Patent Application
20160017468
The invention refers to the development of chemical compositions of structural steels hardened by thermal treatment—through-surface hardening (TSH) implemented under the supervision of K. Z. Shepeliakovsky, Doctor of Technical Sciences, Professor, Honored inventor of the Russian Federation [1]. The first original name of the process was surface hardening with the deep induction heating.
Therewith, it was shown that high mechanical properties can be achieved with carbon- and low-alloy steels with the 1st generation low (LH) and specified (SH) hardenability [2], [3].
These are steels whose hardenability conforms to the effective loaded cross-section of the parts; in this case, as a result of through-surface hardening, the surface layers of this cross-section with an optimum 0.1-0.2 value of the diameter (thickness) have martensite structure with hardness HRC≈60, while the core hardness is HRC=30-45.
Steel hardenability is characterized by the ideal diameter (DI) value that actually defines the optimum hardened layer depth with reference to the specific part shaped as a cylinder, sphere or plate.
In principle, LH and SH steels have the same purpose and just conventionally differ only by the ideal diameter (DI) value: for LH steels as the earlier products, this diameter is equal to 8-16 mm, whereas for modern SH steels it is over 16 mm.
The necessary DI range of LH and SH steels for the specific type of parts was achieved by total restriction in the upper limit of one or a group of admixture elements which led to a lower accuracy and a wider DI range.
The closest analog is the known structural LH steel (see patent RU 2158320) containing:
The disadvantage of the known 2nd generation LH steels is that the required low hardenability was achieved only by strict restriction in the contents of all constant admixtures—Mn, Si, Cr, Cu which made smelting more difficult and resulted in lower accuracy of the specified DI range during the development of the chemical composition and, as a consequence, led to a wider deviation in the hardened layer depth that was outside the allowed tolerance range.
So, for example, LH 80 steel with 0.8% C and containing Mn, Si, Cr, Ni, Cu (<0.1% each) and 0.06-0.12% Ti (LH steels) (see patent RU2158320) provides minimal hardenability—DI<12 mm with grain #10 austenite and smaller—DI<11 mm for #11 and <10 mm for #12, whereas a similar LH 81 steel with 0.8% C; 0.05% Mn; 0.12% Si; 0.11% Cr; 0.25% Ni; 0.3% Cu; 0.05% Al; 0.22% Ti (with a wider range of permanent admixtures of Ni, Cu) has the same DI. The objective to be achieved with this invention is the development of the 3rd generation LH (SH) steels.
The technical result is to obtain an even finer austenite grain ##11-13 GOST5639 (ASTM), even more stable preset hardenability (DI) with a substantially smaller deviation range that strictly corresponds to the hardened layer depth obtained on parts subjected to thermal treatment using the proposed process, the possibility of machining thinner, smaller and other parts with the through-surface through-thickness hardening.
To achieve the technical result, structural steel was proposed for TSH featuring a DI of 6.0-200 mm and more, with a distinction that it contains the following components ratio, weight %:
Iron and the necessary admixtures make the rest, with the DI determined by the mathematical expression:
Dkp.=K·√C·(1+4.1·Mn)·(1+0.65·Si)·(1+2.33·Cr)·(1+0.52·Ni)·(1+0.27·Cu)·(1+3.14·Mo)·(1+1.05·W)·[1+1.5(0.9−C)]·(1−0.45C′)·(1−0.3Ti)·(1−0.35V)·(1−0.25Al),
where
Structural steel with the specified chemical composition and with the same grain size has an ideal DI:
To prevent hot-brittleness, the total content of manganese, titanium and zirconium in the structural steel is more than six times the maximum sulfur content.
The specialty of the proposed steel is that using formula (1) to achieve the given value of the ideal diameter makes it possible to limit the critical concentration, in the steel, of the above-mentioned constant admixtures that drastically increase hardenability, to 0-0.005% or to 0-0.1%, whereas that of other, weaker, constant admixtures may be expanded to 0-0.3% and, sometimes, to 0-0.5% without a deterioration in quality. This simplifies selection of the initial charge during smelting and makes steel cheaper, since the final target is to obtain the specified calculated DI value by combining the composition of the remaining elements-admixtures after steel deoxidation with the number of alloying elements introduced as per formula (1) based on the classical method of calculating hardenability, per Grossman [4], which turned out to be most acceptable for LH and SH steels.
Practical results confirmed the veracity of this method for parts of various shape and sizes.
