High-strengh air cooled steel alloy, and hot worked product.

The invention relates to an air cooled steel, and to a forged product manufactured therewith and characterized by a totally ferritic-pearlitic microstructure without bainite and, accordingly, by a high strength, a particularly high fatigue strength, and good machinability

The use of air cooled microalloyed steels in hot-forged products is economically sensible as the pieces require no hardening and tempering. However, a problem with these microsteels is a modest strength, especially a low yield strength, when compared to tempered alloy steels. For example, in EN 10267:1998 standard, the highest strength microalloyed forging steel is 46MnVS6, having a minimum yield limit of 580 MPa. In practice, however, the most popular grade is 38MnVS6, having an Re = 520 N/mm2.

The strength of air cooled microalloyed steels can be improved by increased alloying, for example by adding manganese and/or chromium. However, increased alloying creates a problem as a result of the formation of bainite in the microstructure of steels. An objective in such steels is to obtain a totally ferritic-pearlitic microstructure capable of providing the desired properties. Even small amounts of bainite in the microstructure undermine mechanical properties by decreasing yield strength and toughness, especially elongation and reduction of area in tensile test, as well as by degrading machinability. An element particularly active with regard to the development of bainite is molybdenum, which in such steels must be considered an impurity and the concentration of which must be often limited to as low as not more than 0,03...0,04 wt-%. Occasionally, in the microstructure is formed not only bainite but also untempered martensite, the effect of which on the above properties is even more deteriorating than that of bainite.

Since, in addition to molybdenum, there are also other effective alloying elements, the tendency to bainite formation can be practically demonstrated by the following summation in wt-%:$$\mathrm{X}=2\mathrm{Mn}+5\mathrm{Mo}+\mathrm{Cr}+\mathrm{Ni},$$ wherein Mn, Mo, Cr and Ni represent concentrations of these particular alloying elements in weight percentage. According to prior experience, if X > 3,1...3,3, bainite develops in air cooling at thin (20 mm) diameters.

One way of upgrading the strength of air cooled forging steels is to subject them to such intensive alloying that their microstructure becomes completely or at least substantially bainitic. An example of such steels is described in the publication US 5820706. Instead of direct, simple air cooling, a problem in this technique is maintaining the temperature range of bainite formation, which hampers the forging process and brings forth one more source of process error. Moreover, there are problems regarding the machinability of such steels, especially because from retained austenite they can also easily develop small amounts of hard untempered martensite in addition to bainite. Generally, especially with forged products of considerable thickness, it is also necessary to increase the alloy content of such steels, thus resulting in high manufacturing costs.

Another example of bainitic steels is a steel as set forth in the published EP application 00850178.5, which is cooled in air. A problem in this case is also a poor and uneven machinability and an increase of alloy content. Since attaining a totally bainitic structure by air cooling without retaining a constant temperature is difficult in practice, in the microstructure usually formed untempered martensite as well. In order to attain even a moderate toughness and machinability, it is generally necessary that the pieces be also annealed for softening hard martensite regions, which increases costs even more.

It is known that a fatigue strength higher than conventional can be achieved in microalloyed steels rich in silicon. A steel like this is disclosed in the publication EP 0572246B1. A problem here is a low yield strength which is also typical of microalloyed steels in general. EP 0572246B1 is the closest prior art document.

The bainite formation in conventional microalloyed steels rich in manganese is sometimes discouraged by decelerating their post-forging cooling by providing for example a tunnel around the cooling line. However, this represents an extra cost and, at the same time, another source of error in the manufacturing process.

It is obvious in light of the above that, by finding a steel alloying based way of precluding bainite formation, the strength of microalloyed steels can be enhanced without disrupting the properties. Thus, it is economically feasible to achieve simultaneously high yield strength, good toughness and good machinability.

In part making usually the greatest individual cost is machining. It is well known that a fine grain size deteriorates machinability of steel. Therefore, a prior austenite grain size coarser than 10 ASTM (greater than about 10 µm in grain diameter) in the material of workpiece is advantageous. Therefore, an object of the invention is to provide a steel alloy having improved machinability combined with high yield stress and toughness.

