This invention relates to making of high strength thin cast strip, and the method for making such cast strip by a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated, internally cooled casting rolls so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal is poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal, supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
In the past, high-strength low-carbon thin strip with yield strengths of 413 MPa (60 ksi) and higher, in strip thicknesses less than 3.0 mm, have been made by recovery annealing of cold rolled strip. Cold rolling was required to produce the desired thickness. The cold roll strip was then recovery annealed to improve the ductility without significantly reducing the strength. However, the final ductility of the resulting strip still was relatively low and the strip would not achieve total elongation levels over 6%, which is required for structural steels by some building codes for structural components. Such recovery annealed cold rolled, low-carbon steel was generally suitable only for simple forming operations, e.g., roll forming and bending. To produce this steel strip with higher ductility was not technically feasible in these final strip thicknesses using the cold rolled and recovery annealed manufacturing route.
In the past, high strength, steel has been made by microalloying with elements such as niobium, vanadium, titanium or molybdenum, and hot rolling to achieve the desired thickness and strength level. Such microalloying required expensive and high levels of niobium, vanadium, titanium or molybdenum and resulted in formation of a bainite-ferrite microstructure typically with 10 to 20% bainite. See U.S. Pat. No. 6,488,790. Alternately, the microstructure could be ferrite with 10-20% pearlite. Hot rolling the strip resulted in the partial precipitation of these alloying elements. As a result, relatively high alloying levels of the Nb, V, Ti or Mo elements were required to provide enough age hardening of the predominately ferritic transformed microstructure to achieve the required strength levels. These high microalloying levels significantly raised the hot rolling loads needed and restricted the thickness range of the hot rolled strip that could be economically and practically produced. Such alloyed high strength strip could be directly used for galvanizing after pickling for the thicker end of the product range greater than 3 mm in thickness.
However, making of high strength, steel strip less than 3 mm in thickness with additions of Nb, V, Ti or Mo to the base steel chemistry was very difficult, particularly for wide strip due to the high rolling loads, and not always commercially feasible. In the past, large additions of these elements were needed for strengthening the steel, and in addition, caused reductions in elongation properties of the steel. High strength microalloyed hot rolled strips in the past were relatively inefficient in providing strength, relatively expensive, and often required compensating additions of other alloying elements.
Additionally, cold rolling was generally required for lower thicknesses of strip; however, the high strength of the hot rolled strip made such cold rolling difficult because of the high cold roll loadings required to reduce the thickness of the strip. These high alloying levels also considerably raised the recrystallization annealing temperature needed, requiring expensive to build and operate annealing lines capable of achieving the high annealing temperature needed for full recrystallization annealing of the cold rolled strip.
In short, the application of previously known microalloying practices with Nb, V, Ti or Mo elements to produce high strength thin strip could not be commercially produced economically because of the high alloying costs, relative inefficiency of element additions, difficulties with high rolling loads in hot rolling and cold rolling, and the high recrystallization annealing temperatures required.
A steel product is disclosed comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20% and having a majority of the microstructure comprised of bainite and acicular ferrite and having more than 70% niobium in solid solution. Alternately, the niobium may be less than 0.1%. The steel product may further comprise at least one element selected from the group consisting of molybdenum between about 0.05% and about 0.50%, vanadium between about 0.01% and about 0.20%, and a mixture thereof.
The steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa. The steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
The steel product may be a thin cast steel strip. Optionally, the thin cast steel strip may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers
The thin cast steel strip may have a thickness of less than 2.5 mm. Alternately, the thin cast steel strip may have a thickness of less than 2.0 mm. In yet another alternative, the thin cast steel strip may have a thickness in the range from about 0.5 mm to about 2 mm.
The hot rolled steel product of less than 3 millimeters thickness is also disclosed comprised, by weight, of less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05% and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20%, and have a majority of the microstructure comprised of bainite and acicular ferrite and capable of providing a yield strength of at least 410 MPa with a reduction of between 20% and 40%. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa. Alternately, the niobium may be less than 0.1%.
Optionally, the hot rolled steel product may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers.
