Patent Application
20090123322
The present invention is about a kind of steel to be used in cutting tools and machining of metals and other materials. The steel at issue has a composition which classifies it as a high speed-type tool steel, with its main feature being the use of a lower content of noble alloy elements, such as vanadium, tungsten and molybdenum, but with properties either equivalent or higher than those of less alloyed conventional high-speed steels and slightly inferior to those of more alloyed conventional high-speed steels. Such arrangement of properties has been obtained from the use of the alternate alloy element niobium and lower cost alloy elements, such as silicon and aluminum.
Cutting tools are applied in a large number of cutting and machining operations. Some examples are cutting operations in tape, automatic or manual saws, drilling, turning, tapping, milling, among other forms of machining steel, nonferrous alloys or other solid-materials. An important example of operation for which the present invention is intended are saws, used in machines or saws for manual cutting, and both can be used under the hard form, entirely in high-speed steel, or bimetallic, with only the areas of high-speed steel teeth and the others made of low alloy mechanical construction steel. Other cutting tools typically employ high-speed steels and they may be made of the steel in the present invention, among them: helicoidal drills, top millings, profile tools, tacks, bit and special drills for high-resistance materials. In addition, thin cutting tools, such as taps, dies and special mills.
The same high-speed steels employed on those tools may be used as conforming tools. Examples are punches, tools for cold forging, blanking dies and plate cutting, coining dies, dies for conforming of postmetallic or ceramic, inserts and other tools for hot and warm forging, as well as tools in other applications in cold, warm or hot conforming, in which the conformed material has temperatures reaching up to 1300° C.
Steels traditionally used in cutting tools are high-speed steels, whose main feature is high resistance to wear and preservation of hardness at high temperatures. Typical examples are the steels of series AISI M or AISI T, being steels AISI M2, M7 and TI highlighted. Less alloyed steels may be used for less demanded tools; the main steels are DIN 1.3333 and steels AISI M50 and M52. The chemical composition of such steels is shown in Table 1, in which emphasis must be placed on tungsten, molybdenum and vanadium, which contribute with a large share to the final cost of the alloy. The effect of these elements on the cost is presented in Table 2, as normalized by the cost of alloys in December 2005. The advantage of less alloyed steels over conventional steels is clear according to these amounts, in terms of alloy cost.
Therefore, high-speed steels always had a strong impact on their cost associated with the costs of raw materials (alloy elements). However, the recent increase in the costs of steels and alloy irons made this even more significant. In many applications, even less alloyed steels had a strong cost impact, increasing the interest in compositions with an even lower content of alloy elements. And, as for conventional steels, the requirement of less alloyed steels which did not have such an expressive loss of properties increased, being hardness the main property of them. The minimum hardness in most applications is 64 HRC and, as shown in Table 1, steels M50 and M52 do not meet this requirement.
ype
ame
.7Mo + 0.4V + 0.3W
inimum hardness
onventional
ess alloyed
indicates data missing or illegible when filed
Thus, the need of a new composition of high-speed steels able to meet the requirements of low content of alloy elements is evident, even lower than currently existing steels, and reaching the minimum hardness of 64 HRC and with the proper distribution of nondissolved carbides, by obtaining then the properties required to the applications.
The steel of the present invention meets such requirements.
The objective of the invention was first of all to study the influence of silicon, aluminum and niobium elements in a composition with a low content of vanadium, molybdenum and tungsten. The important effect of niobium was identified in this study, however not sufficient to evolve hardness towards the levels necessary. Aluminum and especially silicon elements were then employed in the steel of the present invention, showing a significant effect. The definition of the contents of these elements and their adequate working range promotes, therefore, the reduction of costs and the achievement of the properties intended in the material. Such ranges are described and the effect of each element is outlined below.
In order to meet the conditions referred to above, the steel of the present invention has a composition of alloy elements that, in mass percentage, consists of:
0.5 to 1.5 C, preferably 0.8 to 1.1 C, typically 0.87 C.
1.0 to 7.0 C, preferably 3.0 to 5.0 C, typically 4.0 C.
3.0 to 10.0 of Weq (tungsten equivalent), being Weq given by relation Weq=W+2.0MO, preferably 4.0 to 8.0 Weq, typically 6.0 Weq.
