, which was empirically determined to be 10 times higher than that of titanium and phosphorus. In the case of vanadium, part of the aggregate contents (approximately 0.07%) is in the sodium solution at 700 ° C, the temperature at which perlite or ferrite are formed. For this reason, the proportion considers vanadium through the formula (V - 0.10). Titanium also promotes the formation of carbonitrides, but since its solubility is low, titanium is completely considered in proportion. Therefore, we arrive at an explanation for NU, which correlates the joint effect of these elements: NU =% Ti +% P + 10% B + (% V-0.07%) When the total of this proportion is very low, this means that the effect of occupation of the contours of the grain is low, and the diffusion phases must be formed more quickly. The results of several compositions indicate that the minimum amount of this ratio should be 0.02%, typically 0.06%.

However, extremely high contents of phosphorus, boron, titanium or vanadium promote embrittlement for various reasons. Titanium and vanadium are strong carbide formers that, when excessive, can facilitate the propagation of cracking. The excess of the carbides is also undesirable for mold applications, since these damage the machining and polishing capabilities of the material. On the other hand, phosphorus, when it is excessively segregated in the contours of grain and other interfaces, promotes embrittlement by diminishing local cohesion (weakening of chemical bonds between the atoms at the interfaces). Boron in excess can also promote effects similar to that of phosphorus, however, with the greatest disadvantage of providing the formation of carbides in grain contours, promoting the embrittlement of these regions and the material as a whole. For these reasons, the maximum contents of these materials must be controlled with the definition of a limit for the NU ratio. The results shown in the examples indicate that the UN should be below 0.30%, typically below 0.20%.

Titanium: although it has already been described above, titanium has yet another effect on the steel of the present invention - "protecting" boron from reaction with nitrogen) (due to the higher affinity of titanium with nitrogen than boron with nitrogen). Therefore, it allows the boron to have a segregation effect on the contours, avoiding its combination with nitrogen. To achieve this effect, titanium must be greater than 0.10%, typically greater than 0.015%.

B .: Since a stronger embrittlement effect was identified by boron, this element must also be individually limited, with a maximum of 0.010%, preferably a maximum of 0.007% and typically less than 0.004%.

Nickel, Molybdenum and Chromium: these three elements promote an increase in hardenability, due to their effect on the growth of diffusion phases, either distributed in perlite microconstituents or by proeutectoid ferrite. After the training, these phases have balance contents and, to be formed, the diffusion of the excess elements can occur. The time for this diffusion can retard the formation process, being the effect of chromium, molybdenum and nickel related to this. Traditionally, this effect is quantified by the hardenability factors, used in the following equation G = [0.13NÍ + 0.60 Mo + 0.26 Cr] This equation shows the combined effect of three elements when the growth of the formed phase is inhibited. Associated with the previous factor, which inhibits nucleation, it is possible to inhibit the formation of the diffusion phases, in the morphology of the proeutectoid ferrite or perlite, thus generating the formation of bainite - with higher mechanical resistance and hardness. Therefore, the value of G can assume a minimum value of 0.1%, preferably above 0.2%, typically above 0.25%. For thinner calibers (for example, smaller than 400 mm), lower G values may be sufficient, such as values between 0.1% or 0.2%. This is interesting to decrease the final cost of the alloy, since nickel, molybdenum and chromium had a significant value in the last few years. In addition to the cost, the contents of these elements must be controlled to inhibit the formation of martensite. If this phase is obtained, the surface hardness of the blocks or bars will be much higher than the hardness of the core. In other words, excessively high contents of the G ratio promote the loss of homogeneity of the target hardness, in addition to increasing the cost of the alloy. The value of G should be less than 1.0%, preferably less than 0.5% and typically less than 0.4%. The three elements can be replaced with copper which, although it is a significant pollutant of the waste material, has a similar effect on the hardenability. If copper is used, it must replace nickel, molybdenum or chromium in equivalent mass proportions.

Nickel: in addition to those above, the minimum nickel content can be applied to avoid precipitation of carbides and increase toughness.

In these cases, the minimum nickel content should be 0.1%, preferably 0.3%.