The hot rolled steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%. The hot rolled steel product may have a thickness of less than 2.5 mm. Alternately, the hot rolled steel product may have a thickness of less than 2.0 mm. In yet another alternative, the hot rolled steel product may have a thickness in the range from about 0.5 mm to about 2 mm.
Also disclosed is a coiled steel product comprised, by weight, of less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element selected from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having more than 70% niobium and/or vanadium in solid solution after coiling and cooling. Alternately, the niobium may be less than 0.1%.
Optionally, the coiled steel product may have fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers.
The coiled steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The coiled steel product has a thickness of less than 3.0 mm. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa.
Alternately, the coiled steel product has a thickness of less than 2.5 mm. Alternately, the coiled steel product may have a thickness of less than 2.0 mm. In yet another alternative, the coiled steel product may have a thickness in the range from about 0.5 mm to about 2 mm. The coiled steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
An age hardened steel product is also disclosed comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, at least one element from the group consisting of niobium between about 0.01% and about 0.20%, vanadium between about 0.01% and about 0.20%, and a mixture thereof, and having a majority of the microstructure comprised of bainite and acicular ferrite and having an increase in elongation and an increase in yield strength after age hardening. Alternately, the niobium may be less than 0.1%.
The age hardened steel product may comprise, in addition, fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers.
The steel product may have a yield strength of at least 340 MPa, or at least 380 MPa, or at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at least 550 MPa, or at least 600 MPa, or at least 650 MPa, as desired. The steel product may have a tensile strength of at least 410 MPa, or at least 450 MPa, or at least 500 MPa, or at least 550 MPa, or at least 600 MPa, or at least 650 MPa, or at least 700 MPa, as desired. The age hardened steel product has a thickness of less than 3.0 mm. Alternately, the age hardened steel product has a thickness of less than 2.5 mm. Alternately, the age hardened steel product may have a thickness of less than 2.0 mm. In yet another alternative, the age hardened steel product may have a thickness in the range from about 0.5 mm to about 2 mm. The age hardened steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
A steel product comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element selected from the group consisting of niobium between about 0.01% and about 0.20% and vanadium between about 0.01% and about 0.20%, and having a majority of the microstructure comprised of bainite and acicular ferrite and comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Alternately, the niobium may be less than 0.1%. Optionally, the steel product may comprise molybdenum between about 0.05% and 0.50%.
The steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The steel product may have a yield strength of at least 485 MPa and a tensile strength of at least of at least 520 MPa. The steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
An age hardened steel product comprising, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20%, and having a majority of the microstructure comprised of bainite and acicular ferrite and having niobium carbonitride particles with an average particle size of less than 10 nanometers. Carbonitride particles, in the present specification and appended claims, includes carbides, nitrides, carbonitrides, and combinations thereof. Alternately, the niobium may be less than 0.1%.
The age hardened steel product may have substantially no niobium carbonitride particles greater than 50 nanometers. The age hardened steel product may have a yield strength of at least 340 MPa, and may have a tensile strength of at least 410 MPa. The age hardened steel product has a total elongation of at least 6%. Alternately, the total elongation may be at least 10%.
A method is disclosed for preparing coiled thin cast steel strip comprising the steps of:
The method may provide in the steel strip as coiled fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Further, the method may comprise the steps of hot rolling the steel strip, and coiling the hot rolled steel strip at a temperature between about 450 and 700° C. Alternately, the coiling of the hot rolled steel strip may be at a temperature less than 650° C.
The method may further comprise the step of age hardening the steel strip to increase the tensile strength at a temperature of at least 550° C. Alternately, the age hardening may occur at a temperature between 625° C. and 800° C. In yet another alternate, the age hardening may occur at a temperature between 650° C. and 750° C.
Also disclosed is a method of preparing a thin cast steel strip comprised the steps of:
The method may further comprise the step of age hardening the steel strip to increase the tensile strength. Alternately, the age hardening may occur at a temperature between 650° C. and 750° C.