However, this calculation has been refined by the authors in view of the likelihood of its further development. Thus, in the formula, the range of the K-coefficient that depends on the ##11-13 austenite grain size was extended. Besides, multipliers for hardenability as a function of tungsten and boron content were introduced. Also, multipliers were introduced for the ideal diameter as a function of modifying elements of the secondary carbonitride phases that do not enter the austenite solid solution prior to hardening cooling—titanium, vanadium, aluminum, carbon (in the structurally-free cementite of hypereutectoid steels), whereas sulphur and phosphorus were excluded from the formula since their content in the above quantities does not practically affect the DI value.
Under the circumstances, primarily from the economic point of view, introduction of the pre-measured amount of manganese as the most effective and relatively inexpensive component is expedient, either by itself, or alongside with inexpensive silicon in the amount of not more than 1.8% of each, instead of those more expensive elements that earlier were unreasonably added to the steel only with the purpose of increasing hardenability. Qualitative addition of boron into steel in abnormally small quantities of 0.003-0.005% also leads to higher hardenability, which becomes even more effective with a lower content of carbon in steel is (see formula 1). Using the formula allows optimal, but not excessive, alloying of steel when developing steel chemical composition.
Hence, addition of other alloying elements—Cr, Ni in the amount of 0-0.5% only with the purpose of bringing steel hardenability (DI) to the required value is less reasonable since it will not practically change mechanical properties compared to their lower content or to their absence for the same value of DI and austenite grain size.
Addition into steel of modifying elements (weight %)—titanium in the amount of not more than 0.4, vanadium—not more than 0.4, zirconium—not more that 0.4, niobium—not more than 0.1, tantalum—not more than 0.1, aluminum—0.03-1, nitrogen—not more than 0.1, calcium—not more than 0.03 that are present in steel in the form of finely dispersed carbides, nitrides and other inclusions that are insignificantly dissolved in austenite helps to reduce the grain size, widen the optimal temperature range when heating to ensure hardening, elevate strength and plasticity properties of the 3-rd generation LH and SH steels. In this case, the combined content of manganese, titanium and zirconium should be more than six times the maximum sulphur content, since both titanium and zirconium, like manganese, bind sulphur into high-melting sulfides.
Addition of other alloying elements, weight %,—chrome, nickel in the amounts of more than 0.6 (not more than 1.8 each), molybdenum (not more than 0.5) and tungsten (not more than 1.5) individually or together selectively (as a complex), as well as in compliance with the above formula to achieve a specified calculated DI value and improve quality indices, i.e. obtain better mechanical properties, heat resistance, lower cold brittleness threshold, etc.
Given below is the rationale for chemical composition of LH and SH steels used for the proposed hardenability process. The ultimate manganese content of 1.8 weight % is determined by steel susceptibility to overheating when its content is too high; silicon content over 1.8-2.0 weight % is fraught with steel changing from pearlite class to ferrite class which is insusceptible to strengthening by hardening; due to higher brittleness of the martensite-structured hardened layer chrome content for pearlite class steels does not exceed 1.8-2.0 weight % either; the ultimate content of nickel in the amount of 1.8-2.0 weight % is selected based on its high cost compared to that of manganese, silicon, chrome and its relatively low increase in hardenability factor (0.52). Besides, austenite grain reduction to size 10-13 in the inventor's application presented for the steel results in a substantial increase in plasticity and viscosity which rules out the influence of nickel when its content is increased; copper is usually a practically unremovable admixture, its maximum content is usually limited to 0.25%, which may be in some cases be increased to 0.3 0.5 weight % for the proposed steel without affecting the quality and taken into account when making weighted alloying of steel; molybdenum and tungsten also belong to expensive components and are also added to steel in weighed amounts, alongside with chrome and nickel, basically, to increase heat resistance of steels; exceeding the specified content—for molybdenum—0.5% weight % and for tungsten—1.5% weight % can, even with presence of small amounts of manganese and chrome, shift steel to the martensite class, i.e. to through-thickness hardening irrespective of the part size.
Sulphur and phosphor present in steel in the above-mentioned amounts do not practically affect the DI size.
Carbide-forming elements—titanium, vanadium, zirconium, niobium, tantalum in the specified amounts, as well as aluminum and nitrogen that form aluminum nitride contribute to reduction in the austenite grain size, slowing down its growth when subjected to heating for hardening and lowering hardenability. Under the circumstances, the lower aluminum content boundary of 0.03 weight % guarantees reasonably complete deoxidation of steel, and exceeding the upper limit of 0.1 weight % is unreasonable due to the fact that aluminum starts dissolving in austenite, hardenability of steel grows uncontrollably, making it more expensive. Exceeding the ultimate content of nitrogen in steel (0.1 weight %) will result in an irreversible coagulation (coarsening) of aluminum and titanium nitrides which is similar with respect to titanium, vanadium and zirconium carbides with the content of these elements above 0.4 weight %, and niobium and tantalum—above 0.1%, all this also making the steel more expensive.
