Patent Application
20190161838
The present disclosure relates generally to steel alloys, and more particularly, to high-strength steel alloys and casting processes for forming them, as well as components made therefrom, such as crankshafts.
An engine's crankshaft converts reciprocating linear movement of a piston into rotational movement about a crank axis to provide torque to propel a vehicle, such as but not limited to a train, a boat, a plane, or an automobile. Crankshafts are a vital part of an engine, and are a starting point of engine design. Crankshaft design affects the overall packaging of the engine, and thereby the total mass of the engine. Accordingly, minimizing the size and/or mass of the crankshaft reduces the size and mass of the engine, which has a compounding effect on the overall size, mass and fuel economy of the vehicle.
The crankshaft includes at least one crank pin journal that is offset from the crank axis, to which a reciprocating piston is attached via a connecting rod. Force applied from the piston to the crankshaft through the offset connection therebetween generates torque in the crankshaft, which rotates the crankshaft about the crank axis. The crankshaft further includes at least one main bearing journal disposed concentrically about the crank axis. The crankshaft is secured to an engine block at the main bearing journals. A bearing is disposed about the main bearing journal, between the crankshaft and the engine block.
The crankshaft may be formed or manufactured by a casting process, such as but not limited to a green sand casting process or a shell mold casting process, which uses cast iron to form the crankshaft. Alternatively, the crankshaft may be forged from a steel alloy. Steel is stronger than cast iron, and therefore is a more desirable material to use for crankshafts. Although the forging process is more costly than the casting process, most steel alloys exhibit a high shrinkage while cooling, and do not cast well, because the shrinkage that occurs while the cast product cools forms voids in the final cast product. This weakens the final cast product and makes it unsuitable for use in an engine.
This disclosure provides a high-strength steel alloy that is suitable for use in casting a crankshaft. The steel alloy features a medium-low carbon content for sufficiently high hardenability, fine grain sizes, a microstructure featuring a substantial amount of bainite, and with good machinability. The final microstructure may consist largely of lower bainite and/or upper bainite, and as such, subsequent heat treating can be eliminated, if desired. An ultimate tensile strength in the range of 750 to 1100 MPa can be obtained.
The disclosed steel alloy contains iron, carbon, manganese, silicon, sulfur, chromium, nickel, molybdenum, and aluminum. In some forms, boron, vanadium, nitrogen, titanium, and/or niobium may also be included.
In one example, which may be combined with or separate from the other examples and features provided herein, a high-strength steel alloy is provided containing: iron, about 0.24 to about 0.80 weight percent carbon, about 0.40 to about 2.10 weight percent manganese, about 0.20 to about 1.60 weight percent silicon, about 0.05 to about 0.14 weight percent sulfur, about 0.10 to about 12.0 weight percent chromium, about 0.10 to about 2.50 weight percent nickel, and about 0.02 to about 0.07 weight percent aluminum.
In another example, which may be combined with or separate from the other examples and features provided herein, a high-strength steel alloy is provided that consists essentially of: about 0.35 weight percent carbon, about 1.65 weight percent manganese, about 0.45 weight percent silicon, about 0.4 weight percent chromium, about 0.7 weight percent nickel, about 0.25 weight percent molybdenum, and the balance iron.
In yet another example, which may be combined with or separate from the other examples provided herein, a method of forming a steel alloy component is provided. The method includes creating a steel alloy comprising: iron, about 0.24 to about 0.80 weight percent carbon, about 0.40 to about 2.10 weight percent manganese, about 0.20 to about 1.60 weight percent silicon, about 0.05 to about 0.14 weight percent sulfur, about 0.10 to about 12.0 weight percent chromium, about 0.10 to about 2.50 weight percent nickel, and about 0.02 to about 0.07 weight percent aluminum. The method further includes casting in a mold the steel alloy to form the component. The method includes shaking out the mold and air quenching the component until the component has a temperature in the range of 420 to 530 degrees Celsius.
Further additional features may be provided, including but not limited to the following: the high-strength steel alloy further comprising boron in an amount not exceeding 0.005 weight percent; wherein the iron is provided in an amount between about 75.0 and about 98.88 weight percent; the high-strength steel alloy further comprising vanadium in an amount not exceeding 0.20 weight percent; the high-strength steel alloy further comprising molybdenum in an amount not exceeding 0.60 weight percent; the high-strength steel alloy further comprising titanium in an amount not exceeding 0.20 weight percent; the high-strength steel alloy further comprising niobium in an amount not exceeding 0.20 weight percent; the high-strength steel alloy further comprising about 0.001 to about 0.005 weight percent nitrogen; and/or the high-strength steel alloy further comprising about 0.01 to about 0.04 weight percent nitrogen.
In still another variation, the high-strength steel alloy comprises: about 0.24 to about 0.40 weight percent carbon, about 1.50 to about 2.00 weight percent manganese, about 0.40 to about 0.80 weight percent silicon, about 0.05 to about 0.12 weight percent sulfur, about 0.10 to about 0.60 weight percent chromium, about 0.60 to about 0.90 weight percent nickel, about 0.20 to about 0.40 weight percent molybdenum, about 0.02 to about 0.04 weight percent aluminum, and about 0.001 to about 0.005 weight percent boron.