The method may provide the age hardened steel strip having niobium carbonitride particles with an average particle size of less than 10 nanometers. Alternately, the age hardened steel strip has substantially no niobium carbonitride particles greater than 50 nanometers.
The method may provide in the steel strip as coiled fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Further, the method may comprise the steps of hot rolling the steel strip, and coiling the hot rolled steel strip at a temperature less than 700° C. Alternately, the coiling of the hot rolled steel strip may be at a temperature less than 650° C.
The method of preparing a thin cast steel strip may comprise the steps of:
The method may provide in the steel strip as coiled fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. Further, the method may provide the age hardened steel strip having niobium carbonitride particles with an average particle size of less than 10 nanometers. Alternately, the age hardened steel strip has substantially no niobium carbonitride particles greater than 50 nanometers.
The method may comprise the steps of hot rolling the steel strip, and coiling the hot rolled steel strip at a temperature less than 750° C. Alternately, the coiling of the hot rolled steel strip may be at a temperature less than 700° C.
In order that the invention may be described in more detail, some illustrative examples will be given with reference to the accompanying drawings in which:
a is an optical micrograph of a niobium steel strip;
b is an optical micrograph of a standard UCS SS Grade 380 steel strip;
a shows micrographs of the microstructure of a first sample of 0.065% niobium steel after hot rolling;
b shows micrographs of the microstructure of a second sample of 0.065% niobium steel after hot rolling;
The following description of the embodiments is in the context of high strength thin cast strip with microalloy additions made by continuous casting steel strip using a twin roll caster.
As shown in
The twin roll caster may be of the kind that is illustrated and described in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243 or U.S. Pat. No. 5,488,988. Reference may be made to those patents for appropriate construction details of a twin roll caster appropriate for use in an embodiment of the present invention.
A high strength thin cast strip product can be produced using the twin roll caster that overcomes the shortcomings of conventional light gauge steel products and produces a high strength, light gauge, steel strip product. The invention utilizes the elements including niobium (Nb), vanadium (V), titanium (Ti), or molybdenum (Mo), or a combination thereof.
Microalloying elements in steel are commonly taken to refer to the elements titanium niobium, and vanadium. These elements were usually added in the past in levels below 0.1%, but in some cases levels as high as 0.2%. These elements are capable of exerting strong effects on the steel microstructure and properties via a combination of hardenability, grain refining and strengthening effects (in the past as carbonitride formers). Molybdenum has not normally been regarded as a microalloying element since on its own it is a relatively weak carbonitride former, but may be effective in the present circumstances and may form complex carbonitride particles along with niobium and vanadium. Carbonitride formation is inhibited in the hot rolled strip with these elements as explained below.
The high strength thin cast strip product combines several attributes to achieve a high strength light gauge cast strip product by microalloying with these elements. Strip thicknesses may be less than 3 mm, less than 2.5 mm, or less than 2.0 mm, and may be in a range of 0.5 mm to 2.0 mm. The cast strip is produced by hot rolling without the need for cold rolling to further reduce the strip to the desired thickness. Thus, the high strength thin cast strip product overlaps both the light gauge hot rolled thickness ranges and the cold rolled thickness ranges desired. The strip may be cooled at a rate of 10° C. per second and above, and still form a microstructure that is a majority and typically predominantly bainite and acicular ferrite.
The benefits achieved through the preparation of such a high strength thin cast strip product are in contrast to the production of previous conventionally produced microalloyed steels that result in relatively high alloy costs, inefficiencies in microalloying, difficulties in hot and cold rolling, and difficulties in recrystallization annealing since conventional continuous galvanizing and annealing lines are not capable of providing the high annealing temperatures needed. Moreover, the relatively poor ductility exhibited with strip made by the cold rolled and recovery annealed manufacturing route is overcome.