Chromium: in addition to being contained in the G proportion, the maximum chromium contents can be applied to avoid loss of thermal conductivity. Therefore, chromium contents should be limited to 1.5%, preferably below 1.0% and typically between 0.1% and 0.8%.

Sulfur: in the steel of this invention, the sulfur forms manganese sulfide inclusions, which become elongated by a hot forming process. Since these are malleable and liquid at temperatures developed in the machining process, these inclusions facilitate the breaking of the ripple and lubricate the cutting tool, improving machinability. Therefore, the sulfur contents should be greater than 0.001%, preferably higher than 0.005%, typically higher than 0.010%. However, since not all applications require high machining capacity, the use of a sulfur band is optional. Although they help with the machining process, the manganese sulphide inclusions damage the surface quality given by the polishing and also the mechanical properties. Therefore, the sulfur content should be less than 0.20%, preferably less than 0.05%, typically less than O .010%.

Calcium: calcium also has an effect on inclusions, changing the hard inclusions of aluminum, which damage the capacity of machining, decreasing the size (spheroidization) of inclusions in general. However, the control of calcium content is complex, due to its high reactivity. As such, the use of calcium can be considered optional, for those cases where high machining and polishing capacities are needed. When used, the calcium should be in contents above 5 ppm, preferably above 10 ppm, typically above 20 ppm. Excessive calcium contents can promote the attack of the refractories used in the channels and emptying devices, increasing the inclusion fraction excessively. Therefore, when added, the final calcium contents should be below 100 ppm, preferably below 50 ppm, typically below 30 ppm.

Aluminum: since this forms hard aluminum inclusions, the aluminum contents can not be too high, to avoid damage to the machining. This should be below 0.5%, typically below 0.1%, preferably below 0.05%.

Nitrogen: nitrogen is necessary to form titanium and vanadium carbonitrides, which inhibit the grain growth, they help to diminish the free energy of the contour of the grain and they avoid the nucleation of the phases of diffusion. On the other hand, excessive nitrogen can react with boron and inhibit the effect of this element in the decrease of the grain contour energy. Also, excessively high nitrogen contents promote the higher formation of titanium carbonitride, which is harmful to the machinability of the material. Thus, nitrogen should be below 0.1%, typically below 0.05%, preferably less than 0.003% and 0.015%.

Silicon: in addition to its use as a deoxidizer, which is important in situations where the aluminum contents are low, as is the case with the steel of this invention, silicon has an important effect on the formation of carbides. This element inhibits the formation of cementite and, as it is shown in the examples, of other carbides that precipitate in the contours of the grain and deteriorate the material. For all these effects, the silicon contents should be between 0.05% and 3.0%, preferably between 0.1% and 2.0%, typically between 0.3% and 1.5%.

The process of production of the material, more specifically its thermal treatment, is also important. As described, the material was designed to have a very high hardenability and a homogeneous hardening capacity throughout the section. Therefore, the material It can be cooled with air, for most calibers. Such cooling must be employed from a heating temperature above the critical AC3 temperature (approximately 850 ° C), from a furnace or even directly after the hot forming of the material. To obtain better tenacity, a faster cooling must be used, for example, through water, oil or forced air convection, or even sprinkling with water. Therefore, the cooling method during hardening may vary, depending on the equipment and the toughness required for the specific application. This is possible only due to the high hardenability, provided by the fine adjustment of the previously defined chemical composition.

In the following description of experiments performed and in the compositions studied, reference is made to the appended figures, where: Figure 1 refers to the graphic distribution of the compositions studied as a function of the NU and G factors, evaluating the hardness obtained after cooling to 0.05 ° C / second, starting at 1,150 ° C. The hardness between 30 and 34 HRC is considered adequate ("OK"), since this is the main hardness interval where the steels of the state of the art are used; Figure 2 refers to a curve of continuous transformation by cooling (CCT for its acronym in English), which it is typical in the steel of this invention, showing the phases that were formed. Field B indicates bainite, while M and F signify, respectively, martensite and ferrite. Note that for coarse calibers cooled with air, a hardness of around 310 HV is obtained, generating the 32 HRC required per application; Figure 3 refers to a curve of continuous transformation by cooling (CCT for its acronym in English), which is typical in steel DIN 1.2738 of the state of the art, showing the phases that were formed. The Bs and Bi fields indicate, respectively, upper and lower bainite, while M and P mean, respectively, martensite and perlite.

