Low Cost High Performant Tool Steels

The present invention relates to steels, in particular tool steels which present a novel combination of an alloying principle and microstructure designed to decrease the manufacturing cost of the tool steel while providing unprecedented performance leading to further reduction of the cost of the components manufactured with the tools build with the tool steels of the present invention. Given the combination of properties and manufacturing cost, the tool steels of the present invention can also be employed for other applications other than tooling.

For many metal shaping industrial applications where high strength sheets or abrasive components are shaped or cut in complex geometries, a combination of high wear resistance, high resistance against cracking (sudden breakage, chipping, . . . ) and high resistance against plastic deformation are required simultaneously in the tooling materials. Traditionally, the wear problem has been solved by providing the tool materials with a sufficient volume fraction of primary carbides. Iron based primary carbides are not very hard, so traditionally cold work tool steels are highly alloyed with carbide forming elements (Chromium, Molybdenum, Vanadium and Tungsten being the most used ones, specially the first three). While Chromium is the most used carbide forming alloying element in tool steels, chromium primary carbide is not very hard and has a tendency to coarsen, which leads to a vicious cycle where it cannot be used in small volume fractions for high wear resistances due to the low hardness and when used in high volume fractions it coarsens leading to a great decrease of the resistance to breakage. This has been traditionally solved by the employment of the other principal carbide formers together with chromium, principally molybdenum and vanadium. Unfortunately, in the last decades the cost associated to alloying with molybdenum and vanadium has increased exponentially. In particular, alloying cost related to alloying with vanadium has increased almost an order of magnitude in the last decade. Also, the traditional way to increase the volume fraction of primary carbides while not allowing them to coarsen is through the powder-metallurgical (PM) route, which provides exceptional mechanical property combinations but has also a very high associated cost.

For many metal shaping industrial applications where there is a heat extraction from the manufactured product, thermal conductivity is of extreme importance; when this heat extraction is discontinuous, it becomes crucial. Thermal conductivity is related to fundamental material properties like the bulk density, specific heat and thermal diffusivity. Traditionally for tool steels, this property has been considered opposed to hardness and wear resistance since the only way to improve it was by means of decreasing alloying content. During many hot work applications, like plastic injection, hot stamping, forging, extrusion, metal injection, light alloy casting and composite curing among many others, extremely high thermal conductivity is often simultaneously required with wear resistance, strength and toughness. For many of these applications, big cross-section tools are required, for which high hardenability of the material is also necessary.

Until the moment, it was believed that high toughness levels were just attainable for low levels of hardness, the same applying for thermal conductivity, decreasing other properties like wear resistance. Moreover, for dies which afterward will need to undergo a surface hardening treatment, like for example nitriding or surface coating, it is normally necessary that substrate base material has enough hardness in order to support the coating, and again hardness should not decrease when exposing the material to the coating/surface treatment required application temperature.

For some other applications like most of plastic injection for the automotive industry, thick tools are used, especially when sufficient strength is required as for to require a thermal treatment. In this case, it is also often convenient to have a good hardenability to be able to achieve the desired hardness level on surface and, preferably, all the way to the core. Hardenability is inherent of each material and is given by the time available to go from a high temperature, normally above austenization temperature, to low temperatures, normally below martensitic start transformation without entering in any stable phase region like ferrite-pearlite zone and/or the bainitic zone. It is well known that pure martensitic structures present higher toughness values once tempered than mixed microstructures with stable phases. For that, the use of severe quenching mediums is needed in order to go from temperatures typically above 700° C. down to temperatures typically below 200° C. (in this document if no otherwise indicated, degrees are ° C.). For this reason, on the other hand, such treatments are very costly. The alloying elements required to achieve such hardenability also increase the cost of the tool steel and often have a negative effect in either toughness or thermal conductivity.

There are many other desirable properties, if not necessary, for tool steels that do not necessarily influence the longevity of the tool, but their production costs, like: ease of machining, capability to be polished, capability of being welded or repaired in general, support provided to the coating, costs . . . . Steels of the present invention present some clear advantages in some of these properties while not presenting significant disadvantages in any.