0.5 to 3.0 Nb, preferably 0.8-1.8 Nb, typically 1.2 Nb, and Nb may be partially or fully replaced with Zr, Ti, Ta or V, in a relation in which 1.0% of Nb corresponds to 0.5% V or Ti, and 1.0% Nb corresponds to 1.0% Zr or Ta.
0.3 to 2.0 V, preferably 0.5-1.0 V, typically 0.7 V, and V may be partially or fully replaced with Nb, in a ratio in which 1.0%. Nb corresponds to 0.5% V. In case of replacement of V with Nb, the content of the final Nb in the alloy must be calculated through this relation and added to the already specified content for the alloy.
0.3 to 3.5 Si, preferably 0.7 to 2.0 V, typically 1.0 Si, and Si may be partially or fully replaced with Nb, at the 1:1 ratio.
8% Co at maximum, preferably 5% cobalt at maximum, typically 2% Co at maximum.
As described below, aluminum may be added to the steel of the present invention, promoting property advantages. However, compositions with no addition of aluminum can also be employed in the steel of the present invention, since it is easier in terms of alloy manufacture. Therefore, the aluminum content should be dosed as follows:
1.0 Al at maximum, preferably 0.5 Al at maximum, typically 0.2 Al at maximum for compositions with Al as residual element. In this case, Al should be treated as impurity.
0.2 to 3.5 Al, preferably 0.5 to 2.0 Al, typically 1.0 Al, plus the Si content described above, for compositions which require Al to improve the performance.
Balance in iron and metallic or nonmetallic impurities, inevitable to the steel mill process, in which said impurities include but are not limited to the following elements, in mass percentage:
1.5 Mn at maximum, preferably 0.8 Mn at maximum, typically 0.5 Mn at maximum.
0.10 P at maximum, preferably 0.05 P at maximum, typically 0.03 P at maximum.
0.10 S at maximum, preferably 0.020 S at maximum, typically 0.008 S at maximum.
0.1 N at maximum, preferably 0.05 N at maximum, typically 0.5 N at maximum.
0.5 Ce at maximum or other rare-earth elements. The elements of the lanthanoid or actinoid families in the periodic table, as well as La, Ac, Hf and Rf elements are considered rare-earth elements. The Ce content should be preferably lower than 0.1 and typically lower than 0.06.
See below the reasons of the specification for the composition of the new material. The percentages shown relate to the mass percentage.
C: Carbon is the main responsible for the response to heat treatment and the formation of primary carbides. Its content should be lower than 1.5%, preferably 1.1% at maximum, so that the presence of austenite retained is not very high after quenching. This is important in less alloyed steels, as the one of the present invention, because carbon tends to form less carbides of alloy elements, in the form of primaries and eutectics; thus, a higher content of free carbon is obtained after quenching, contributing to a significant increase in the fraction of retained austenite. However, the carbon content should be sufficient to form primary carbides, especially in combination with niobium, as well as secondary carbides during tempering and promote the hardening of martensite after quenching. Thus, the carbon content should not be lower than 0.5%, being carbon higher than 0.8% preferable.
Cr: The chromium content should be higher than 1%, preferably higher than 3%, because this element contributes to quenching characteristics and precipitation of secondary carbides during tempering and annealing. Together with carbon, chromium also determines the formation of M7C3-type primary carbides, which are not desirable for high-speed steels, since they reduce rectification capacity and toughness. Thus, the chromium content should be limited to 10%, preferably lower than 7%.
W and Mo: Tungsten and molybdenum have analog effects on high-speed steels, present especially in M2C- or M6C-type primary carbides and secondary carbides of the same type, being the latter formed during tempering or under gross solidification condition. Thus, they may be jointly specified through the equivalent tungsten relation (Weq), given by the sum W+2Mo, that normalizes the differences of atomic weight of both elements. For the present invention, the use of molybdenum and tungsten is intended especially for formation of secondary carbides during tempering, promoting thus temper hardness. Therefore, for an adequate volume of secondary precipitation and hardness after tempering, Weq must be higher than 3%, preferably higher than 4%. On the other hand, such elements contribute significantly to the cost of alloy and, thus, the reduction of these elements is one of the main aspects of the steel of the present invention. Therefore, the content of Weq should be lower than 10.0%, preferably lower than 8.0%.