Figures 4a and 4b refer to the hardness measurements in two industrial blocks, in two different calibers, showing high uniformity of hardness.

Figure 5 refers to the evaluation of different alloys 18 to 21, regarding the tenacity towards impacts, calibres and micrographs are shown, in order to relate the values obtained to the carbide precipitation.

Figures 6a to 6d refer to the micrographs obtained for compositions 25 to 28, with different silicon contents. The compositions are shown in table 5.

Figures 7a to 7d refer to micrographs obtained for compositions 29 to 32, with different phosphorus contents. The compositions are shown in table 5.

Figures 8a to 8d refer to the micrographs obtained for compositions 33 to 36, with different boron contents. The compositions are shown in table 5.

Figure 9 refers to the microstructures and the tenacity of the samples that were subjected to thermal treatment by solubilization, followed by slow cooling at temperatures of 950, 850, 750 and 600 ° C. Attack: 2% Nital. Amplification: 200X.

Figures 10a and 10b refer to the evaluation of the proportion obtained for the hardness compared to the carbon contents: (Figure 10a) comparison of the precise calculated values and the measured hardness; (Figure 10b) equations that provide the hardness values between the upper and lower limits, for a variation of + 20 HB.

EXAMPLE 1: In order to define the steel compositions of this invention, several alloys were produced and compared to those of the state of the art. Experimental bars were produced, and the chemical compositions that were obtained are shown in Table 2, henceforth being called by their sequence number; for comparison, a study of a typical DIN 1.2738 steel composition (widely used in plastic molds and other applications in tool bases). Before discussing the results of the hardness, it is interesting to note in Table 3 the significant decrease of the elements of the alloy in the compositions of this invention, which translates into a lower cost.

Table 2 shows the values of NU and G, from the proportions previously described, related to the inhibition of nucleation and the growth of diffusion phases. For each composition, dilatometry studies were performed, and the hardness obtained for the cooling speed of 0.05 ° C / second is also shown in table 2, this cooling being equivalent to a 400 mm block cooled with air. The objective of such hardness is within the range of 30 to 34 HRC, for typical applications of plastic molds and mold carriers. Therefore, the hardness in this range is called "OK", the resistance being outside this range called high or low.

When these results are in the graphic form, as shown in Figure 1, the ideal work fields of the alloys of this invention are determined; in other words, fields with a combination of NU and G generate hardness within the target range. Therefore, this causes working in the limits of the alloys of this invention, in terms of the elements that form NU (Titanium, Phosphorus, Boron and Vanadium), and G (Chromium, Nickel and Molybdenum).

Table 2: alloys of this invention studied with different values of titanium, vanadium, boron, nickel chromium and molybdenum, to determine the effect of the proportions of NU and G 5 fifteen Table 3: Composition similar to that of DIN 1.2738 steel, studied in the example.

The explanation for this result is directly related to the mechanisms of nucleation and growth. Firstly, the elements that promote the reduction of the energy in the contour of the grain are fundamental and, therefore, they avoid the formation of diffusion compounds, which cause less hardness (be it ferrite proeutectoide or ferrite and cementite in the morphology of pearlite). Such paper is provided by the elements that form the NU factor of the formula. Titanium and vanadium tend to form compounds precipitated in the contours of the grain (carbides or carbonitrides), decreasing the free energy of these regions. At the 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%, approximately equivalent to vanadium in the solid solution at 700 ° C. On the other hand, phosphorus and boron tend to to segregate and concentrate in these regions, also causing the decrease of its energy, thus avoiding the nucleation of the diffusion phases. The effect of boron was empirically determined to be about 10 times higher than the effect of phosphorus and, therefore, its factor is multiplied by 10. In addition, the intrinsic effect of titanium in the formation of carbonitrides, these compounds eliminate the free nitrogen of the matrix, which tends to react with boron and eliminates the important effect of boron when it is segregated in the contour of the grain.