In previous conventionally produced microalloyed steels, elements such as niobium and vanadium could not remain in solid solution through solidification, hot rolling, coiling and cooling. The niobium and vanadium diffused through the microstructure forming carbonitride particles at various stages of the hot coil manufacturing process. Carbonitride particles, in the present specification and appended claims, includes carbides, nitrides, carbonitrides, and combinations thereof. The formation and growth of carbon and nitrogen particles in the hot slab and subsequent coiling of previous conventionally produced microalloyed steels further reduced the grain size of austenite in the hot slab, decreasing the hardenability of the steel. In these prior steels, the effect of particles in the hot slab had to be overcome by increasing the amount of microalloying elements, reheating the cast slabs to higher temperatures, and lowering carbon content.
In contrast to the previous conventionally produced steels, the present high strength thin cast steel strip product was produced comprising, by weight, less than 0.25% carbon, between 0.20 and 2.00% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20%, and having a microstructure comprising a majority bainite. The steel product may further comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. The steel product may further comprise a more even distribution of microalloys through the microstructure than previously produced with conventional slab cast product.
Alternately, the high strength thin cast steel strip product may comprise, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and niobium between about 0.01% and about 0.20%, and having a majority of the microstructure comprising bainite and acicular ferrite and having more than 70% soluble niobium.
In another alternate, a coiled steel product may comprise, by weight, less than 0.25% carbon, between 0.20 and 2.0% manganese, between 0.05 and 0.50% silicon, less than 0.01% aluminum, and at least one element selected from the group consisting of niobium between about 0.01% and about 0.20% and vanadium between about 0.01% and about 0.20%, and a combination thereof, and having more than 70% soluble niobium and vanadium, as selected, after coiling and cooling. The coiled high strength thin cast steel strip product may have more than 70% soluble niobium and vanadium, as selected, particularly after hot rolling reduction and subsequent coiling and before age hardening. The microstructure may be a mixture of bainite and acicular ferrite. Alternately, the microstructure of the hot rolled and subsequently coiled and cooled steel may comprise bainite and acicular ferrite with more than 80% niobium and/or vanadium remaining in solid solution, and alternately may have more than 90% remaining in solid solution.
Alternatively or in addition, the steel product may have a total elongation greater than 6% or greater than 10%. The steel product may have a yield strength of at least 340 MPa (about 49 ksi) or a tensile strength of at least 410 MPa, or both, exhibiting satisfactory ductility. The relationship between yield strength and total elongation in the hot rolled product is shown in
After hot rolling the hot rolled steel strip may be coiled at a temperature in the range from about 500-700° C. The thin cast steel strip may also be further processed by age hardening the steel strip to increase the tensile strength at a temperature of at least 550° C. The age hardening may occur at a temperature between 550° C. and 800° C., or between 625° C. and 750° C., or between 675° C. and 750° C. Conventional furnaces of continuous galvanizing or annealing lines are thus capable of providing the age hardening temperatures needed to harden the microalloyed cast strip product.
For example, a steel composition was prepared by making a steel composition of a 0.026% niobium, 0.04% by weight carbon, 0.85% by weight manganese, 0.25% by weight silicon that has been cast by a thin cast strip process. The strip was cast at 1.7 mm thick and inline hot rolled to a range of strip thickness from 1.5 mm to 1.1 mm using a twin roll caster as illustrated in
As shown in
The hardenability of the present steels is shown in
The high strengths were achieved by utilizing the niobium microalloying addition to increase the hardenability of the steel through the suppression of the formation of proeutectic ferrite.
The effect of hot reduction on yield strength is reduced in the present niobium steel. In previous C—Mn products, there is typically a decrease in strength with increasing hot reduction. In contrast, as shown in
To investigate this effect further, the austenite grain size was measured at each thickness in the 0.026 Nb steel. Where the base steel tended to be fully recrystallized above about 25% hot reduction, the 0.026 Nb steel showed only limited recrystallization even at 40% reduction. This indicates that the niobium in solid solution reduced the effect of hot reduction on the strength properties by suppressing static recrystallization of the deformed austenite after hot rolling. This is shown in
The thin cast strip niobium steel product had consistent yield and tensile strength levels over the range of hot rolling applied, and capable of providing a yield strength of at least 410 MPa with a reduction of between 20% and 40%. The prior austenite grain size was determined for each strip thickness. The austenite grain size measurements indicated that only very limited recrystallization had occurred at high hot rolling reductions, whereas in the comparable base steel strip, the microstructure almost fully recrystallized at hot rolling reductions over about 25%. The addition of niobium to the cast steel strip suppressed the recrystallization of the coarse as-cast austenite grain size during the hot rolling process, and resulted in the hardenability of the steel being retained after hot rolling and retention of niobium in solution.