The inventor has discovered that the problem to simultaneously obtain very high performance in tool steels while reducing the manufacturing cost can be attained through the simultaneously application of a smart alloying principle and microstructure optimization.

For cold work applications, low cost tool steels with excellent relevant property compromise like the one provided by powder-metallurgical steels or even combinations of properties equaling or exceeding the wear resistance of powder-metallurgical tool steels while approaching or even surpassing the breakage resistance of matrix steels can be attained at a fraction of the manufacturing cost of PM steels.

For hot work and plastic injection molding applications: thermal conductivity, wear resistance, easy machinability and uniformity of properties, together with good levels of toughness at low cost, can be attained simultaneously with improved polish-ability and weldability amongst others. Some of the selection rules of the alloying principles within the range and thermo-mechanical treatments required to obtain the microstructure described in the present invention and also the levels of thermal conductivity indicated in the present invention are presented in the detailed description of the invention section. Obviously, a detailed description of all possible combinations is out of reach. The thermal diffusivity is regulated by the mobility of the heat energy carriers, which unfortunately cannot be correlated to a singular compositional range and a thermo-mechanical treatment. In fact, the thermal diffusivity is the best macroscopic property to measure the attaining of the correct microstructure at the atomic-placement level described in the present invention.

A specific thermal diffusivity value cannot be derived from a steel composition; actually thermal diffusivity is a parameter describing a structural feature in the sub-nanometric scale (atomic arrangement, regarding the optimization of density of states and mobility of carriers in all phases). When writing the application, the applicant referring to the Guidelines C-ll, 4.11 (nowadays Guidelines 2012, Part F, Chapter lV, point 4.11, “Parameters”) realized that almost all parameters (available) to describe this structural feature in the sub-nanometric scale are unusual parameters and that would be prima facie objectionable on grounds of lack of clarity. The sole exception for unequivocally describe mentioned structural feature in the sub-nanometric scale is thermal diffusivity and therefore this parameter is chosen to reasonably describe the structural feature.

In the meaning of this document, the values of thermal diffusivity refer to measures at room temperature, otherwise indicated. In the meaning of this document, room temperature refers to 23° C., unless context clearly indicates otherwise. In an embodiment, the thermal diffusivity is measured at room temperature by means of the Flash Method. In an embodiment, the thermal diffusivity is measured at room temperature according to ASTM-E1461-13. In an embodiment, the thermal diffusivity can alternatively be measured at room temperature according to ASTM-E2585-09(2015).

AISI D2 (or the closely related SKD-61 in Asia and 1.2379 in Europe), have managed to become the reference cold work tool steels worldwide. In recent years, the colloquially called 8% Cr steels (like 1.2965 or other cold work tool steels with 7 to 8.5% by weight % Cr with % V anywhere between 1 and 3.5% by weight, % C between 0.7 and 1.3 by weight and other alloying elements like % Si, % Mn, % Mo, % W, . . . ) have managed to substitute some of the AISI D2 for applications requiring high wear resistance or a better compressive yield/resilience compromise. For applications requiring higher toughness, the so-called matrix steels, with little or even absence of primary carbides, have also found a place in the market despite their lower wear resistance which sometimes is somewhat minimized by applying superficial hardening or carburizing in the wear prone areas. Typical examples of matrix steels are AISI A8, W.Nr.-1.2358). Finally, for the most demanding applications powder-metallurgical tool steels are employed, which present a very good combination of resistance against plastic deformation, resilience and wear resistance, specially against galling or adhesive wear, they also provide good support to coatings but unfortunately, they are even much costlier to produce that the other two.

