Low-alloyed steel and components made thereof

Abstract

A low-alloyed steel, comprising about 0.3 to about 0.50 wt. % carbon, about 2.0 to about 5.0 wt. % silicon, and a remainder of iron, optionally containing low amounts of molybdenum, titanium and/or boron, with up to about 0.5 wt. % impurities. The low-alloyed steel is useful for making structural components having a tensile strength of greater than about 1000 to about 2000 MPa, a yield strength of greater than about 700 to approximately 950 MPa; a break elongation of greater than about 17% and a scaling resistance of greater than about 650 C.

Claims

1. A structural component made of a low-alloyed steel consisting of: 0.3 to 0.50 wt. % carbon; 4 wt. % silicon; 1. 0 to 1.2 wt. % chromium; 0.7 to 0.9 wt. % manganese; 0.15 to 0.3 wt. % molybdenum; 0.002 to 0.005 wt. % boron; 0.02 to 0.04 wt. % titanium; a remainder of iron; and wherein the structural component has a tensile strength of greater than 1000 to 2000 MPa, a yield strength of greater than 700 to approximately 950 MPa; a break elongation of greater than 17% and a scaling resistance of greater than 650 C.

2. The low-alloyed steel according to claim 1, which has 0.35 to 0.4 wt. % carbon.

3. The low-alloyed steel according to claim 1, which has 1.1 to 1.2 wt. % chromium.

4. The low-alloyed steel according to claim 1, which has 0.2 to 0.3 wt. % molybdenum.

5. The low-alloyed steel according to claim 1, which has 0.03 to 0.04 wt. % titanium.

6. The structural component according to claim 1, selected from a group comprising pistons, crank shafts, connecting rods, steering parts, valve parts, conveyor parts, power plant components, replacement parts for heat-resistant areas, steam turbine parts, combustion chamber parts for gas and oil burners, and exhaust system components.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a cut of two samples which have been annealed in an oven for 5 hours at 700 C., respectively, in a controlled oxygen atmosphere.

(2) FIG. 2 shows a cut of two steel samples which have been annealed in an oven for 5 hours at 750 C., respectively, in a controlled oxygen atmosphere.

(3) FIG. 3 is a representation of the notch impact strength, tensile strength, necking of steel samples against the silicon content of different 42CrMo4 alloys which have been tempered at different temperatures.

DETAILED DESCRIPTION OF THE INVENTION

(4) The steels according to the invention contain at least about 92.00 wt. % iron, preferably at least about 96.00 wt. % iron, and uncharacteristically from about 2.0 to about 5.0 wt. % silicon. It is advantageous to keep impurities and unavoidable elements at a concentration of under about 0.10% weight, preferably under about 0.05% weight.

(5) Due to the addition of Si, costs for the steel according to the invention are approximately the same as for 42CrMo4 . However, in the case of the former, at the same time a considerable increase in scaling resistance by about 100 C. to about 150 C. and more as well as in yield strength is achieved. In steels according to the invention, yield strength is increased by approximately 100 MPa, accompanied by a slight decrease of the fracture strength. Machinability is not affected and can be performed by using the usual tools and methods.

(6) A typical steel according to the invention has the following composition:

(7) TABLE-US-00003 Chemical Min max composition (wt. %) (wt. %) C 0.38 0.45 Cr 0.9 1.20 Mo 0 0.3 Ti 0.020 0.04 Si 3.0 6.0 B 0.002 0.005

(8) Typical representatives of this group are steels designated herein as 42TBSi and 41TBSi.

(9) The now newly introduced alloying components have the following effects.

(10) Silicon increases the scaling resistance, is a mixed-crystal-solution hardening agent and inhibits the formation of carbide. During steel manufacturing, it renders the molten mass more fluid and also acts as a reducing agent. Further, it increases tensile strength, yield strength as well as scaling resistance and has a ferrite stabilizing effect. Added in too high amounts, it reduces malleability of the alloy.

(11) Through TiC formation, titanium prevents the inter-crystalline corrosion in iron alloys. Being a powerful nitride binder, it serves, among other things, for the protection of boron through the reaction with nitrogen. For example, when nitrogen is bound with titanium, a satisfying hardenability in the temperature range up to about 1000 C. occurs when the steel contains approximately 5 to 20 ppm boron. Ti is used for deoxidation of the steel and for fixation of C and N in the form of TiC or TiN, respectively. Therefore, Ti content should be at least about 0.02%. However, because a saturation effect occurs with regard to the action caused by Ti addition as soon as the Ti content exceeds about 0.08%, the upper limit of the Ti content is defined as about 0.08%.

