Patent Application
20150240324
Hot rolled high strength structural steels containing not more than 0.22 weight percent (wt %) carbon (C) in combination with not more than 1.70 wt % manganese (Mn) and less than 0.3 wt % silicon (Si), and exhibiting strength levels of 350 megapascals (MPa) yield strength and 520 MPa tensile strength are known. Higher strength levels can be obtained with the addition of microalloying elements such as niobium (Nb), vanadium (V), and/or titanium (Ti). However, it is appreciated that the use of such microalloying elements increases the cost of the alloy. In addition, such alloys are known to have a high sensitivity to hot rolling processing parameters. In particular, coiling temperatures must be within +/−30° C. in order to obtain acceptable variations in the mechanical properties of the hot strip produced from such alloys.
In addition to the above, multiphase or complex phase steels have been proposed for serving or meeting desired high strength levels that exceed maximum levels of conventional high strength steels. However, such steels require not only accurate coiling temperatures but demand a specific time-temperature regime during finishing rolling, travel along a run-out table prior to coiling and during cooling. For example, hot rolled transformation-induced plasticity (TRIP) steels are not commercially available due to their strong sensitivity to hot strip processing temperature variations.
In contrast, fully martensitic steels can be easily processed if cooling capacity is sufficient to provide coiling temperatures of less than 200° C. Such steels can have tensile strengths of 1000 MPa and greater; however, ductility of these martensitic steels is typically less than 10%.
Given the above, an alloy for hot rolling that provides high strength and high ductility and yet is relatively insensitive to hot strip processing variations would be desirable.
A process for making a hot rolled strip from silicon alloyed steel is provided. The process includes providing a steel slab having a chemical composition in weight percent within a range of 0.06-0.30 carbon (C), 0.3-2.0 manganese (Mn), 0.6-3.5 silicon (Si), and iron (Fe) plus incidental melting impurities. The steel slab is hot rolled and hot rolled steel strip is produced. The hot rolled steel strip is coiled at temperatures between 600-100° C., has a microstructure containing at least 90 volume percent (vol. %) ferrite plus pearlite, a yield strength of at least 400 megapascals (MPa), a tensile strength of at least 600 MPa, and a tensile elongation of at least 20%. In addition, the coiled hot rolled strip has the above stated microstructure and mechanical properties irrespective of the coiling temperature between 100-600° C.
In some instances, the chemical composition of the steel slab can have up to 0.1 phosphorus (P), 0.08 maximum (max) aluminum (Al), 0.6 max chromium (Cr), 0.3 max nickel (Ni), 0.6 max copper (Cu), 0.08 max niobium (Nb), 0.6 max molybdenum (Mo), 0.10 max titanium (Ti), and 0.015 max sulfur (S). The steel alloy may or may not have a Si/Mn ratio between 1.3-3.5. In addition, the steel alloy can have a Si/Mn ratio between 0.8-3.5 when 0.03-0.05 P is present. In the alternative, the steel alloy can have a Si/Mn ratio between 0.3-3.5 when 0.05-0.1 P is present.
In some instances, the steel slab is hot rolled in an austenitic finishing mode and cooled at a cooling rate between 10-100 Kelvin per second (K/s) when the hot rolled strip has exited the last hot rolling stand and is between 950° C. and 400° C. at the onset of such accelerated cooling. In addition, the hot rolled strip can be subjected to a cooling interruption within a range of 0-20 seconds, e.g. within 3-20 seconds, when the hot rolled strip is between a finishing rolling station and coiling. In the event that the hot rolled strip is subjected to the cooling interruption, the strip is cooled at the cooling rate between 10-100 K/s before or after the cooling interruption and when the strip is at temperatures between 950-400° C.
In other instances, the steel slab is hot rolled in a partially ferritic finishing rolling mode and cooled at a cooling rate between 10-100 K/s when the hot rolled strip is between 850-400° C. at the onset of such accelerated cooling. Similar to hot rolling of the steel strip in the austenitic finishing roll mode, the partially ferritic finished hot rolled strip can be subjected to a cooling interruption within a range of 0-20 seconds between the finishing rolling station and coiling. Also, in the event that the partially ferritic finished hot rolled strip is subjected to the cooling interruption, the strip is cooled at the cooling rate between 10-100 K/s before or after the cooling interruption and when the strip is at temperatures between 850-400° C.
In some instances, the coiled hot rolled strip has a tensile strength of at least 650 MPa, while in other instances the hot rolled strip has a tensile strength of at least 700 MPa. In addition, the tensile strength (TS) of the coiled hot rolled strip obeys the equation:
TS(MPa)=164.9+634.7×C %+53.6×Mn %+99.7×Si %+651.9×P %+3339.4×N %+11/√Dα,Accuracy±25 MPa Eqn. 1
where Dα is the ferrite grain diameter (mm). In this manner, silicon alloyed steels with a desired tensile strength are produced.
A silicon alloyed steel for hot rolling and a process of hot rolling the silicon alloyed steel is provided. As such, the present invention has use as a structural material.
The silicon alloyed steel is suitable for hot rolling within a broad range of processing parameters. In addition, the steel alloy has a chemical composition, in wt %, within the range of 0.06-0.30 C, 0.3-2.0 Mn, 0.6-3.5 Si, and Fe plus other incidental melt impurities known to those skilled in the art. Optionally, the alloy has a phosphorus P content of less than or equal to 0.1, an Al content less than or equal to 0.08, a Cr content less than or equal to 0.6, a Mo content less than or equal to 0.6, a Ni content less than or equal to 0.3, a Cu content less than or equal to 0.6, a Nb content less than or equal to 0.08, a Ti content less than or equal to 0.10, and/or a S content less than or equal to 0.015. In some instances, the ratio of Si/Mn is between 1.3-3.5. In other instances, the ratio of Si/Mn is between 0.8-3.5 when the P content is between 0.03-0.05. Still in other instances, the ratio of Si/Mn is between 0.5-3.5 when the P content is between 0.05-0.1.