Until the development of high thermal conductivity tool steels (EP1887096A1), the only known way to increase thermal conductivity of a tool steel was keeping its alloying content low and consequently, showing poor mechanical properties, especially at high temperatures. Tool steels capable of surpassing 42 HRc after a tempering cycle at 600° C. or more, were considered to be limited to a thermal conductivity of 30 W/mK and a structure at the atomic level (atomic arrangement) prescribed in the present invention whose implementation can be monitored by a thermal diffusivity value greater than of 8 mm/s and 6.5 mm/s for hardness above 42 HRc and 52 HRc respectively.

The inventor has discovered that the problem of having simultaneously very high performant tool steel with low manufacturing cost can be solved with a steel with the features of claim. Inventive uses and preferred embodiments follow from the other claims. Some further inventive features and preferred embodiments can be encountered in the text, since they have not been incorporated in the claims at this time.

This invention is the result of several years of investigations trying to reduce the alloying cost of tool steels while improving the performance for certain applications. The compositional rules and guidelines to be followed and microstructures to be favored have been determined for several tooling applications when using less expensive alloying elements or at least replacing some of the most expensive alloying elements.

Two main important discoveries have been made regarding the carbides in the tool steels. On the one hand, it has been found that for several of the applications of interest in the present invention size of the carbides plays an important role, as was to be expected, but very surprisingly the desirable tendency is exactly the opposite one when considering primary or secondary carbides. Traditionally, the size of carbides has been considered inversely proportional to toughness related properties. The literature reports, that vanadium and chromium-vanadium alloyed secondary carbides can be obtained with very small size and therefore such kind of carbides have been used for most hot work tool steels. Literature also reports that smaller primary carbides have a more negative effect on toughness related properties, but in more recent publications, size has been rated as second rank to other properties, like coherence (or adhesion of the carbide to the matrix) and even fracture toughness of the carbide. Therefore, almost all cold work tool steels with primary carbides, employ carbides with good adhesion to the matrix, and in last years with high fracture toughness despite their tendency to coarsen and thus presenting large sizes when large cross sections are involved. In the studies made by the inventor leading to the present invention, it was found that for several applications of the present invention, in primary carbides size is of first rank importance and thus measures have to be taken to assure primary carbides do not tend to coarsen, even if some fracture toughness of the carbide or coherence to the matrix is sacrificed and even more surprising, coarse secondary carbides are preferable. This has a limit since excessively coarse secondary carbides are also not desirable. Many other relevant observations were made by the inventor in these studies preceding the present invention, some of which will be further analyzed in the following paragraphs. The observations described in this paragraph, lead to some alloying rules for the steels of the present invention. To have the right size of secondary carbides, at least a certain amount of molybdenum-like alloying elements have to be employed, even if this results in an increase of the alloying price. To provide good guidelines in this aspect the concept of equivalent molybdenum (% Mo=% Mo+½*% W) can be of help for several of the applications. In this invention when not otherwise indicated, the alloying element fractions refer to weight fractions and the carbide content fractions refer to volume fractions. Besides the required presence of % Mosome further alloying guidelines derive from this first observations. For applications where primary carbides are not desirable or necessary, a bainite containing microstructure is desirable which should contain % Mn and/or % Ni but not too much % Si, extremely low contents of % P are preferred and boron can be present in small amounts or even larger amounts but then together with % Zr or another equivalent boride former. For applications where primary carbides are desirable or necessary, a martensite containing microstructure is desirable which can contain higher levels of % Si, and % Ti primary carbides should be prioritized, often (but not always) complex % Ti carbides which also contain shape modifying addition like % Nb. As is to be expected, also in this document when a characteristic, guideline, rule or any other is described as something which is desirable, often used, several times occurring or something of the like, the meaning is the literal one, so while it might be indicating an important part of the invention, it is not one that is mandatory for the whole scope of the invention although it might be mandatory for a reduced scope of the invention like some certain applications but not the whole invention—in the event of reducing the scope of the invention, then they can become mandatory for the remaining scope-. Clearly differentiated from other instances where a characteristic, guideline, rule or any other is described as mandatory for the whole scope of the invention where it is implied that that particular characteristic is not only very important or capital to the invention but also obligatory for the whole scope.