In still another variation, the high-strength steel alloy comprises: about 0.25 to about 0.50 weight percent carbon, about 1.50 to about 2.00 weight percent manganese, about 0.30 to about 0.60 weight percent silicon, about 0.05 to about 0.12 weight percent sulfur, about 0.20 to about 0.60 weight percent chromium, about 0.50 to about 0.90 weight percent nickel, about 0.15 to about 0.40 weight percent molybdenum, about 0.02 to about 0.04 weight percent aluminum, and about 0.001 to about 0.005 weight percent boron.
A crankshaft for an automotive propulsion system is provided, which may be created from any of the variations of the high-strength steel alloy provided herein.
Other additional features may include, but are not limited to: the high-strength steel alloy having an ultimate tensile strength in the range of 750 to 1100 MPa; the high-strength steel alloy having an ASTM grain size number in the range of 5 to 8; the method further comprising holding the component at an isothermal temperature in the range of 420 to 530 Celsius starting immediately after the step of air quenching and continuing for a time period in the range of 1.5 to 3.5 hours; wherein the step of holding the component at the isothermal temperature is performed immediately after the step of air quenching without cooling the component to a temperature below 420 degrees Celsius between the step of air quenching and the step of holding the component at the isothermal temperature; and wherein the step of creating the steel alloy further comprises creating the steel alloy comprising boron in an amount not exceeding 0.005 weight percent.
Further aspects, advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings are provided for illustration purposes only and are not intended to limit this disclosure or the claims appended hereto.
High strength steel alloys having a substantially bainite microstructure are provided. In comparison to other steel alloys, these steel alloys exhibit improved material strength and hardness, with relatively fine grain size and adequate ductility, as well as desirable castability and machinability. The steel alloys disclosed herein are useful for forming automotive components that undergo large loads and fatigue, such as crankshafts.
These steel alloys have a low to medium carbon content for sufficiently high hardenability, fine grain sizes, favorable bainitic microstructure, and ease of machining. The final microstructure consists primarily of lower to upper bainite, which can be achieved through a cooling process that eliminates the need for subsequent heat treatment, as will be described in further detail below. An ultimate tensile strength in the range of 750 to 1150 MPa with an elongation greater than 8% can be obtained. The disclosed steel alloys have equivalent Young's moduli to forged steel counterparts with better machinability. The steel alloy may be used in gas or diesel engine components, such as crankshafts, by way of example.
The steel alloys disclosed herein contain iron, carbon, manganese, silicon, sulfur, chromium, nickel, and aluminum. In some versions, boron, molybdenum, vanadium, nitrogen, titanium, and/or niobium may also be included.
The steel alloys disclosed herein may be high-strength steel alloys and may include iron and by weight about 0.24 to about 0.80 weight percent carbon; about 0.40 to about 2.10 weight percent manganese; about 0.20 to about 1.60 weight percent silicon; about 0.05 to about 0.14 weight percent sulfur; about 0.10 to about 12.0 weight percent chromium; about 0.10 to about 2.50 weight percent nickel; and about 0.02 to about 0.07 weight percent aluminum. For example, Table 1 shows this first example of the steel alloy, which contains iron, carbon, manganese, silicon, sulfur, chromium, nickel, and aluminum.
Further, in some variations, the steel alloy may include boron, molybdenum, vanadium, nitrogen, titanium, and/or niobum as follows: 0.0004-0.005 weight percent boron, 0.10-0.60 weight percent molybdenum, 0.05-0.20 weight percent vanadium, 0.02-0.20 weight percent titanium, 0.03-0.20 weight percent niobium, and 0.01-0.04 weight percent nitrogen. Thus, Table 2 shows the additional elements that may be added to the elements in Table 1 to form a new steel alloy as disclosed herein.
In some variations, the ranges of the steel alloy may be further refined to include iron and by weight about 0.24 to about 0.40 weight percent carbon; about 1.50 to about 2.00 weight percent manganese; about 0.40 to about 0.80 weight percent silicon; about 0.05 to about 0.12 weight percent sulfur; about 0.10 to about 0.60 weight percent chromium; about 0.60 to about 0.90 weight percent nickel; about 0.20 to about 0.40 weight percent molybdenum; about 0.02 to about 0.04 weight percent aluminum; and about 0.001 to about 0.005 weight percent boron. For example, Table 3 shows this example of the steel alloy, which contains these elements: iron, carbon, manganese, silicon, sulfur, chromium, nickel, molybdenum, aluminum, and boron.
In another variation, the ranges of the steel alloy may include iron and by weight about 0.25 to about 0.50 weight percent carbon; about 1.50 to about 2.00 weight percent manganese; about 0.30 to about 0.60 weight percent silicon; about 0.05 to about 0.12 weight percent sulfur; about 0.20 to about 0.60 weight percent chromium; about 0.50 to about 0.90 weight percent nickel; about 0.15 to about 0.40 weight percent molybdenum; about 0.02 to about 0.04 weight percent aluminum; and about 0.001 to about 0.005 weight percent boron. For example, Table 4 shows this example of the steel alloy, which contains these elements: iron, carbon, manganese, silicon, sulfur, chromium, nickel, molybdenum, aluminum, and boron.