The higher strength of the present steel strip after hot rolling was mostly due to the microstructure formed. As shown in
The yield and tensile strength results from the trial steels, shown in Table 2 below, in the as-hot rolled condition are summarized in
In contrast to the previous conventionally produced steels, the present high strength thin cast steel strip product was produced comprising, by weight, less than 0.25% carbon, between 0.20 and 2.00% manganese, between 0.05 and 0.50% silicon, less than 0.06% aluminum, and at least one element selected from the group consisting of titanium between about 0.01% and about 0.20%, niobium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20%, and having a microstructure comprising a majority bainite. The steel product may further comprising fine oxide particles of silicon and iron distributed through the steel microstructure having an average particle size less than 50 nanometers. The steel product may further comprise a more even distribution of microalloys through the microstructure than previously produced with conventional slab cast product.
The transmission electron microscopy (TEM) examination may be used to determine the size, identity and volume fraction of niobium carbonitride particles present in the steel. The absence of any niobium carbonitride particles upon TEM examination supported the view that the observed strength was largely attributable to the microstructure being largely bainite rather than ferrite. The subsequent observed strengthening increment arising from an age hardening heat treatment therefore leads to the conclusion that niobium had been substantially in solution in the hot rolled strip. After determining the volume fraction of carbonitride particles in the microstructure using TEM analysis, the amount of microalloy element in solid solution can be concluded.
Thin foils or carbon replicates may be evaluated by TEM in determining the amount of the present carbonitride particles. In our analysis, a JEOL 2010 transmission electron microscope was used. However, from our experience with this instrument, Nb particles below 4 nanometers may not be resolvable in heavily dislocated ferrite.
For thin foil analysis, a foil is prepared. The foil is cut and ground to a thickness of 0.1 mm. The sample is then thinned to electron transparency by electro-polishing using a 5% perchloric acid, 95% acetic acid electrolyte in a Tenupole-2 electro-polishing unit. The sample can then be directly transferred to the TEM.
For carbon replication, a desired sample may be prepared by etching a polished sample in Nital (a solution of alcohol and nitric acid) after etching, coating the samples with carbon, and then scoring the carbon coating into appropriate dimensions (for example 2 mm square) for TEM analysis. After scoring, carbon replicas may be liberated from the sample by dissolving the ferrite matrix in 3% Nital. The carbon replica samples are collected on 3 mm diameter support grids, then repeatedly washed in ethanol/water solutions. The carbon extraction replica with the supporting grid can then be transferred to the TEM.
An additional factor believed to account for the absence of niobium carbonitride particles in the hot rolled cast strip relates to the nature of the dispersion of niobium with the rapid solidification of the strip during its formation by the method of continuously making cast strip described. In previously made microalloyed high strength strip, relatively long time intervals were involved in the solidification with slab cooling, slab reheating and thermo-mechanical processing that permitted opportunities for pre-clustering and/or solid state precipitation of carbonitride particles such as (Nb, V, Ti, Mo)(CN), that enabled the kinetics for subsequent precipitation through the stages of the manufacturing process. In the present process described, where the cast strip is continuously formed from a casting pool between casting rolls, the extremely rapid initial solidification in forming the cast strip (in about 160 microseconds) is believed to inhibit pre-clustering and/or solid state precipitation of carbonitride particles, and in turn, slow and reduce the kinetics for precipitation of the microalloys in subsequent processing including rolling and coiling operations. This means that the microalloys of Nb, V, Ti, and Mo are relatively more evenly distributed in the austenite and ferrite phases, than in thin steel strip previously made by conventional slab casting and processing.