The inventor has found that the problem of obtaining high performant low cost tool steels can be solved with the guidelines provided in the following paragraphs.

In this document, when not otherwise indicated, equivalent carbon (% C) is defined as follows:

In this document, when not otherwise indicated, equivalent molybdenum (% Mo) is defined as follows:

This is because in many instances of the present invention % Mo can be replaced partially or completely with % W obtaining the same technical effect. Alloying with % W is currently considerably more expensive and thus less desirable, therefore in some applications it will be preferred to not have any intentional addition of % W. On the other hand, % W tends to promote harder carbides and therefore some instances where wear resistances is one of the top priorities might benefit from the usage of % W. When it comes to % C, in some instances of the present invention it is desirable to alter the shape of the hard particles, and it has been found by the inventor that often the partial replacement of % C with % B and/or % N is advantageous. Moreover, in some applications the partial replacement of % C with % B and/or % N changes the friction coefficient and the wear behavior, which can be capitalized for better performance. This raises a language issue since replacing some of the % C with % B and/or % N more often than not leads to the hard particles becoming mixtures of carbides, borides and/or nitrides sometimes pure and sometimes as mixtures (carbo-borides, carbo-boro-nitrides, carbo-nitrides, boro-nitrides), but for the sake of simplicity in this document when not otherwise indicated the terms: carbides, primary carbides, secondary carbides (also in singular and any other variation) refer to the hard particles present which often are not pure carbides (for example in a steel of the present invention with significant % B present, the steel might have primary carbo-borides which will be referred as primary carbides). That does not apply to the terms borides and nitrides.

An implementation of the present invention can be made through providing a steel, in particular a tool steel, having the following composition, all percentages being in percentage by weight (% wt):

The rest consisting of iron and trace elements, wherein

Trace elements refers to several elements, unless context clearly indicates otherwise, including but not limited to, H, He, Xe, F, Ne, Na, P, S, Cl, Ar, K, Br, Kr, Sr, Tc, Ru, Rh, Pd, Ag, I, Ba, Re, Os, Ir, Pt, Au, Hg, Tl, Po, At, Rn, Fr, Ra, Ac, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rf, Db, Sg, Bh, Hs, O, Li, Be, Mg, Ca, Rb, Zn, Cd, Ga, In, Ge, Sn, Pb, Bi, Sb, As, Se, Te, Th, Ds, Rg, Cn, Nh, Fl, Mc, Lv, Ts, Og and Mt. In an embodiment, trace elements comprise at least one of the elements listed above.

Trace elements may be added intentionally to attain a particular functionality to the steel, such as reducing the cost of production and/or its presence may be unintentional and related mostly to the presence of impurities in the alloying elements and scraps used for the production of the steel.

The inventor has found that it is important for some applications limit the content of any trace element to amounts of less than 1.8% by weight, less than 0.8% by weight, less than 0.3% by weight, less than 0.1% by weight, less than 0.09% by weight and even below 0.03% by weight.

There are several applications wherein the presence of trace elements is detrimental for the overall properties of the steel. In different embodiments the sum of all trace elements in the steel is below 2.0% by weight, below 1.4% by weight, below 0.8% by weight, below 0.4% by weight, below 0.2% by weight, below 0.1% by weight and even below 0.06% by weight. There are even some embodiments for a given application wherein trace elements are preferred being absent from the steel.

There are several applications wherein the presence of trace elements is preferred. In different embodiments, the sum of all trace elements is above 0.0012% by weight, above 0.012% by weight, above 0.06% by weight, above 0.12% by weight and even above 0.55% by weight.