In order to accomplish the objectives of the invention, a steel of the invention is characterized by what is set forth in the characterizing clause of claim 1.

With respect to the inventive steel, the simultaneous use of silicon alloying and molybdenum alloying results economically a high-strength steel, having a structure which is purely ferritic-pearlitic with neither bainite nor martensite nor drawbacks inflicted thereby.

Further, the use of chromium concentrations within the range of appr. 0,5 wt-% to appr. 1,5 wt-% enables an improvement in the nitriding properties of steel. The chromium concentration is preferably within the range of appr. 0,5 wt-% to appr. 0,8 wt-%.

Being a major nitride producer, the chromium alloying causes development of chromium nitrides in the ferritic diffusion layer of a nitrided surface, thus increasing the layer's hardness. For example, 1 wt-% of chromium provides a hardness of about 600 HV (K-E. Thelning, Steel and its Heat Treatment, Butterworths 1975 pp. 86-87). This hardness in the diffusion layer is sufficient even for highly demanding mechanical engineering. As nitride producers, titanium and vanadium also provide activities similar to that of chromium, thus enhancing the latter's effect.

Compared with unalloyed steel or steel with no alloying elements capable of forming nitrides, the diffusion layer in chromium-alloyed steel has a hardness which is several hundred HV degrees higher, which provides a solid base for an extremely hard compound layer present in the outermost surface. Vanadium and titanium alloying enhances this effect. A surface layer nitrided as described is more resistant to a surface pressure without a risk of cracking, which makes it possible to use the steel e.g. for gears and shafts. A hard diffusion layer means usually also a greater consolidated layer thickness for improved fatigue strength. This is favourable, for example in nitrided crankshafts.

On the other hand, an excessively high chromium concentration reduces the toughness of forged ferritic-pearlitic steel, which is why it is advisable to retain the concentration at less than 1,5 wt-% and in certain cases at less than 1 wt-%.

Still another object of the invention is a forged product, which is characterized by what is set forth in the characterizing clause of the independent claim 2.

The invention relates also to a hot rolled steel bar, which is characterized by what is set forth in the characterizing clause of the independent claim 3.

The following table illustrates mechanical properties and microstructures measured for test steel alloys of the invention as well as corresponding properties in standard reference steels. Steel C Si Mn P S Cr Ni Mo V Ti Als N Ref. 1 0,37 0,60 1,31 0,008 0,034 0,13 0,09 0,02 0,11 0,025 0,018 0,011 Ref. 2 0,39 0,55 1,44 0,007 0,037 0,22 0,17 0,04 0,09 0,020 0,021 0,016 Test 1 0,35 1,49 1,57 0,017 0,057 0,18 0,11 0,03 0,12 0,023 0,024 0,014 Test 2 0,36 1,26 1,08 0,010 0,059 0,20 0,14 0,03 0,12 0,014 0,016 0,011 Test 3 0,35 1,46 1,39 0,020 0,059 0,26 0,13 0,06 0,13 0,004 0,007 0,014 Test 4 0,36 1,43 1,33 0,017 0,056 0,27 0,11 0,06 0,16 0,010 0,009 0,021 Test 5 0,36 1,47 1,34 0,017 0,055 0,27 0,11 0,06 0,22 0,010 0,010 0,028 Steel Re A5 Z KCU2 X Microstructure Austenite grain size Ref. 1 580 16 50 72 2,94 F + P Ref. 2 682 8 10 12 3,47 F + P + B Test 1 717 15 45 50 3,58 F + P ASTM 6 (50 µm) Test 2 651 15 42 39 2,65 F + P ASTM 4 (100 µm) Test 3 658 10 20 38 3,47 F+ P Test 4 735 14 30 40 3,34 F + P Test 5 820 13 27 19 3,36 F + P Steel Re A5 Z KCU2 Microstructure Austenite grain size Ref. 1 555 18 48 46 F + P Ref. 2 591 14 34 12 F + P Test 1 647 15 42 19 F + P ASTM 4 (100 µm) Test 2 598 15 42 F + P ASTM 4 (100 µm) Test 3 665 16 38 27 F + P Test 4 695 13 34 20 F + P Test 5 744 13 33 15 F + P

As indicated by the tables, there is no bainite present in test alloys regardless of high alloying rate and molybdenum concentrations. In test alloys, the sum value representing bainite formation is as high as 3,58 without the presence of bainite.