In addition to avoiding nucleation, the presence of 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 graph of figure 1 shows these elements, counted by the G factor, which are the multiplying indexes obtained from the classical hardening results of the elements. Manganese is not counted in G, because it is constant in all alloys. At very high contents, these elements promote excessive hardenability, generating the formation of martensite and increasing the hardness. And in very low quantities, the hardness becomes very low. This occurs because, even when nucleation is inhibited by high values of NU, the high growth tendency generates the formation of a significant amount of ferrite or perlite, decreasing the hardness.

From 17 alloys shown in the graph of table 1, some represent very well the effect of the elements of the alloy studied, as explained below. Alloys 1 and 2 show the effect of the phosphorus contents that, when they are very low, generate low values of NU, and do not reach the required hardness. However, when this is greater than 0.020% (alloy 2), the hardness is very close to that required. The alloys 5 and 17 have low chromium, nickel or molybdenum contents, thus damaging the G ratio and, consequently, not achieving the required hardness. On the other hand, alloys 7, 8 and 9 show that the excessively high contents of the chromium and nickel elements result in high values of G, causing excessively high hardness (due to the formation of the martensite part). Alloy 15 shows the importance of vanadium which, at low contents, generates a significant decrease in the NU value and, consequently, a significant decrease in hardness. Therefore, vanadium can be considered absolutely necessary for the alloy.

A last and important comment refers to the alloy 10. This alloy is the only one that is outside the proposed list, but the reason is simple. The alloy 10 has low titanium content, which could cause the decrease of the NU value. However, the decrease in hardness was much more significant than expected. This happens due because the lack of titanium generates loss of boron effect, since the lack of titanium leaves more nitrogen free to react with boron and, therefore, to promote the loss of its effect (described by some authors as effective boron) . This synergistic effect can not be explained by the equations NU and G and, therefore, the alloys of this invention have a special requirement for titanium.

To give an example, Figure 2 shows the CCT curve of a typical composition of this invention, which can be compared (in Figure 3) to the CCT curve of DIN 1.2738 steel of the state of the art.

Once the best composition is defined by the pilot studies described in example 1, several industrial batches were produced, with different geometries, as shown in table 4. Figures 4a and 4b show the hardness profile and a photograph of two large blocks produced according to the previous composition. The hardness around 285 to 310 HIB (30 to 34 HRC) was obtained with both, without any tendency to fall in the core regions.

EXAMPLE 2: despite the homogeneous hardness within the adequate range, the industrial heats, particularly in blocks with sections greater than 400 mm, showed a tenacity significantly lower than DIN 1.2738 steel (the reference for this application), with values for the impact test without inserts of approximately 200 joules (test specimen with 7x10 mm). The comparison between the tenacity values to the microstructure of the material showed that the main cause of such low values is the precipitation of the carbides of the contours of the grain, as shown in figure 5. Therefore, alternatives for the alloys were developed. of this invention to avoid the precipitation of these carbides and the consequent embrittlement of large blocks.

Regarding the chemical composition, it was observed that the amount of carbides increases as the boron content increases and decreases when the silicon content increases, without significant effect for phosphorus contents; Table 5 shows the chemical compositions used in this evaluation. The conclusions can be based on the compositions of Table 6, with the results shown in Figures 6a to 8d. It was also observed that nickel has an important effect, as shown by the comparison in figure 5 of alloys 18 and 19, for the same caliber; alloy 18 showed lower amounts of carbides due to the lower nickel contents.

Table 4: Alloy of the present invention, produced in industrial quantities, in blocks with different dimensions, showing the values for the hardness obtained, as well as the proportions of G and UN. 10 fifteen In the case of boron, although it is important for hardenability (example 1), the excessive contents help with the formation of these carbides: note that for the amount 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 the high boron contents are concentrated in the contours of the grain, helping with the carbide precipitation.

The phenomenon is stronger for large bars, with intense microegregation effects, generating an increase in the local concentration of boron.

Figures 8a to 8d show this effect, being very clear the increase in the amount of carbide in the samples with higher boron contents (the precipitation was promoted by a treatment that simulates the cooling of the blocks with larger core than the section of 800 mm, with very slow cooling at 36 ° C / hour).

A similar, albeit less intense, effect occurs with the decrease in silicon contents, as shown in Figures 7a to 7d; The use of silicon content above 0.40% tends to reduce the formation of these carbides. However, as shown in Table 6, the increase in the contents of silicon reduces the hardenability of the material of this invention (the fundamental property), particularly for contents above 1.0% (high volume of ferrite in the alloy with 2% silicon, according to table 6).