(12) Even when added in only very small amounts, boron increases the yield strength and the strength of the steel. It also acts as a neutron absorber and makes the steel suitable for nuclear power plant applications and the like. Addition of boron in an amount of up to about 0.01% in austenitic steels also enhances their high thermal stability. Boron steels are high-quality cold-forming steels. The alkaline effect of boron in steel results in an enhanced hardenability, which already has an effect at very low concentrations of about 0.0010% boron. In small amounts of up to about 100 ppm, boron also increases hardenability more than other, more expensive elements which have to be used in much higher amounts.

(13) An outstanding feature of boron steels is the enhanced hardenability effected by the addition of even minute amounts of boron between about 3 and about 15 ppm. The amount of boron is critical, as an excessive amount thereof (>30 ppm) can lower the toughness and lead to embrittlement and hot shortness. The effect of boron on the hardenability also depends on the amount of carbon contained in the steel, with the effect of boron increasing inversely proportional to the percentage of the present carbon.

(14) Boron can also be ineffective if its condition is altered through faulty heat treatment. For example, a high austenitization temperature, and temperature ranges, in which specific boron precipitates occur, are to be avoided.

(15) Generally, the hardenability of steel is to a great extent ascribable to the effects of oxygen, carbon and nitrogen in steel. Boron reacts with oxygen to become boron trioxide (B.sub.2O.sub.3); with carbon to become iron boron cementite (Fe.sub.3(CB)) and iron boron carbide (Fe.sub.23(CB).sub.6) and with nitrogen to become boron nitride (BN). Loss of boron can occur through oxygen. The hardenability of boron steel is also closely connected to the austenitic conditions and normally decreases through heating to over 1000 C. Boron steels also have to be tempered at a lower temperature than other alloyed steels with the same hardenability.

(16) The use of boron steels is advisable when the basic mass meets the mechanical requirements (toughness, wear resistance, etc.), but the hardenability is not sufficient for the planned cut size. Instead of higher alloyed and thus more expensive steel, the corresponding amounts of boron can be used, so that a suitable hardenability can be achieved.

(17) A typical application for the steels of the present invention is for structural components, especially machine components having a tensile strength of >950 to about 1250 MPa, a yield strength of >700 to approximately 770 MPa, a break elongation of >10% and a scaling resistance of approximately 600 C. to about 650 C. and more.

(18) Typically, such components include machine components, such as combustion engine components including but not limited to pistons, crank shafts, connecting rods, and valve parts, or other automotive components such as steering parts, conveyor parts especially for warm parts, power plant components, replacement parts for heat-resistant areas, steam turbine parts, combustion chamber parts for gas and oil burners, and exhaust systems and their related parts. The steels according to the invention are used for many other applications, such as wear-resistant materials and as high-strength steels. Examples are cutting tools, spades, knives, saw blades, safety carriers in vehicles etc.

(19) The properties of the steels according to the invention as compared to those of known steels are:

(20) TABLE-US-00004 Property 42CrMo4/41CrS4 41TBSi 42TBSi tensile strength >900 to 1100 950 to 1150 1000 to 1200 (MPa) yield strength >650 >700 >750 (MPa) fracture strain >12% >10% >10% Scaling resistance up to approx. 550 C. approx. 600 C. approx. 650 C. heat treatment QT QT QT machinability good good good friction welding good properties analysis DIN EN 10083 DIN EN 10083 plus

(21) Specific advantages of the steels according to the invention are good cold formability, prolonged tool service life for tools made thereof, better weldability due to the lower carbon equivalents, and lower annealing temperatures. This results in energy savings and good case hardening.

(22) Exemplary Embodiment 1

(23) A cast steel billet made of 41TBSi is forged into a piston for a combustion engine in the course of a forging process at 1150 C. The motor piston thus manufactured is equipped with a head in the usual manner and built into a hybrid motor (HVV motor). After 1500 operating hours, no scaling of the steel surface of the piston showed in the ignition area is detectable. In comparison, a different cylinder which was made of 42CrMo4, but was otherwise identical, showed considerable scaling signs after 800 operating hours.

(24) Exemplary Embodiment 2

(25) A cast steel billet made of 42TBSi is forged into a piston in the course of a forging process at 1150 C. The piston thus manufactured is deployed in the usual manner as a combustion chamber for a gas engine.

(26) After a burning time of several months, no scaling of the steel surface of the piston showed in the firing/ignition area. In comparison, a different piston which was made of 42CrMoS4, but was otherwise identical, showed clear scaling signs after 70% of this runtime.