Different applications require different levels of equivalent carbon (% C). Also, the level of equivalent carbon together with the rest of the alloying, with special mention to carbide formers, determines the plausible volume fractions of primary carbides or the absence thereof. For some applications, where excessive primary carbides are rather not desirable like is the case in applications where the toughness requirements are more detrimental than the wear resistance ones, % Cshould not be too high. In different embodiments, % Cis 1.69% by weight or less, 1.49% by weight or less, 0.98% by weight or less, 0.59% by weight or less, 0.55% by weight or less, 0.48% by weight or less and even 0.44% by weight or less. Some applications, like sometimes applications requiring a rather good polishing ability in the tool steel or good weldability it is often interesting to keep even lower levels of % C. In different embodiments, % Cis 0.39% by weight or less, 0.34% by weight or less, 0.29% by weight or less and even 0.19% by weight or less. In contrast, in some applications higher contents of % Ceq are preferred. In different embodiments, % Ceq is 0.17% by weight or more, above 0.21% by weight, above 0.32% by weight, above 0.43% and even above 0.71% by weight. For some applications, if abundant primary carbides are desirable, then the % Ceq content should be higher. In different embodiments, % Ceq is 0.81% by weight or more, 0.91% by weight or more, 1.01% by weight or more, 1.12% by weight or more and even 1.26% by weight or more.

Different applications require different levels of carbon (% C). For some applications, % C should not be too high. In different embodiments, % C is 1.67% by weight or less, 1.49% by weight or less, 0.94% by weight or less, 0.59% by weight or less, 0.53% by weight or less, 0.51% by weight or less, 0.42% by weight or less and even 0.39% by weight or less. Some applications, like sometimes applications requiring a rather good polishing ability in the tool steel or good weldability it is often interesting to keep even lower levels of % C. In different embodiments, % C is 0.34% by weight or less, 0.31% by weight or less, 0.24% by weight or less and even 0.16% by weight or less. In contrast in some applications higher contents of % C are preferred. In different embodiments, % C is above 0.16% by weight, above 0.23% by weight, above 0.31% by weight, above 0.58% by weight and even above 0.66% by weight. For some applications, if abundant primary carbides are desirable, then the % C content should be higher, in different embodiments, % C is 0.91% by weight or more, 1.01% by weight or more, 1.11% by weight or more, 1.22% by weight or more and even 1.36% by weight or more.

It has been found that for some applications the presence of nitrogen (% N) is desirable while yet for other applications it is rather an impurity. In different embodiments, % N is 0.002% by weight or higher, 0.01% by weight or higher and even 0.12% by weight or higher. In contrast, in some applications an excessively high content of % N is rather detrimental. In different embodiments, % N is 0.3% by weight or less, less than 0.26% by weight, less than 0.18% by weight, less than 0.09% by weight, less than 0.009% by weight, less than 0.0059% by weight, less than 0.0019% by weight, and even less than 0.00095% by weight. Obviously, there are cases where the desired nominal content is 0% by weight or nominal absence of the element as occurs with all optional elements for certain applications.

It has surprisingly been found that for some applications, a strict control on the level of % N leads to a marked improvement of the mechanical properties increasing both strength and toughness related properties. In an embodiment, % N is between 2 ppm and 190 ppm by weight. In different embodiments, the lower limit for the controlled % N content is 11 ppm by weight, 16 ppm by weight, 21 ppm by weight, 32 ppm by weight, 140 ppm by weight and even 98 ppm by weight. In different embodiments, the upper limit for the controlled % N content is 68 ppm by weight, 48 ppm by weight and even 33 ppm by weight. In an embodiment, the upper limit for the controlled % N content is 19 ppm by weight. In an embodiment, % N refers to total % N present. In an embodiment, % N refers only to free nitrogen. In an embodiment, % N refers to the nitrogen in solid solution at room temperature. In an embodiment, % N refers to the maximum nitrogen in solid solution during austenitization. In an embodiment, very special care is taken during the degassing of the material to assure the specially low % N levels specified in some of the embodiments in this application. In an embodiment, vacuum degassing is applied to the melt. In an embodiment, the vacuum degassing is performed with a vacuum level reaching 9*10mbars at some point in the vacuum degassing process. In another embodiment, the vacuum degassing is performed with a vacuum level reaching 9*10mbars at some point in the vacuum degassing process. In another embodiment, the vacuum degassing is performed with a vacuum level reaching 9*10mbars at some point in the vacuum degassing process.