V: For the present steel, vanadium should have a function equivalent to the one described for molybdenum and tungsten-action on the secondary hardening, forming thin carbides at tempering. Vanadium also can form primary carbides, but this is not the main purpose of its addition to the steel in the present invention. Vanadium also has, further, a significant influence on the control of growth of austenitic grains, during austenitization. For such effects, vanadium should be higher than 0.3%, preferably higher than 0.5%. Since this is also an important agent in the alloy cost, the content of vanadium in the present invention should be lower than 2.0%, preferably lower than 1.0%.
Nb: Niobium has an important effect for the steel of the present invention. This element forms mainly MC-type highly hard eutectic carbides and, therefore, they are important for resistance to the wear of the tools produced. Another interesting effect of niobium is that the MC carbides formed dissolve a little tungsten, molybdenum and vanadium, enabling these elements to be free, after austenitization and quenching, for secondary precipitation. Thus, the high-speed steel linked to niobium allows for the use of a lower amount of molybdenum, tungsten and vanadium and, therefore, this element operates significantly to reduce the alloy cost. However, its performance is ensured by the fraction of thin and highly hard MC carbides, formed by niobium. On the other hand, the content of niobium cannot be higher than 3%, because it forms primary and coarse carbides under these situations, hardly refined by the hot conforming process. So, an excessive content of niobium may harm toughness and rectification capacity of the alloy, in addition to increasing its cost. Therefore, the niobium content in the steel of the present invention should be between 0.5 and 3.0%, preferably between 0.8 and 1.8%.
Si: Silicon is one of the main element for the steel of the present invention. This element has an usually undesirable effect on both primary and secondary carbides of more alloyed high-speed steels. Among them, the increase in the volume of primary carbides is one of the main effects, harming the rectification capacity and the response to heat treatment, and the decrease in the resistance to tempering. This occurs for the effect of silicon on the volume of delta ferrite during solidification, and the reduction in the volume of high-stability MC- and MC2-type secondary carbides. So, it is not added higher than 0.5% in usual compositions. However, the steel of the present invention does not have negative problems as for the introduction of silicon, since it is a less alloyed steel. On the contrary, this element causes a significant increase in the temper hardness. This effect is not fully explained, but it must result from the effect of silicon on the elimination of the cementite precipitated upon tempering, promoting an increase in the amount of MC- and M2C-type carbides. Thus, despite the reduction of elements which promote secondary hardening, such as tungsten, molybdenum and vanadium, the increase in silicon content in the steel of the present invention promotes the recovery and elevation of hardness, until values acceptable for high-speed steels. For such effect, the content of silicon must be higher than 0.3%, preferably higher than 0.7%. However, the content of this element must be lower than 3.5%, since it reduces the austenitization range and causes an expressive hardening of ferrite when annealed. The content of silicon must be preferably lower than 2.0%,
Al: The addition of aluminum is optional for the steel of the present invention. Slight property gains, such as resistance to tempering, may be achieved with content higher than 0.3%, preferably higher than 0.7%. However, in order to promote high hardening of ferrite, high reactivity in liquid steel and increase of AC1 and AC3 temperatures, aluminum must be lower than 3.5%, preferably lower than 2.0%. Even in content close to 1.0%, aluminum still causes these undesirable effects. The variation of AC1 and AC3 temperatures makes the conditions for annealing of material especially difficult, requiring significantly higher temperatures. And, the reactivity of the liquid metal makes the works of steel mills and cleaning difficult, in term of nonmetallic inclusions of the end steel obtained. Thus, the steel of the present invention can be also produced with residual contents of aluminum. In this case, aluminum must be lower than 1.0%, preferably lower than 0.5%.
Residuals: Other elements, such as manganese, nickel and copper and those usually obtained as typical residuals from the preparation process of liquid steel, must be regarded as impurities, related to the process of deoxidation in steel mill or inherent to manufacturing processes. Therefore, the content of manganese, nickel and copper is limited to 1.5%, preferably lower than 1.0%. Elements such as phosphorus and sulfur segregate on grain contours and other interfaces. Thus, phosphorus must be lower than 0.10%, preferably lower than 0.5%, and sulfur must be lower than 0.050%, preferably 0.020% at maximum.
As described, the alloy can be produced in the form of products rolled or forged by whether conventional or special processes, such as powder metallurgy, spray conforming or continuous casting, in products such as wire rod, bars, wire, plates and strips.
