Patent Application
20180251871
The present application relates to an improvement in hot-rolled steel products. Hot-rolled steels are produced by subjecting an ingot of a predetermined thickness to a series of rollers to progressively decrease the thickness of the ingot. Throughout the rolling process, the steel is maintained at a very high temperature that is generally above the recrystallization temperature; final reduction passes may occur at temperatures below the recrystallization temperature of austenite. Once the rolling process is complete, the steel is coiled as it is cooling. The final steel coil is then cooled to ambient temperature.
In some circumstances, it can be desirable to increase the strength of steel materials used in hot-rolling processes. For instance, hot-rolled steels can be used in the context of automotive frames. However, the automotive industry continually seeks more cost-effective materials that are lighter for more fuel-efficient vehicles. While thinner steel materials can meet this need, higher strength is necessary to accommodate these thickness reductions. Thus, it is desirable to increase the strength of steel materials used in hot-rolling processes.
The steels of the present application solve the problem of poor weldability and low elongation in hot-rolled steels by a novel alloying strategy that incorporates transition metal elements that increase the propensity of martensite formation after hot-rolling processes despite relatively low cooling rates encountered during the hot-rolling processes.
The present embodiment involves a high strength, hot-rolled steel that exhibits an ultimate tensile strength of approximately 1500 MPa. Although the steel of the present example is produced in a relatively heavy gauge, or high thickness, of greater than 3 mm, it should be understood that in other embodiments various other suitable thicknesses may be used.
As described above, the present embodiment exhibits generally high strength. To achieve this high strength, the steel of the present example includes a predominately martensitic microstructure after hot-rolling, coiling, and cooling to ambient temperature. To achieve this martensitic microstructure, the steel of the present embodiment has sufficient hardenability or susceptibility to thermal heat treatment. The term “sufficient hardenability” is defined by the formation of martensite during coiling and after hot rolling.
It should be understood that martensite is generally more likely to form in response to relatively fast cooling rates. However, in the present embodiment the hardenability of the steel is sufficiently high such that martensite forms even with the relatively slow cooling rates that are present in commercial hot-rolling and coiling operations.
Carbon is generally understood to have a direct relationship with hardenability. In other words, increasing carbon additions to a steel can likewise increase hardenability. However, in some circumstances it may be undesirable to rely exclusively on carbon content to obtain desired hardenability. For instance, when carbon additions exceed certain levels, the weldability and the elongation to fracture of the steel can be reduced. In the present embodiment, these detrimental characteristics are avoided while also increasing hardenability of the steel through use of substitutional or transition metal elements in lieu of increasing carbon substantially. By way of example only, these substitutional or transition metal elements can include manganese, molybdenum, niobium, vanadium, chromium, or some combination thereof.
In embodiments of the present alloys, manganese is the primary alloying addition used to increase hardenability of the steel while avoiding other detrimental conditions such as reduced weldability and reduced elongation to fracture. Other elements such as molybdenum, niobium, chromium, and/or vanadium can also be similarly used to increase hardenability.
In the present embodiment, carbon is held at a relatively low level that will be described in greater detail below. Meanwhile, as described above, certain substitutional or transition metal elements are added to increase hardenability. The particular amount of increased hardenability is determined by the increase required to promote the formation of martensite despite the relatively slow cooling rates encountered during coiling and subsequent ambient air cooling. In some embodiments, the cooling rate can be approximately 0.05 to 2° C./s. Of course, in other embodiments different cooling rates can be used while still promoting the formation of martensite.
In addition to iron and other impurities incidental to steelmaking, the embodiments of the present alloys include manganese, silicon, chromium, molybdenum, niobium, vanadium, and carbon additions in concentrations sufficient to obtain one or more of the above benefits. The effects of these and other alloying elements are summarized as:
Carbon is added to reduce the martensite start temperature, provide solid solution strengthening, and to increase the hardenability of the steel. Carbon is an austenite stabilizer. In certain embodiments, carbon can be present in concentrations of 0.1-0.50 weight %; in other embodiments, carbon can be present in concentrations of 0.1-0.35 weight %. In still other embodiments, carbon can be present in concentrations of about 0.22-0.25 weight %.
Manganese is added to reduce the martensite start temperature, provide solid solution strengthening, and to increase the hardenability of the steel. Manganese is an austenite stabilizer. In certain embodiments, manganese can be present in concentrations of 3.0-8.0 weight %; in other embodiments, manganese can be present in concentrations of 2.0-5.0 weight %; in still other embodiments, manganese can be present in concentrations greater than 3.0 weight %-8.0 weight %; and in still other embodiments, manganese can be present in concentrations greater than 3.0 weight %-5.0 weight %.