In an embodiment, the melt is vacuum degassed during 31 minutes or more. In another embodiment, the melt is vacuum degassed during 46 minutes or more. In an embodiment, the melt is vacuum degassed during 61 minutes or more. In another embodiment, the melt is vacuum degassed during 91 minutes or more. In another embodiment, the melt is vacuum degassed during 121 minutes or more. This special care in keeping a low % N would be considered madness for such kind of alloys which normally address a very price sensitive market, but the inventor has found with great surprise that this extra cost can be compensated by the increase in properties.

In some applications it has been found that some alloying elements, affect the quantity of desirable % N, to better describe this effect, the concept of an equivalent nitrogen (% Neq) will be introduced:

Where % AC refers to the sum of actinides

And where % LA refers to the sum of lanthanides (% La+% Ce+% Pr+% Nd+% Pm+% Sm+% Eu+% Gd+% Tb+% Dy+% Ho+% Er+% Tm+% Yb+% Lu).

In an embodiment, % Neq is between 0.2 ppm and 140 ppm by weight. In different embodiments the lower limit for the controlled % Neq content is 1.2 ppm by weight, 6 ppm by weight, 11 ppm by weight and even 22 ppm by weight. In different embodiments, the upper limit for the controlled % Neq content is 89 ppm by weight, 78 ppm by weight, 58 ppm by weight, 48 ppm by weight and even 28 ppm by weight. In another embodiment, the upper limit for the controlled % Neq content is 19 ppm by weight. In another embodiment, the upper limit for the controlled % Neq content is 9 ppm by weight.

In some applications it has been found that the % Neq has a combined effect with % Mn, % Ni and in some cases % Si. For such applications and to make understanding easier, the NMN parameter has been developed, where:

NMN=% Mn+1.7*% Ni−20*% Neq

In an embodiment, NMN is between 0.125 and 1.8. In different embodiments, the upper limit for the controlled NMN parameter is 1.4. In different embodiments, the upper limit for the controlled NMN parameter is 0.94, 0.74, 0.68 and even 0.49. In different embodiments, the lower limit for the controlled NMN parameter is 0.22, 0.32, 0.41 and even 0.52. In an embodiment, when NMN is 0.34 or larger, then % Si has to be 0.28% by weight or lower. In an embodiment, when NMN is 0.44 or larger, then % Si has to be 0.18% by weight or lower. In an embodiment, when NMN is 0.51 or larger, then % Si has to be 0.14% by weight or lower. In an embodiment, when NMN is 0.54 or larger, then % Si has to be 0.09% by weight or lower.

It has been found that for some applications the presence of boron (% B) is desirable while yet for other applications it is rather an impurity. In different embodiments, % B is above 1 ppm by weight, above 11 ppm by weight, above 21 ppm by weight, above 26 ppm by weight, above 31 ppm by weight, above 32 ppm by weight, above 41 ppm by weight, above 42 ppm by weight, above 0.002% by weight, and even above 0.0032% by weight. In some applications if primary borides or carbo-nitro borides are desirable, then the % B content should be higher, in different embodiments, % B is 0.01% by weight or higher, 0.02% by weight or higher, 0.04% by weight or higher, 0.1% by weight or higher, 0.26% by weight or higher and even 0.36% by weight or higher. In contrast, in some applications an excessively high content of % B is rather detrimental. In different embodiments, % B is 0.49% by weight or less, less than 0.49% by weight, less than 0.26% by weight, less than 0.2% by weight, less than 0.18% by weight, less than 0.09% by weight, less than 0.035% by weight, less than 0.009% by weight, less than 0.0058% by weight, less than 0.002% by weight and even less than 0.0004% by weight. Obviously, there are cases where the desired nominal content is 0% by weight or nominal absence of the element as occurs with all optional elements for certain applications.