The steel alloys shown in Table 3 or Table 4 may also contain vanadium in an amount not exceeding 0.20 weight percent, titanium in an amount not exceeding 0.20 weight percent, niobium in an amount not exceeding 0.20 weight percent, and nitrogen, wherein the nitrogen may be provided in the range of 0.01 to 0.04 weight percent.
In another example, a steel alloy may be provided that consists essentially of the following: about 0.35 weight percent carbon, about 1.65 weight percent manganese, about 0.45 weight percent silicon, about 0.4 weight percent chromium, about 0.7 weight percent nickel, about 0.25 weight percent molybdenum, and the balance iron. A small amount of boron, such as 0.001 to about 0.005 weight percent boron, could also be included. Preferably, sulfur should also be included, such as 0.05 to 0.12 weight percent sulfur. For example, Table 5 shows this example of the steel alloy, which contains these elements: iron, carbon, manganese, silicon, chromium, nickel, and molybdenum, and optionally boron and sulfur.
The new steel alloy may exhibit a time-temperature-transformation (TTT) featured diagram 100 as illustrated conceptually in
At the highest temperatures, such as above D7, the steel alloy is already solidified and transformed into an austenite microstructure as indicated in section 106. As the steel alloy is cooled, it may undergo various phase transformations as a function of time. Before the time reaches time u1, corresponding to the nose 108 of the phase diagram 100, the steel alloy remains in an austenite form. Were the steel alloy to be cooled slowly through region 110, a ferrite and pearlite microstructure would be created. Were the steel alloy to be cooled a bit faster through region 112, a finer ferrite and pearlite microstructure would be created. In region 114, ferrite and coarse pearlite are formed. In region 116, ferrite and pearlite are formed. In region 118, fine pearlite is formed. A bainite formation temperature is indicated at 120. In region 122, 50% fine pearlite and 50% upper bainite are formed. In region 124, upper bainite is formed. In region 126, lower bainite is formed. On the left side of the diagram 100, metastable austenite is created in region 128, as when the steel alloy is quickly cooled to a temperature between D5 and D3, in an amount of time less than u1. Once the region 128 is entered, an isothermal process may be used to enter the bainite regions 124 and/or 126. Entering temperature D3 would cause martensite to begin to form in region 130, which would include martensite and slightly retained austenite until room temperature is reached.
To achieve a desired bainitic microstructure, the steel alloy may be cooled along the lines 136 and 138, wherein the surface of the cast steel alloy is cooled along the line 136, and the center of the cast steel alloy is cooled along the line 138. As such, the steel alloy is quickly cooled from temperature D7 to lower than D5 before time u1. Once the steel alloy is cooled to region 128, it is held at an approximately constant temperature D4 past the bainite start line 140 into region 112 and past the bainite finish line 142 into the bainite region 126 until bainitic transformation completes. At about time u2 or shortly thereafter in the region 128, the steel alloy begins to form a bainite microstructure. At line 142, bainite transformation completes.
Referring now to
Referring now to
The process or method 300 then includes a step 306 of pouring the hot liquid steel alloy into the mold and solidifying the cast steel alloy in the mold. The solidifying takes place above temperature T0 and before corresponding time x0, in
Once the cast steel alloy is solidified and cooled to temperature T1, the method 300 includes a step 308 of shaking the cast steel alloy component out of the mold. With reference to
The method 300 then includes a step 310 of air quenching the cast steel alloy component down to the temperature T2. The temperature T2 is in the range of 420-530 degrees Celsius. After the temperature T2 is met through air quenching, the cast steel alloy component is held constant at the temperature T2 in a step 312 for a period of time between x2 and x3 as shown in
Thus, the step 312 of holding the component at an isothermal temperature T2 in the range of 420 to 530 Celsius may start immediately after the step 310 of air quenching and continue for a time period in the range of 1.5 to 3.5 hours. In some variations, the step 312 is performed immediately after the step 314 without cooling the component to a temperature below 420 degrees Celsius between the step 310 of air quenching and the step 312 of holding the component at the isothermal temperature T2. In this way, the heat that already exists in the part as it is cooling after solidification can be used without wasting the heat and having to reheat the part to create bainite.
Thus, the new steel alloy is already strong and hard, with a bainite microstructure, without the need for additional reheating, quenching, austempering, and tempering. Accordingly, time and cost are saved from not having to perform reheating, quenching, austempering, and tempering.
The fine grain steel alloys described herein may be used to manufacture a steel automotive component. Therefore, it is within the contemplation of the inventors herein that the disclosure extend to steel automotive components, including but not limited to crankshafts, transmission shafts, transmission cases, half shafts, axle shafts, and the like. For example, referring to
Furthermore, while the above examples are described individually, it will be understood by one of skill in the art having the benefit of this disclosure that amounts of elements described herein may be mixed and matched from the various examples within the scope of the appended claims.
It is further understood that any of the above described concepts can be used alone or in combination with any or all of the other above described concepts. Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope and content of this invention.