With such an alloy composition, the mechanical properties of the alloy are remarkably stable or uniform for a wide range of coiling temperatures. For example, the alloy can exhibit a spread of yield strength of less than 70 MPa, a spread of tensile strength less than 50 MPa, and a spread of percent elongation to failure less than 10% for coiling temperatures between 600-100° C. As such, a silicon alloyed steel suitable for hot rolling within a wide range of thermo-mechanical parameters is provided.
Turning now to
In other instances, the austenized steel is subjected to a partially ferritic finishing mode which refers to finishing the material at a temperature below the Ar3 temperature for the alloy. Finishing the material in the partially ferritic mode still results in a microstructure with greater than 90 vol % ferrite plus pearlite before being coiled at a temperature that is less than or equal to 600° C. The material is also cooled between 10-100 K/s when the material is between 850° C. and the coiling temperature. Finally, and similar to the material hot formed in the austenitic finishing mode, the material hot formed in the partially ferritic finishing mode can be subjected to a cooling interruption of 0-20 seconds before or after accelerated cooling.
A steel alloy according to the embodiment of the present invention and processed as stated above has a yield strength of at least 400 MPa, a tensile strength of at least 600 MPa, and an elongation to failure of at least 20%. In addition, such properties are obtained when the hot rolled strip is coiled at any temperature or temperatures between 100-600° C. Stated differently, different coils of the alloy exhibit relatively constant mechanical properties despite coiling temperatures that can vary by as much as 400° C. As such, the steel alloy is relatively insensitive to processing parameters and exhibits mechanical properties that are relatively or essentially insensitive to coiling temperatures below 600° C. It is appreciated that such insensitivity to processing parameters naturally provides minimal head to tail variations in mechanical properties for coiled hot rolled strip and thus leads to exceptional yield for the material.
In order to provide specific examples of the steel alloy and yet not limit the scope of the invention in any way, two inventive alloy compositions and two comparison alloy compositions subjected to a range of finishing temperatures and coiling temperatures are discussed below.
A first inventive steel alloy (A) having a composition of 0.21 C, 1.01 Mn, 2.0 Si, and 0.06 Al was cast into a 160 by 160 mm thick ingot. The cast ingot was forged into a 20 mm specimen which was austenized at 1150° C. for 20 minutes and then subsequently subjected to four consecutive strokes of forging having a strain of 0.3 and a strain rate of 10/s in order to produce a final hot formed sample with a thickness of 6 mm.
A second inventive steel alloy (B) having a composition of 0.16 C, 1.00 Mn, 2.01 Si, and 0.06 Al was cast and forged in a similar manner as the first alloy composition.
The 6 mm specimens were cooled at accelerated cooling rates between 10-50 K/s between the finishing temperature and coiling temperature shown in the Table 1. In addition, a time period of approximately 10 seconds was present between the onset of accelerated cooling from the finishing temperature to the coiling temperature. Finally, the mechanical properties shown in Table 1 were produced from round tensile specimens having a diameter of 4 mm and a test length of 33.8 mm.
A first comparison alloy (C) having a composition of 0.201 C, 0.83 Mn, with the remainder being traces and unavoidable residuals was processed according to an embodiment illustrated in
As shown in Table 1, both alloy A and alloy B steel samples were subjected to a wide range of finishing temperatures and coiling temperatures. However, the mechanical properties exhibited by the tensile specimens were remarkably constant. For example, alloy A exhibited an average yield strength of 564 MPa, with a range or spread of 55 MPa, and average tensile strength of 719 MPa, with a range of 31 MPa, and an average tensile elongation of 29% with a range of 8% —even though samples were finished at temperatures between 800-890° C. and coiled at temperatures between 350-450° C.
Alloy B exhibited an average yield strength of 518 MPa, with a range of 67 MPa, an average tensile strength of 666 MPa, with a range of 40 MPa, and an average tensile elongation of 31% with a range of 12% —even though samples were finished at temperatures between 790-850° C. and coiled temperatures between 100-450° C.
In contrast, alloy C exhibited an average yield strength of 328 MPa, with a range of 68 MPa, and an average tensile strength of 470 MPa with a range of 44 MPa when coiled at temperatures that provided acceptable ductility, i.e. an average tensile elongation of 31.6% with a range of 12.2% —all for a range in coiling temperatures between 661-668° C. However, when alloy D was coiled at temperatures approaching 400° C., the average yield strength increased to 657 MPa, with a range of 184 MPa, the average tensile strength increased to 767 MPa, with a range of 123 MPa, ductility dropped to an average of 11.8%, with a range of 6% —all for a range of coiling temperatures between 503-514° C.
With regards to
As illustrated by Table 1 and
The inventive alloys also exhibit acceptable or good weldability. Again, not being bound by theory, the inventive alloys do not exhibit excessive martensitic and hard phases in a heat affected zone (HAZ) due to their cooling transformation behavior. As such, and with the absence of martensite and hard phases, hydrogen induced cracking is reduced.
Given the above data and results, it is appreciated that a hot rolled high strength structural steel alloy that is insensitive to hot rolling finishing treatment temperatures and coiling temperatures is provided. It should also be appreciated that changes, modifications, and the like can be made to the alloy compositions and processing parameters and yet fall within the scope of the invention. As such, it is the claims and all equivalents thereof that define the scope of the invention.