Atom probe analysis of niobium cast strip made by forming from a casting pool between casting rolls as above described has verified the more even distribution of microalloys (indicating reduced pre-clustering and/or solid state precipitation) in both the as cast and the hot rolled strip when coiled at about 650° C. or lower. This more even distribution of elements is believed to be inhibiting the formation of carbonitrides in the coiling operation under conditions where fine coherent precipitation of such elements occurred in previous conventionally made and processed microalloyed slab cast steel. The reduction or absence of pre-clustering and/or solid state formation of carbonitrides in the microalloyed cast strip made by twin roll casting also slows the kinetics of formation of carbonitrides during subsequent thermo-mechanical processing such as annealing. This then permits the opportunity for age hardening at temperatures higher than those where the particles in previously conventionally processed strip lost their strengthening capacity through coarsening (Ostwald ripening) mechanisms.
With an age hardening heat treatment, higher tensile strength was found to be achievable. For example, with a 0.026% niobium addition, an increase of at least a 35 MPa (about 5 ksi) increase in yield strength from 410 to 450 MPa (about 60-65 ksi) was observed. With a 0.05% niobium addition, it is contemplated that with a age hardening, an increase of at least 10 ksi is expected, and a with 0.1% niobium addition, it is contemplated that with a age hardening, an increase of at least 20 ksi is expected. The microstructure of the present age hardened steel product may have niobium carbonitride particles with an average particle size of 10 nanometers and less. The microstructure of the age hardened steel product may have substantially no niobium carbonitride particles greater than 50 nanometers.
Laboratory ageing heat treatments were conducted on samples of 0.026% niobium steels at various temperatures and times to induce action of the niobium, that was believed retained in solid solution in the hot rolled strip. As shown in
The results, as shown in
Recently, in addition to producing the 0.026 wt % niobium steel, steels with niobium additions of 0.014 wt % and 0.065 wt % have been successfully produced via the present process. Heat compositions are shown below in Table 2.
The yield strengths achieved for steel C and steel F are shown in
Additionally, in contrast to previous conventionally produced microalloyed steel, we have found that the microalloy addition suppresses the formation of carbonitride particles in the hot rolled and subsequently coiled and cooled steel. Instead, the microstructure of the hot rolled and subsequently coiled and cooled steel comprises bainite and acicular ferrite with more than 70% niobium and/or vanadium remaining in solid solution. Alternately, the microstructure of the hot rolled and subsequently coiled and cooled steel may comprise bainite and acicular ferrite with more than 80% niobium and/or vanadium remaining in solid solution, and alternately may have more than 90% remaining in solid solution.
Thus, it has been shown that the niobium cast strip results in light gauge, high strength, steel product. The niobium addition firstly is capable of suppressing the austenite recrystallization during hot rolling, which enhances the hardenability of the steel by retaining the relatively coarse as cast austenite size. The niobium being retained in solid solution in austenite after hot rolling, directly increases the steel's hardenability, which assists in transforming the austenite to a final microstructure comprised mostly of bainite, even at relatively high coiling temperatures. The formation of a bainitic microstructure promoted the retention of the niobium addition in solid solution in the hot rolled strip.
Further improvement in properties may be obtained by age hardening the present steels. In previous microalloyed and non-microalloyed steels, an increase in strength could be obtained by age hardening, but in such prior steels, a decrease in elongation occurs with an increase in strength. We have found that both an increase in elongation and an increase in strength may be obtained by age hardening the present steels.
It was determined that the retention of the microalloying elements such as niobium and vanadium in solid solution by the prior processing conditions provided considerable hardenability for subsequent age hardening cycle. Such an age hardening cycle can be produced using a suitable continuous galvanizing line or continuous annealing facility. Hence a microalloyed steel strip made using a thin strip casting process, combined with an age hardening heat treatment provided by a suitable galvanizing line or annealing line, is a unique manufacturing path providing a unique strengthening approach for this type of steel product.