Therefore, for the production of large bars with high tenacity and adequate hardenability, the use of high silicon contents (between 0.2 and 1.0%) and minimum boron contents is more appropriate, this minimum being defined by the NU factor described in Example 1 Table 5: alloys of the present invention produced to study the effect of the contents of phosphorus, boron and silicon in the precipitation of the carbides that generate brittleness in the blocks with large dimensions The metallurgical reasons for such effects They have different explanations, which can be discussed in this document. In high contents, boron tends to concentrate in the contours of the grain, forming complex carbides, particularly with iron and chromium. Through the analysis of electron microscopy performed on the steels of the present invention, these two elements were found, as well as the molybdenum traces. Therefore, a decrease in the boron content eliminates the carbides at the source of the problem. However, this decrease can not be excessive, because to avoid the formation of ferrite, the presence of boron in the contours of the grain is required, 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 iron contents, the mechanism of silicon can be understood as the same as occurs with cementite.

Table 6: quantitative measurements of the volumetric fraction of the - carbides in the contours of the grain, by means of the image analysis method composed of manually identified carbides. The analysis of 10 fields per sample with amplification of ????, totaling an exploration of 14 tnm? for each sample. Prior to the measurement, the samples were subjected to solubilization at 1150 ° C and low cooling at 36 ° C / hour. The representative images are shown in figures 6a to 8d. * heats with variations of boron, alloys 32 to 35, have lower carbon contents (see table 5); therefore, these can not be compared with those of other variations, but these can be compared between them, showing the effect of boron.

EXAMPLE 3: In addition to the change in chemical composition one way to avoid such a situation is to promote rapid cooling from high temperature, where carbides are not yet present. These tests were performed as shown in Figure 9; note that below 800 ° C the drop in toughness is more significant, particularly between 750 ° C and 600 ° C, with this fall followed by carbide precipitation.

In order to reduce such intense precipitation, after forging or after the austenitizing / solubilizing treatment, the block can be rapidly cooled. This process was designed based on the results of numerical simulation, and can be applied by cooling in oil or water. In the case of water, to avoid cracks, some steps can be introduced into the air, reducing the difference in temperature between the surface and the core. Table 7 shows the results of such experiments, observing a significant increase in resistance when the cooling rate is higher. Obviously, this process must be applied to large blocks, where cooling rates are inherently low, or in situations that require high resistance. In opposite cases, cooling with air can be applied.

Table 7: Simulation data and results obtained in impact test bodies, for different cooling conditions of the steel blocks of the present invention. The values for the impact energy refer to the test bodies without inserts, section 7 x 10 mm, in the transverse direction The proposed process in water can be carried out in oil; for the caliber of 400 mm x 1000 mm, with operation for 60 minutes in oil. Ts = surface temperature EXAMPLE 4: In the previous examples, the chemical composition and heat treatment of the steel proposed for this invention were defined. Due to the use of titanium in chemical composition, hard carbide particles are formed - resulting in higher tool wear, and damaging the machinability of the material. For applications in molds, machining aspects are essential.

To avoid this, alloys of the present invention were studied in terms of higher sulfur and calcium contents. These two elements influence the formation of inclusions. Sulfur forms manganese sulphides, which have a low hardness and help break up the chips and the lubrication of the tool. On the other hand, calcium changes the hard inclusions of aluminum, generating complex inclusions with better machinability. The addition of calcium also spheroidizes the inclusions, generating better polishing conditions, which is also an important operation for plastic molds.

Table 8 shows the machining results for the steel of this invention, with this change in the sulfur and calcium contents in the alloy 19, and without this change in the alloy 18. For comparison, the same test was performed with the steel DIN 1.2738 (reference for application in mold). There is a perceptible increase in the volume of tools, with a change in the contents of calcium and sulfur (or alloys 18 and 19).