Reference is made to the figures attached in the following description of the experiments carried out, where:
Experimental ingots of two steels of the art were produced, named ET1 and ET2, for comparison with the experimental ingots of the present invention, named PI1, PI2 and PI3. Steel ET1 corresponds to DIN 1,3343, similar to high C AISI M2, broadly employed in tools made of high-speed steels and, for this reason, used as reference to the material of this invention. On the other hand, steel ET2 is a less alloyed steel, able to reach 64 HRC and very employed for saws, at cutting blades. The chemical compositions are shown in Table 2. The sum of higher cost elements, such as tungsten, molybdenum and vanadium was also quantified, as normalized by cost.
Table 2 shows the significant reduction in the elements of steel alloy of the present invention, which are converted into a lower cost alloy—as compared in Table 3, calculated for amounts of December 2005. The reduction which occurs from the steel of art ET1 to ET2 can be observed, as well as the reduction at the same proportion of steel ET2, since this is a less alloyed steel, for the steels of the present invention. Thus, such results show that the steel of this invention is a second step towards the reduction of alloy costs, concerning already existing less alloyed steels, such as steel ET2. And, as for steel ET1, the difference in alloy cost is twice as large.
The ingot fusion was made at a close procedure for such five alloys, in a vacuum induction oven, and poured into iron cast moulds, resulting in a 55-kg ingot. After solidification, the ingots were subcritically annealed and such five compositions were initially classified concerning the fusion gross microstructure. Firstly, one can see the higher quantity of primary carbides in ET1 alloy, a result from its higher content of alloy elements. Secondly, the concentration of vanadium, molybdenum and tungsten elements is clear, in accordance with the density of points in the X-ray image, and it is significantly higher at primary carbides in ET1 and ET2 alloys, concerning PI1, PI2 and PI2 alloys. On the hand, the latter tend to form predominant niobium elements. Such carbides are MC type, and highly hard: therefore, they can replace well carbides of higher cost elements, such as tungsten, molybdenum and vanadium. And, added to such effect, niobium carbides have an interesting feature: they do not have expressive amounts of other elements, especially molybdenum, tungsten and vanadium.
Thus, they provide these elements with more freedom to form secondary carbides that, during tempering, are important to check the high hardness required to the applications of the material.
In short,
bs.
elat.
indicates data missing or illegible when filed
In addition to primary carbides, the hardness after the heat treatment is essential for high-speed steels. Therefore, the experimental ingots were rolled for 34-mm diameter round bars and annealed, with level at 850° C. for ET1, ET2 and ET3 alloys, and level at 980° C. for PI3 alloy. Afterwards, they were submitted to quenching treatment, with austenitization between 1185 and 1200° C. for 5 minutes and two temperings, between 450 and 600° C. for 2 hours each.
Table 4 shows hardness after quenching and tempering of ET1, ET2, PI1, PI2 and PI3 steels, which, in form of a chart, is presented in
The results in Table 4 and
The increase in the hardness of PI1 and PI2 alloys for PI3 alloy is evident in
lloys
lloys
indicates data missing or illegible when filed
The size of austenitic grains for ET2, PI1, PI2 and PI3 alloys was also evaluated for several austenitization temperatures. The results are shown in Table 5. Steels PI1, PI2 and PI3 have grain size slightly larger than steel ET2, because it has a high vanadium content—quite efficient to control the growth of the size of austenitic grains. However, PI1, PI2 and PI3 alloys have grain size still refined, especially until 1185° C., and considering that 33-mm gauge is relatively large for high-speed steels. Therefore, this austenitization temperature seems the most suitable for the steel of the present invention.
2.6 ± 0.9
2.1 ± 1.2
2.4 ± 2.1
2.4 ± 1.2
0.7 ± 1.2
0.8 ± 1.6
2.1 ± 1.6
1.5 ± 1.0
.9 ± 1.6
0.9 ± 1.1
0.4 ± 2.1
1.6 ± 2.3
indicates data missing or illegible when filed
The primary carbides of steel ET2 and the steel of the present invention—PI1, PI2 and PI3, in addition to the assessment at the fusion gross condition, were also assessed after hot conforming. The results were obtained via image-computing analysis. Such results are shown in Table 6 and
Steel ET2 has a total volumetric fraction of carbides equivalent to the one of steels PI1 and PI3; steel PI1 has a slightly higher volumetric fraction. As for size, steel ET2 has fewer total carbides, but it has a higher number of coarse carbides (over 8 μm).