(27) Exemplary Embodiment 3

(28) A forged steel billet made of conventional 42CrMo4 as well as a steel billet of steel according to the invention (42CrMo4+4% Si+0.04 wt. % in Ti; and 0.005 wt. % in B) were transferred into an electric air circulating furnace and annealed in the oven for 5 hours at 700 C. The controlled circulating air atmosphere of ordinary air in the oven ensured that the oxygen content was kept constant. Two more samples made of conventional 42CrMo4 and the steel according to the invention were annealed for 5 hours in the same oven under the same conditions, but at 750 C. The tested steel billets both came from cast, forged ingots which had been forged down to 45 mm in diameter. FIG. 1, which in its top part depicts a cut of the 42CrMo4 steel after the annealing treatment at 700 C., and in its bottom part depicts a cut through the steel alloy according to the invention which was annealed under the same conditions, clearly shows that the scale layer is considerably thinner in the steel according to the invention than in the conventional 42CrMo4 steel without silicon addition (8 micrometers as compared to 30 micrometers), demonstrating that scaling in the Si steel material occurs considerably slower and to a lesser extent.

(29) FIG. 2 shows the same steel billet submitted to an annealing treatment of 5 hours at 750 C. in the same air convection oven, where the upper sample is the 42CrMo4 steel, which has developed a thickened scale layer of max. 44 micrometers compared to the treatment at 700 C., while the steel according to the invention shows a thin scale layer of max. 5 micrometers.

(30) This suggests that the silicon steel according to the invention is significantly less oxidized by oxygen at higher temperatures than conventional low-alloyed CrMo4 steel. This means that the steels according to the invention reach a scaling resistance which so far could only be achieved by using costly additives.

(31) FIG. 3 graphically represents a list of characteristics of 42CrMo4 steels with silicon additions up to 4% as a function of the silicon content and the annealing temperature. The abscissa indicates the Si content of a basic alloy 42CrMo in wt. %, while the left ordinate shows the tensile strength UTS in MPa. The right ordinate indicates the notch impact strength (KU). Curves for necking RoFa (%) of the steel according to the invention are shown for low as well as high Si content. It is shown that necking and notch impact strength decrease, while the tensile strength values increase. The notch impact strength starts decreasing rapidly at Si contents of more than 2.5 wt. %. The characteristics also depend on the annealing temperature (low tempering/high tempering). The high annealing temperature was 680 C. around approximately 0.5% Si, while the low annealing temperature was 630 C. When approximately 2.5 wt. % in Si is added, the high annealing temperature was 730 C. and the low annealing temperature was 680 C. It becomes clear that with increasing Si contenteven independently of the annealing temperaturethe tensile strength increases while the necking and notch impact strength decrease. A higher annealing temperature lowers notch impact strength and necking RoFa, while the necking RoFa at a low silicon content is higher for steel tempered at a higher temperature than for a steel tempered at a lower temperature. This ratio of RoFa of steel tempered at a higher temperature and the RoFa of steel tempered at a higher temperature is reversed with increasing Si content, while at higher silicon content the notch impact strength becomes almost independent of the annealing temperature. The tensile strength increases with rising annealing temperature and Si content.

(32) Accordingly the invention also relates to machine components or structural components with a tensile strength of about 1000 MPa and more for alternating mechanical strains up to a temperature of about 630 C., which are formed from a thermally quenched and tempered steel alloy. In particular, the invention relates to motor and/or drive components of vehicles.

(33) In modern technology, other machine components with alternating mechanical and thermal strain are also exposed to increasing loads that reach the limits of the respective material resistance. This particularly applies to motors, as the weight reductions obtained here can also be used for saving fuel etc. The materials these components are made of have to comply with high requirements regarding the property profile, toughness, hardness and ductility values in the thermally quenched and tempered state, as these property values are of vital importance for the dimensional design of the parts. Because of the failure of parts in long-term operation, it has become evident that the properties of material fatigue also have to be considered in order to attain a high degree of operational safety.

(34) Now the low-alloyed heat-treatable steels according to the invention can be used advantageously for parts with significant mechanical stress variation in the rail, automobile and aviation sectors. Use of steel alloys which have a composition that corresponds to those of heat-treatable steels of the previously mentioned kind has proven successful in the manufacture of highly stressed machine components, where their fatigue characteristics and thermal stability is adequate for alternating mechanical stress in the limit value range of the used materials.

(35) The description of the invention is only exemplary in nature and variations which will be apparent to a person skilled in the art are intended to be within the scope of the invention, as defined by the claims.