An alternative to improve the machining capacity could be to reduce the volumetric fraction of the carbonitrides, thus reducing the cause of the accelerated wear of the tools. The combination of the carbonitride volume decrease effect plus the use of high sulfur and calcium contents was used in alloy 37. Although it is similar in composition to alloy 19, there was a significant increase in machinability, associated with a drastic decrease in the volume of titanium carbonitride. In this case, the decrease in carbonitrides was made by increasing the solidification rate, through the use of a smaller bar. However, the same can occur by reducing the nitrogen or titanium contents. Since titanium is important for the NU factor, discussed in Example 1, the use of low nitrogen content, as a carbonitride volume controller, proves to be the most important.

Table 8: Comparison of the machinability of the alloys 18, 19 and the alloy of the state of the art, measured by the volume of the tools at the end of the life cycles of the tools (VB = 0.20 mm). Test conditions: tool = P25 hard metal coated with TiN, with diameter of 25 mm, cutting speed = 270 m / min, feed = 0.25 mm / tooth, depth of cut = 0.75 mm and work penetration = 10 mm. Next are the results of the chemical compositions. The volume fraction of carbonitride was measured by computer image analysis in 20 fields at an amplification of 500 x in each sample, totaling a scan of 0.56 mm2 Meaciór C Si Mn P s Cr Mo Ni V TI Al B N Ca (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) ET1 0.39 0.4 1.66 0.011 16 1.87 0.17 0.74 < 0.01 < 50 90 < 10 44 17 18 0.22 0.40 1.90 0.050 20 0.58 0.25 0.34 0.15 320 100 30 80 6 19 0.22 0.44 1.89 0.051 90 0 56 0.07 0.32 0, 13 310 100 38 130 15 37 0.21 0.41 1.88 0.026 70 059 0 26 0.34 0 10 260 74 27 74 22 EXAMPLE 5: The complete design of the previous alloy was based on providing hardness of 30 to 34 HRC, since this is the main use interval for mold steels. For conventional steels, a higher hardness can be obtained by using different annealing treatment conditions. In the steel of the present invention, with direct hardening through the forge, this can not be realized. Therefore, this invention also attempted 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 core in a block with a section of approximately 400 mm (speed of 0.05 ° C / second). The results in Table 9 and Figures 10a and 10b show that the highest hardness can be obtained when higher carbon contents are used. Based on these data, a proportion was experimentally obtained for carbon contents and hardness after cooling slow. This is the proportion: Hardness HV = 450% C + 210.

(Hardness obtained after cooling to 0.05 ° C / second, equivalent to air cooling of a block with a thickness of 400 mm.

Table 6: HV hardness values obtained after cooling to 0.05 ° C / second in compositions with different carbon contents.

Therefore, this example shows that it is possible to assign different hardness to the alloy of this invention, by using the carbon contents. For example, for hardness 315 HV (approximately 32 HRC), the interval obtained in example 1 is confirmed, with 0.23% carbon being necessary. On the other hand, for hardness of 400 HV (approximately 40 HRC), carbon contents of 0.42% may be necessary.

As shown in Table 1, the steels of the prior art for this same hardness range have significantly higher carbon contents: DIN 1.2738, hardness of 32 HRC and 0.36% carbon, and DIN 1. 2711, hardness of 40 HRC and 0.52% carbon. This fact has an interesting consequence for welding processes, which are widely used in molds. Since these operate with lower carbon contents, the hardness of the heated region will be much lower in the steel of the present invention, compared to the steels of the state of the art. For carbon contents of 0.23%, the steel of the present invention generates an approximate hardness of 45 HRC in the region affected by the welding, while this hardness is approximately 60 HRC for the steel DIN 1.2738 and 64 HRC for the DIN 1.2711. This fact helps in many machining operations after welding, as well as in the appearance after polishing or texturing.

Small variations in the indexes of previous equations can produce adequate results, within the range of hardness necessary for applications. For a variation greater than or equal to ± 20 HB commonly accepted in the industry, the ratio may vary according to Figure 10b, being described by the following relationships. Upper hardness = 590% C + 165 and lower hardness = 310% C + 255. Therefore, a final equation for hardness as a function of carbon contents can be described as follows: Hardness HV = (450 ± 140)% C + (210 ± 45).

Therefore, depending on the application of the required hardness, the carbon contents in the steel of the present invention should be calculated by the above equation.