Steels PI1, PI2 and PI3 have carbides concentrated in thinner ranges, both in absolute number and relative values. As obtained through quantitative analysis, such results can be also qualitatively seen in the microstructure of materials, shown in
For high-speed steels, the existence of thinner carbides is interesting, because they promote larger points of resistance and wear, and they operate in order to increase toughness. Thin carbides are also important to promote better machining capacity, making high-speed steel easier to be processed when manufacturing tools. Therefore, more refined carbides obtained at steels PI1, PI2 and PI3 are very interesting for application in cutting tools. They result especially from niobium eutectics which, after hot conforming, have thinner morphology than primary carbides of ET2 alloy, especially those rich in vanadium.
verage (%)
tandard
ize range (μm)
rom 0 to 1
rom 1 to 3
rom 3 to 5
rom 5 to 8
ver 8
otal
rom 0 to 1
rom 1 to 3
rom 3 to 5
rom 5 to 8
ver 8
indicates data missing or illegible when filed
Therefore, the steels described in the present invention, especially steels ET2 and ET3, have properties quite adequate for high-speed steel tools used in low demanding situations. Manual saws or saws used in machines are examples, in addition to cutting tools such as drills and milling devices, employed in situations with low working life demands.
The properties of the steel of the present invention allows for its use as replacement for steels such as ET2 in all of such applications, with equivalent properties and a significant cost reduction (see Table 3). The steel of the present invention can also replace more alloyed steels, herein represented by steel ET1, probably with lower performance, but the cost reduction is extremely significant.
Such combination of cost and properties is achieved only through an alloy design using lower cost elements, with the purpose of enhancing the effect of nobler elements—tungsten, molybdenum and vanadium.
In order to assess the behavior for industrial applications, the steel of the present invention were tested in performance tests. Cutting tools of the “hard manual saws” type were manufactured and cutting tests were carried out. Such testes were performed in accordance with standard BS 1919, in three blades of each one of ET2, PI1, PI2 and PI3 alloys.
The alloys of the present invention, PI1, PI2 and PI3, were produced from 55-kg experimental ingots, hot rolled until 2.8×12 mm2 dimensions and, then, rolled again for the final dimension of the saw. Steel ET2 was obtained from an industrial batch for reference purposes. Alloy ET2 was chosen for comparison purposes, because this is the material traditionally employed in manual saw blades.
The test consisted of 10 cuts per blade on a bundle of stainless steel UNSS304,00, with dimensions of 2.60×25.00 mm2, 180-HV hardness. The speed was constant, 70 strokes per minute, and the cutting powers were precalibrated equally for all the saw blades. The tests were carried out in a proper machine. The performance indicators were: average wear rate and total average cutting time. The wear rate is characterized by the evolution in the number of strokes required to make each cut. It is calculated through the first order derivative of the chart on number of strokes per cut in view of the number of cuts. A lower rate of wear means that the saw cuts with fewer strokes, what is felt by users as better performance. The same thing occurs for cutting time—the shorter, the better the saw blade performance. The results obtained at the performance test are shown in Table 7, for the materials under two tempering conditions.
ear Rate
utting Time
ear Rate
utting Time
indicates data missing or illegible when filed
The most important condition is 540° C., since this is the most used in saws produced currently. The results achieved are interesting for the alloys of the present invention, once they show results either equivalent or even higher than those of the steel of the art (ET2), especially for PI2 and PI3 alloys. For tempering at 540° C., the alloy with PI3 has the lowest wear rate; and, as well as PI2 alloy, it results in shorter cutting time than ET2 alloy.
Therefore, PI2 and PI3 alloys can be deemed as interesting for application, since they result in a significant reduction in the content of alloy elements and, notwithstanding, they promote a suitable cutting performance. Such performance, as shown in Table 7, may be even higher than the steels of the art. As discussed in Example 1, this occurs by means of the proper development of the chemical composition-especially through the combination of Nb and Si elements, something that promotes high hardness and refined carbides, entailing the total reduction of the more expensive alloy elements Mo, W and V.
| Number | Date | Country | Kind |
|---|---|---|---|
| PI0601679-0 | Apr 2006 | BR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/BR2007/000023 | 2/2/2007 | WO | 00 | 10/23/2008 |