Minimal DI value of 6 mm was obtained by the authors in an experiment for LH 40 steel of the following composition, weight %: 0.41 C; 0.03 Mn; 0.04 Si; 0.06 Cr; 0.05 Ni; 0.3 Cu; 0.05 Al; 0.22 Ti (the calculated DI is 5.4 mm for #13 grain size austenite).
Further decrease in DI<6 mm leads to drastic growth in the critical hardenability speed (Vcr) to over 1500° C./sec, to using extremely pure steels that are free of permanent admixtures which makes it very complicated.
The most important distinctive feature of steels proposed in this method is that when developing chemical composition of steel subjected to hardening it is possible to theoretically predetermine the DI value with sufficient accuracy.
Tables 1-4 below list typical chemical compositions of LH and SH steels with their DI and examples of technological processes based on the proposed method.
LH 81 steel with the following chemical composition, weight %: 0.78 C; 0.04 Mn; 0.08 Si; 0.07 Cr; 0.15 Ni; 0.08 Cu; 0.04 Al; 0.15 Ti; 0.015 S; 0.018 P—has a calculated DI=7.8 mm when treated for #12 grain.
An 8-mm wall bearing ring made from this steel was through-heated in an induction coil up to 850° C. for 20 sec and then subjected to cooling with a sharp water shower and tempered in a furnace at 150° C. with 2-hr soaking. As a result, the hardened layer on the outer and inner surfaces of the ring was 1.7 mm and 1.5 mm, i.e. 0.18-0.2 of the wall thickness, which corresponds to the through-surface hardening (TSH) and the real critical diameter of 8 mm; the hardened layer microstructure was cryptocrystalline martensite (#1 size), hardness was 65-66HRC, the core was troostite, troostosorbite and sorbite with 38-45HRC hardness.
SH 61 steel with the following chemical composition, weight %: 0.61 C; 0.5 Mn; 0.08 Si; 0.13 Cr; 0.25 Ni; 0.03 Cu; 0.04 Al; 0.05 Ti; 0.015 S; 0.018 P—has a calculated DI=22.5 mm when treated for #11 grain. A 45 mm dia cylindrical center pin was made from this steel. It was heated in an induction heater up to 900° C. for 50 seconds and then subjected to cooling with a sharp water flow and tempered in a furnace at 180° C. with 2-hr soaking. As a result, the hardened layer on the part surface was 5 mm, i. e. 0.11; in this case, the actual DI was 21 mm; the hardened layer microstructure was fine-needled martensite (#2 size), hardness was 56HRC, the core was fine pearlite (troostite), troostosorbite, sorbite with 30-40HRC hardness.
A 30 mm dia grinding ball made from this steel was through-heated in a furnace up to 850° C. and then subjected to cooling with a sharp water flow and self-tempered at 180° C. for 5 seconds, followed by final cooling with a water flow. As a result, the hardened layer on the part surface was 12 mm, i.e. 0.4 of the diameter, which conforms to hardening corresponding to the calculated value; the hardened layer microstructure was fine-needled martensite (#2 size, #11 grain size), hardness was 64HRC, the core was troostomartensite with, 48-50HRC hardness.
SH 50 steel with the following chemical composition, weight %: 0.5 C; 0.1 Mn; 0.15 Si; 1.0 Cr; 0.8 Ni; 0.03 Cu; 0.05 Al; 0.35 V; 0.5 W; 0.015 S; 0.018 P—has a calculated DI=47 mm when treated for #10 grain. A parallelepiped-shaped part measuring 150×200×200 mm made from this steel was through-heated in a furnace up to 850° C. and then subjected to cooling with a sharp water flow and self-tempered twice at 180° C. for 5 seconds, followed by final cooling with a water flow and tempering in a furnace at 450° C. with 3-hr soaking. As a result, the hardened layer along the part surface perimeter was 9 mm, i.e. 0.06 of the thickness (150 mm) which corresponds to the actual DI=50 mm.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011109067 | Mar 2011 | RU | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/RU2011/000280 | 4/28/2011 | WO | 00 | 9/30/2015 |