Isothermal ageing treatments of the hot rolled 0.026% Nb cast strip material were carried out for 20 minutes at 600° C. and 650° C. (1110° F. and 1200° F.), inducing formation of niobium carbonitrides, or Nb(C,N), as confirmed by TEM examination. This resulted in an increase in yield strength of the material, as shown in
The increase in the hardenability provided by the microalloy addition through the suppression of the ferrite transformation significantly lowers the austenite decomposition temperature into the bainite/acicular ferrite temperature range. This lower transformation start temperature provides the potential to retain the vast majority of the microalloy addition in solid solution by applying conventional run out table cooling rates and appropriate coiling temperatures.
The microalloying elements, such as niobium and vanadium, in solid solution are available for age hardening during a subsequent heat treatment to increase strength. Laboratory age hardening studies established that substantial strengthening could be achieved even with relatively short heat treatment cycles, such as available with continuous annealing lines and galvanizing lines. The results from laboratory simulated continuous annealing cycles applied to trial Steel C (0.026% Nb), Steel F (0.065% Nb), and Steel G (0.084% Nb) are shown in
The results from full scale plant trials with steels B and F, using the heat treatment conditions established from the laboratory study are given in
Samples of steel F were age hardened on using the age hardening conditions found on a galvanizing line. As shown in TABLE 3, the age hardened steel had a strength of almost 70 MPa, and the elongation increased from 11.47% to 14.16%. The relationship between yield strength and total elongation for the presently disclosed niobium steels in the as hot rolled condition and in the age hardened and galvanized condition (longitudinal test direction) is shown in
As shown in
In the case of longer time heat treatments, lower temperatures in the range of 500° C. to 650° C. may be used. From
Transmission electron microscopy (TEM) was carried out on samples of steels C and F, which had been given a heat treatment of 60 minutes at 650° C. Fine particles in the size range of 4 to 15 nanometers were found. These fine particles were found to include niobium carbonitrides, indicating that the strengthening may be attributed to age hardening by fine niobium carbonitride particles.
The microstructure of the age hardened microalloyed steel product may have niobium carbonitride particles, with an average particle size of 10 nanometers and less. The microstructure of the age hardened steel product may have substantially no niobium carbonitride particles greater than 50 nanometers. Samples of the present niobium steel were inspected using TEM evaluation, and portions of the microstructure had no measurable amount of niobium carbonitride particles.
We believe that the enhanced strength/elongation relationship in the present age hardened steel may be due to portions of the microstructure being substantially free of particles greater than 5 nanometers in size, or “precipitate free zones,” and nano-clusters. The development of precipitate free zones in the vicinity of grain boundaries may influence the strength and tensile elongation relationship by providing reduced hardness regions adjacent to grain boundaries. The relaxation of stress concentrations in precipitate free zones has been reported to enhance strength and elongation. The beneficial effects of precipitate free zones on elongation and strength may appear in circumstances where the precipitate free zones are narrow and the size of grain boundary precipitates is small.
In the present steel, the element additions may provide for increased elongation with increased strength after age hardening by producing smaller precipitate free zone width and smaller change in hardness than in conventionally produced niobium steels. Because of the more even dispersion of elements in rapidly solidified steels, the kinetics of age hardening can be retarded so as to effectively expand the time-temperature window over which the formation of nano-clusters can be stably controlled. The element nano-clusters may provide strengthening in the early stages of age hardening. Cluster strengthening may be due to the extra energy required for dislocations to cut the diffuse boundary of the cluster of solute species. The clusters may provide substantial strengthening without reducing ductility because their elastically soft boundaries do not severely inhibit dislocation movement or cause pile-ups in the way that normal second phase particles do.
In the present steels, a more even distribution of elements remains in solid solution during the rapid solidification of steel. In contrast to previous conventionally produced niobium and vanadium steels, the microstructure of the hot rolled and subsequently coiled and cooled steel comprises bainite and acicular ferrite with more than 70% niobium and/or vanadium addition remaining in solid solution and substantially no niobium carbonitride particles greater than 50 nanometers. Alternately, the microstructure of the hot rolled and subsequently coiled and cooled steel may comprise bainite and acicular ferrite with more than 80% niobium and/or vanadium addition remaining in solid solution, and alternately may have more than 90% remaining in solid solution.