It is noted that in relation to this date better method known by the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (13)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A bainitic steel for molds, characterized in that it has a chemical composition of elements that consists, in mass percentage, of carbon between 0.05 and 1.0; silicon up to 1.0, manganese between 0.5 and 5.0; phosphorus, boron, titanium and vanadium given by the ratio NU = [Ti + P + 10 B + (V - 0.10)], the values of NU being between 0.02 and 0.30, with titanium always above 0.005, boron always below 0.010 and vanadium may be partially or totally replaced with niobium, in the ratio of two parts by mass of niobium per one part of vanadium, nickel, molybdenum and chromium given by the ratio G = [0.13 Ni + 0.60 Mo + 0.26 Cr], with values of G above 0.10 and below 1.0; sulfur up to 0.20; silicon between 0.05 and 3.0; nitrogen less than 0.10; calcium with contents up to 0.010; aluminum below 0.5, cobalt below 2.0, the remainder being substantially iron and impurities that can not be avoided in the manufacturing process.

2. The bainitic steel for molds according to claim 1, characterized in that it has a chemical composition of elements consisting, in mass percentage, of carbon between 0.10 and 0.6; silicon up to 1.0, manganese between 0.8 and 3.0; phosphorus, boron, titanium and vanadium given by the ratio NU = [Ti + P + 10 B + (V - 0.10)], with the values of NU between 0.08 and 0.30, with titanium always above 0.005, boron always below 0.010, titanium between 0.005 and 0.10, and vanadium may be partially or totally replaced with niobium, in the proportion of two parts by mass of niobium per one part of vanadium, nickel, molybdenum and chromium given by the proportion G = [0.13 Ni + 0.60 Mo + 0.26 Cr], with G values above 0.20 and below 0.50; in addition to this proportion, the chromium contents should be between 0.1 and 1.5, and the nickel contents above 0.3; sulfur to 0.05; silicon between 0.05 and 3.0; nitrogen below 0.05; calcium contents up to 0.005; aluminum below 0.1, cobalt below 1.0, the rest being substantially iron and impurities that can not be avoided in the manufacturing process; The material can be produced in blocks up to 850 mm thick, the hardness being obtained between 250 and 450 HV through cooling with air from a temperature above 700 ° C, and the value of this hardness is given by the equation HV = (450 ± 140)% C + (210 ± 45).

3. The bainitic steel for molds according to claim 2, characterized in that it has a chemical composition of elements consisting, as a percentage in mass, carbon between 0.10 and 0.6; silicon between 0.05 and 0.6; manganese between 1.3 and 3.0; phosphorus, boron, titanium and vanadium given by the ratio of NU = [Ti + P + 10 B + (V-0.10)], with the values of NU between 0.10 and 0.20, with titanium always above 0.010, boron always below 0.0050, and vanadium may be partially or totally replaced with niobium, in the proportion of two parts by mass of niobium per one part of vanadium; nickel, molybdenum, and chromium given by the ratio G = [0.13 Ni + 0.60 Mo + 0.26 Cr], with G values higher than 0.25 and lower than 0.40; In addition to this proportion, the chromium contents should be between 0.1 and 1.0, and the nickel contents between 0.2 and 1.0; Sulfur between 0.001 and 0.010; the silicon between 0.20 and 1.5; nitrogen between 0.0040 and 0.0150; calcium with contents between 0.0005 and 0.0030; aluminum below 0.05, cobalt less than 1.0, the rest being substantially iron and impurities that can not be avoided in the manufacturing process; the material can be produced in blocks of up to 850 mm thick, the hardness being obtained between 280 and 450 HV through cooling with air, directly after the hot forming, with the value of this hardness given by the equation HV = (450 ± 140)% C + (210 ± 45).

4. The bainitic steel for molds according to any of claims 1 to 3, characterized because it has a chemical composition of elements that consists essentially of, in mass percentage, carbon between 0.18 and 0.52, chromium between 0.30 and 0.60, molybdenum between 0.10 and 0.50, nickel between 0.30 and 0.50, vanadium between 0.04 and 0.10; boron between 0.0010 and 0.0030; Sulfur between 0.0010 and 0.0100; calcium between 0.005 and 0.030; nitrogen between 0.0030 and 0.0100; where the hardness of final use is obtained directly after forging or rolling, with relatively high calibers, thicknesses between 100 and 1000 mm, without the need to use hardening processes with oil or water; the thermal treatment must be cooling with calm air with forced convection, being the Vickers hardness value determined by the carbon contents of alloy, according to the following proportion: HV = (450 ± 140)% C + (210 ± 45) , for hardness values between 280 and 420 HV, equivalent to 29 and 42 HRC.

5. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it comprises a G ratio less than 0.10, for applications in gauges smaller than 400 mm in thickness, being G calculated by the following ratio: G = [0.13 Ni + 0.60 Mo + 0.26 Cr], where the symbols represent the contents in mass percentage of the relevant elements.

6. The bainitic steel for molds according to any of claims 1 to 3, characterized because it comprises manganese contents, partially or totally replaced with nickel or copper, in equal amounts, in mass percentage.

7. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it contains in mass percentage, the niobium, zirconium or tantalum elements that replace, partially or totally, the titanium or vanadium elements, in a ratio of two parts of niobium corresponding to a part of vanadium or titanium, a part of tantalum or zirconium corresponding to two parts of vanadium or titanium.

8. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it shows, in mass percentage, boron between 0.0015 and 0.0030; silicon between 0.40 and 1.2.

9. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it shows, in mass percentage, sulfur between 0.002 and 0.090 and calcium between 0.0005 and 0.0030.

10. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it comprises a final hardness obtained by cooling with air, directly after the hot forming or through the previous heating in the furnace, being the final hardness obtained (in the scale Vickers) given by the equation: HV = (450 ± 70)% C + (210 ± 22), or even an equivalent equation via a hardness conversion by measurements of other scales.

11. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it has, in parts per million in mass, sulfur between 0.002 and 0.30, and calcium between 0.0005 and 0.010, or because it has in its microstructure a volume fraction of carbonitrides less than 0.25%, applied to situations where high machining capacity is necessary.

12. The bainitic steel for molds according to any of claims 1 to 3, characterized in that it has an increase in tenacity via rapid cooling, after hot forming or heating at temperatures above 900 ° C.

13. The bainitic steel for molds according to any of claims 1 to 12, characterized in that it has an increase in tenacity by means of rapid cooling, after hot forming or heating at temperatures above 900 ° C, this being cooling process given by the following heat treatment: cooling with air to the temperature of 700 ° C, then going to a water tank for 30 minutes (keeping the water temperature below 80 ° C), followed by cooling with air up to room temperature; in the case of the parts susceptible to cracks, the cooling time with water can be replaced with 60 minutes in cooling with oil, keeping all the other conditions of the heat treatment constant. SUMMARY OF THE INVENTION A composition of alloying elements is described which consists of, in percentages by mass, carbon between 0.05 and 1.0; manganese between 0.5 and 3.0; phosphorus, boron, titanium and vanadium given by the ratio NU = [Ti + P + 10% B + (% V-0.10)], the values of NU being between 0.02 and 0.30, with the titanium always above 0.005, the boron always below 0.010 and vanadium may be partially or totally replaced with niobium, in the proportion of two parts by mass of niobium per part vanadium; nickel, molybdenum and chromium given by the proportion with G = [0.13 Ni + 0.60 Mo + 0.26 Cr], with G values above 0.10 and below 1.0; with nickel above 0.1; with molybdenum between 0.07 and 0.27; with vanadium from 0.1 to 0.15; Sulfur up to 0.10; silicon between 0.05 and 3.0; nitrogen below 0.10; calcium with contents up to 0.02; aluminum below 0.5, chromium below 1.5; silicon between 0.05 and 3.0; phosphorus below 0.075, the rest being substantially iron and impurities that can not be avoided in the manufacturing process; for its production the final hardness can be obtained by cooling with calm air, directly after the hot forming or by previous heating in the oven, even in blocks with section up to 1000 mm; the hardness values, in the Vickers scale, are defined by the equation: HV = (450 ± 140)% C + (210 ± 45), for values between 280 and 450 HV (30 to 45 HRC); for high tenacity applications, the steel of the present invention can also be produced with rapid cooling, from temperatures above 900 ° C, in water or oil media.