Silicon is added to provide solid solution strengthening. Silicon is a ferrite stabilizer. In certain embodiments, silicon can be present in concentrations of 0.1-0.5 weight %; in other embodiments, silicon can be present in concentrations of 0.2-0.3 weight %.
Molybdenum is added to provide solid solution strengthening, to increase the hardenability of the steel, and to protect against embrittlement. In certain embodiments, molybdenum can be present in concentrations of 0-2.0 weight %; in other embodiments, molybdenum can be present in concentrations of 0-0.6 weight %; in still other embodiments, molybdenum can be present in concentrations of 0.1-2.0 weight %; in other embodiments, molybdenum can be present in concentrations of 0.1-0.6 weight %; in yet other embodiments molybdenum can be present in concentrations of 0.4-0.5 weight %; and in yet other embodiments molybdenum can be present in concentrations of 0.3-0.5 weight %.
Chromium can be added to reduce the martensite start temperature, provide solid solution strengthening, and increase the hardenability of the steel. Chromium is a ferrite stabilizer. In certain embodiments, chromium can be present in concentrations of 0-6.0 weight %; in other embodiments, chromium can be present in concentrations of 2.0-6.0 weight %; in other embodiments, chromium can be present in concentrations of 0.2-6.0 weight %; and in other embodiments chromium can be present in concentrations of 0.2-3.0 weight %.
Niobium can be added to increase strength and improve hardenability of the steel. In some embodiments niobium can also be added to provide improved grain refinement. In certain embodiments, niobium can be present in concentrations of 0-0.1 weight %; in other embodiments, niobium can be present in concentrations of 0.01-0.1 weight %; and in other embodiments, niobium can be present in concentrations of 0.001-0.055 weight %.
Vanadium can be added to increase strength and improve hardenability of the steel. In certain embodiments, vanadium can be present in concentrations of 0-0.15 weight %; and in other embodiments, vanadium can be present in concentrations of 0.01-0.15 weight %.
Boron can be added to increase the hardenability of the steel. In certain embodiments, boron can be present in concentrations of 0-0.005 weight %.
The hot-rolled steels can be processed using conventional steel making, roughing, and finishing processes. For example, the steels can be continuously cast to produce slabs of approximately 12-15 cm in thickness. Slabs are then reheated at temperatures of 1200-1320° C., and hot-rolled to a final gauge of ≥2.5 mm, with the final reduction pass occurring at a temperature of approximately 950° C. Scale on the hot-rolled steel coil can be removed by pickling and/or abrasive blasting using processes that are known in the art.
The alloys of the present application can be as-hot-rolled (that is, bare or uncoated) or they can also be coated with an aluminum-based coating, a zinc-based coating (either galvanized or galvannealed), after hot-rolling and scale removal. Such coating can be applied to the steel sheet using processes known in the art, including hot dip coating or electrolytic coating.
Various steel samples were prepared with the compositions shown below in Table 1. Generally, carbon was held at a fixed concentration. Meanwhile, the concentration of various substitutional or transition metal elements was varied while carbon remained constant to test the impact of these elements. These elements included manganese, chromium, molybdenum, and/or niobium.
Ingots were formed for each composition described above in Table 1. The ingots were formed by vacuum melting each composition in an induction furnace to cast 11-kg ingots. The as-cast ingots had an initial thickness of 45 mm. Once formed, the ingots were reheated to 1316° C. and rolled to a final thickness of approximately 3.6 mm. The rolling of each ingot was completed in eight passes. On the final rolling pass, a temperature measurement was taken and it was observed that the temperature of each ingot was <955° C. After rolling, coiling was simulated by subjecting each ingot to furnace equilibration at approximately 566° C. with a range of 450 to 650° C. and subsequent cooling to ambient temperature.
After the ingots were subjected to the simulated rolling and coiling processes described above in Example 2, micrographs were prepared using a Nital etch.
Ingots made with compositions of references 4339-1, 4339-2, and 4340-1 were observed to include varying amounts of ferrite, pearlite, and bainite. A martensitic microstructure was observed in ingots made with compositions of references 4340-2, 4341-1, 4341-2, 4342-1, and 4342-2. The presence of martensite in these samples was unexpected when considering the cooling rates applied to each ingot. As described above, relatively slow cooling rates generally favor the formation of ferrite, pearlite, and bainite over the formation of martensite. However, martensite formation was observed even though the expectation was ferrite, pearlite, bainite, and/or other non-martensitic constituents.