In reference alloy 2 of the prior art, as expected, bainite is present at the rate of X = 3,47 wt-%. It is further noted that, as a result of bainite, the elongation, reduction of area and impact ductility of said reference alloy are lower than those of the inventive steels. Because of the absence of bainite it is also clear that the inventive steel is better than the reference steels in terms of machinability.

Raising the carbon concentration to higher than 0,4 wt-% increases strength, but the effect on tensile strength is lesser than on yield strength. On the other hand, at a lower carbon concentration, e.g. 0,15...0,25 wt-%, it is possible to establish a higher yield ratio, which is beneficial in some cases.

By virtue of silicon alloying, the inventive steel has a fatigue strength which is better than that of standard quenched and tempered and micro-alloy steels, as disclosed in the publication EP 0572246B1.

In a steel of this invention, the silicon alloying particularly reduces the tendency to bainite formation as evident by comparing a prior known reference alloy (Ref. 2) with test alloys 2...5 of the invention (Tables 1...3), the sum expression thereof giving the value X of higher than 3,3 wt-%. Table 2 also shows how the yield strength, elongation, reduction of area and impact strength of a bainite containing reference alloy (Ref. 2) are distinctly weaker than those of test alloys containing more silicon.

Manganese and chromium increase strength, but add to the risk of bainite formation at high concentrations.

An alloying element with a particularly powerful strengthening effect and at the same time promoting bainite formation is molybdenum. In a steel of the invention, molybdenum alloying, together with silicon alloying, has been utilized for increasing strength without drawbacks resulting from bainite. At small dimensions (20 mm), the inventive steel tolerates 0,06 wt-% of molybdenum with no problems, but at large dimensions the cooling rate is slower and higher molybdenum concentrations (e.g. 0,1...0,2 wt-%) are possible without a risk of bainite.

Vanadium is an effective precipitation hardener. Provided that hot working temperatures are not overly high, vanadium is also functional as a grain-size growth inhibitor. At rather high concentrations, higher than 0,3 wt-%, the use of vanadium is uneconomical and, in addition, toughness is reduced by vanadium. For these reasons it is in some cases advisable to omit vanadium completely.

Nitrogen is an effective hardener, either as such or together with vanadium. On the other hand, high concentrations, those higher than 0,03...0,04 wt-%, may nevertheless degrade the surface quality of a hot-rolled bar.

Niobium functions as a precipitation hardener the same way as vanadium.

Titanium nitrides are capable of withstanding, without dissolving, extremely high temperatures, even higher than 1200 C, which is why a minor addition of titanium is preferred especially in hot forging to inhibit an excessive growth of grain size and to improve toughness. Oversized additions of titanium, however, result in a structure developing large primary TiN particles, which precipitate as early as during solidification and which are ineffective in terms of grain growth and which, by functioning as a crack initiator, undermine toughness and fatigue strength. Moreover, they deteriorate machinability especially at higher cutting speeds. For the maximum machinability Ti should be kept lower than 0,008 wt-%.

Strength can be enhanced in hot forging by accelerating the cooling rate of the forged product in flowing air, in water-air mist or in some other flowing gas. The inventive steel is highly suitable for such a process by virtue of its minor tendency to bainite formation.

The range of application for steels of the invention covers hot-forged products, for example parts of automotive engines, such as crankshafts, connecting rods and pistons.

In addition, such steels are especially applicable for parts of a vehicular chassis, such as suspension arms, steering arms, front axle beams, etc.

Chromium-alloyed steel, in particular, is highly applicable for nitrided components, such as crankshafts, gear wheels and pinions.

In addition, steels of the invention can be used directly in hot rolled condition without forging or heat treatment. Thus, such steels can replace steel bars heat-treated by hardening and tempering. Intended applications include vehicular parts and machine components, for example drive shafts, steering components, fasteners, etc.