It has been found that for some applications the presence of niobium (% Nb) is desirable while yet for other applications it is rather an impurity. In different embodiments, % Nb is above 0.001% by weight, above 0.04% by weight, above 0.11% by weight, above 0.21% by weight, above 0.31% by weight and even above 0.41% by weight. In contrast, in some applications an excessively high content of % Nb is rather detrimental. In different embodiments, % Nb is less than 0.9% by weight, less than 0.49% by weight, less than 0.39% by weight, less than 0.29% by weight and even less than 0.19% by weight. Obviously, there are cases where the desired nominal content is 0% by weight or nominal absence of the element as occurs with all optional elements for certain applications.

It has been found that for some applications the presence of chromium (% Cr) is desirable while yet for other applications it is rather an impurity. For example in some systems % Cr increases the concentration of atomic placement defects on carbides, when properly manufactured. This can be an advantage in some applications and a disadvantage in others. In different embodiments, % Cr is 0.001% by weight or higher, 0.01% by weight or higher, 0.12% by weight or higher, 0.63% by weight or higher, 1.1% by weight or higher, 1.6% by weight or higher, 2.1% by weight or higher, 2.3% by weight or higher, 3.1% by weight or higher, and even 4.6% by weight or higher. For some applications higher levels are preferred. In different embodiments, % Cr is above 6.1% by weight, above 7.1% by weight, above 8.1% by weight and even above 10.1% by weight. In contrast, in some applications an excessively high content of % Cr is rather detrimental. In different embodiments, % Cr is less than 12.8% by weight, less than 9.6% by weight, less than 8.4% by weight, less than 5.9% by weight, less than 3.8% by weight, less than 2.3% by weight, 1.9% by weight or less, less than 1.4% by weight, less than 0.9% by weight, less than 0.12% by weight, less than 0.06% by weight and even less than 0.02% by weight. Obviously, there are cases where the desired nominal content is 0% by weight or nominal absence of the element as occurs with all optional elements for certain applications.

It has been found that for some applications the presence of nickel (% Ni) is desirable while yet for other applications it is rather an impurity. For example % Ni can increase toughness but also decrease it depending on final microstructure. It has been found that % Ni sometimes affects both the ferritic and bainitic hardenability but in a different way depending on concentration of other elements and processing. In different embodiments, % Ni is 0.001% by weight or higher, 0.01% by weight or higher, 0.12% by weight or higher, 0.58% by weight or higher, 0.69% by weight or higher, 1.19% by weight or higher, 1.64% by weight or higher, 2.1% by weight or higher and even 2.6% by weight or higher. In contrast, in some applications an excessively high content of % Ni is rather detrimental. In different embodiments, % Ni is less than 3.8% by weight, less than 2.9% by weight, less than 2.3% by weight, less than 1.8% by weight, less than 1.4% by weight, less than 0.9% by weight, and even less than 0.46% by weight. Obviously, there are cases where the desired nominal content is 0% by weight or nominal absence of the element as occurs with all optional elements for certain applications.

It has been found that for some applications the presence of silicon (% Si) is desirable while yet for other applications it is rather an impurity. In different embodiments, % Si is 0.001% by weight or higher, 0.02% by weight or higher, 0.16% by weight or higher, 0.52% by weight or higher, 0.61% by weight or higher, and even 0.92% by weight or higher. In contrast, in some applications an excessively high content of % Si is rather detrimental. In different embodiments, % Si is less than 1.4% by weight, less than 0.86% by weight, less than 0.49% by weight, and even less than 0.46% by weight. For some applications lower levels are preferred. In different embodiments, % Si is less than 0.44% by weight, less than 0.28% by weight, less than 0.14% by weight, less than 0.09% by weight and even less than 0.04% by weight. Obviously, there are cases where the desired nominal content is 0% by weight or nominal absence of the element as occurs with all optional elements for certain applications.