Based on the observations above, it was found that a martensitic microstructure can be formed when manganese is at least 5 wt. % while other substitutional elements are minimal and the carbon content is approximately 0.23 weight %. Less manganese can be present while still forming a martensitic microstructure if other substitutional elements are included. For instance, for steels containing approximately 4 wt. % manganese, additions of molybdenum, niobium, and/or vanadium can still promote the formation of a martensitic microstructure. Similarly, for steels containing approximately 3 wt. % manganese, an addition of 3 wt % chromium can still promote the formation of a martensitic microstructure.
After the ingots were subjected to the simulated rolling and coiling processes discussed above in Example 2, mechanical testing was also performed. Table 2, shown below, provides the results of the mechanical testing for each composition provided in Table 1.
As can be seen in Table 2, the compositions noted above in Example 4 as being susceptible to formation of martensitic microstructure after hot-rolling and relatively slow cooling also exhibited tensile strengths of approximately 1500 MPa. Ultimate tensile strengths in excess of 1400 MPa were achieved using several alloy strategies that produced martensitic microstructure in the as-hot-rolled condition. As described above in Example 4, this could include alloying with only manganese (e.g., reference 4340-2), alloying with a combination of manganese, molybdenum, and niobium (e.g., reference 4341-1), alloying with a combination of manganese, molybdenum, niobium, and vanadium (e.g., reference 4341-2), alloying with a combination of manganese, and chromium (e.g., reference 4342-1), and alloying with a combination of manganese chromium, molybdenum, and niobium (e.g., reference 4342-2).
For the compositions noted above as producing martensite in the as-hot-rolled condition, it was expected for the martensite to provide a hard and strong steel. The data provided above in Table 2 confirms that the martensite containing steels were strong with tensile strengths of approximately 1500 MPa. However, unexpectedly, the martensite containing steels exhibited relatively high elongation given the expected hardness of the steels. As can be seen above, total elongation was approximately 8-10%.
A high strength steel comprising by total weight percentage of the steel:
(a) from 0.1% to 0.5%, preferably from 0.1% to 0.35%, more preferably from 0.22-0.25%, Carbon;
(b) from 2.0% to 8.0%, preferably from greater than 3.0% to 8%; more preferably from 2.0 to 5.0%, and more preferably from greater than 3.0% to 5.0%, Manganese; and
(c) from 0.1% to 0.5%, preferably from 0.2% to 0.3%, Silicon.
A high strength steel of Example 6 or any one of the following Examples, further comprising from 0.0% to 6.0%, preferably from 0.0% to 2.0%, more preferably 0.1% to 6.0%, more preferably 0.1% to 2.0%, more preferably 0.1% to 0.6%, and more preferably 0.4% to 0.5%, Molybdenum.
A high strength steel of either one of Examples 6 and 7, or any one of the following Examples, further comprising from 0% to 6.0%, preferably 0.2% to 6.0%, more preferably 2.0% to 6.0%, and more preferably 0.2% to 3.0%, Chromium.
A high strength steel of any one of Examples 6 through 8, or any one of the following Examples, further comprising from 0.0% to 0.1%, preferably 0.01% to 0.1%, more preferably 0.001 to 0.055% Niobium.
A high strength steel of any one Examples 6 through 9, or any one of the following Examples, further comprising from 0.0% to 0.15%, preferably 0.01% to 0.15%, Vanadium.
A high strength steel of any Examples 6 through 10, or any one of the following Examples, further comprising from 0% to 0.005% Boron.
A high strength steel of any one of Examples 6 through 11, or any one of the following Examples, wherein the steel has, after hot-rolling and coiling, an ultimate tensile strength of at least 1480 MPa and a total elongation of at least 6%.
A high strength steel of any one of Examples 6 through 12, or any one of the following Examples, wherein the steel has, after hot-rolling and coiling, an ultimate tensile strength of approximately 1500 MPa and a total elongation of approximately 8 to 10%.
A high strength steel of any one of Examples 6 through 13, wherein the steel is coated with an aluminum-based coating or a zinc-based coating (either galvanized or galvannealed), after cold rolling and before hot stamping.
This application claims priority to U.S. Provisional Application Ser. Nos. 62/465,527 filed Mar. 1, 2017, entitled “Hot-Rolled Steel with Very High Strength and Method for Production,” the disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62465527 | Mar 2017 | US |