The elements remain trapped in solution in the hot rolled coil and do not precipitate if the coiling temperature is below about 650° C. Formation is effectively retarded because the prior associations of atoms (such as in the form of particles) that normally occur in conventional slab casting and reheating for hot strip rolling are prevented in the present process. The observed increase in strength that occurs in the hot rolled coils may thus be largely attributable to hardenability and solid solution hardening effects.
Formation of carbonitride particles can be activated during heat treatment. Additionally, during age hardening, pre-precipitation clusters and finer particles are stable over an extended range of time and temperature because of the significant amount of niobium and/or vanadium in solid solution prior to age hardening. The precipitate free zones that form near grain boundaries as a normal precipitation phenomenon are narrower and contain more evenly dispersed nano-clusters and finer precipitates than for conventionally produced steels. Thus the hardness changes in the precipitate free zones relative to the grain interior are relatively small for the present steels. We believe that narrower precipitate free zones and small hardness changes across precipitate free zones reduce stress concentrations in the precipitate free zones reducing microcracking from preferential deformation in the precipitate free zones. We believe that the cluster strengthening may be characterized by a strength increase without a deterioration in ductility since dislocation pile-up does not occur at clusters. The combination of narrow precipitate free zones and cluster strengthening mechanisms is believed to lead to precipitate free zones of the present steels. This results in improved elongation because cracks are more difficult to initiate and less constrained to the grain boundary precipitate free zone region. Further, the nano-clusters may co-exist with distinct particles within the grain interior regions over a certain annealing temperature/time combinations.
An annealing furnace may be used to perform the age hardening, which is not a current strengthening approach for processing such products. The annealing condition may be a continuous annealing cycle with a peak temperature of at least 650° C. and less than 800° C. and better 675° C. to 750° C. Alternately, strengthening may be achieved in a production environment using a very short age hardening cycle available with conventional annealing furnaces incorporated in continuous galvanizing lines. The final strength levels recorded in the full scale plant trials were similar to that produced with the laboratory heat treatments of the respective steels.
Similar results are contemplated with niobium between about 0.01% and about 0.20%, as well as with titanium between about 0.01% and about 0.20%, molybdenum between about 0.05% and about 0.50%, and vanadium between about 0.01% and about 0.20%.
The composition of the present steel utilizing vanadium is shown as steel H in Table 2. The yield strength of steel H is shown in
The yield strength of steel H in the as-hot rolled and galvanized conditions are presented in
Also shown in
This thin cast strip enables production of new steel product types including:
1. A high strength, light gauge, galvanized strip by utilizing a microstructure that has bainite as the major constituent and age hardening during the galvanizing process. The annealing section of the galvanizing line can be used to induce age hardening of the niobium and/or vanadium of the thin cast strip that has been hot rolled.
2. A high strength, light gauge, uncoated strip by utilizing a microstructure that is majority bainite and age hardened during processing on a continuous annealing line. The high temperature furnace of the conventional continuous annealing can be used to induce activation of the niobium and vanadium elements retained in solid solution by the bainite microstructure after hot rolling of the thin cast strip.
3. A high strength, light gauge, hot rolled cast strip product where the strength levels are insensitive to the degree of hot rolling reduction applied. The bainitic microstructure produces a relatively high strength product (YS≧380 MPa (˜55 ksi)). The suppression of austenite recrystallization during or after hot rolling can provide final strength levels insensitive to the degree of hot rolling reduction. The final strength levels will be consistent across a range of thicknesses that can be produced by a thin cast strip process.
While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described, and that all changes and modifications that come within the spirit of the invention described by the following claims are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Modifications may be made without departing from the spirit and scope of the invention.
This application is a continuation-in-part application of application Ser. No. 11/744,881, filed May 6, 2007, which is a continuation-in-part of application Ser. No. 11/255,604, filed Oct. 20, 2005. The application also claims the benefit of provisional application Ser. No. 60/943,781, filed Jun. 13, 2007.
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Number | Date | Country | |
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Parent | 11744881 | May 2007 | US |
Child | 12116039 | US | |
Parent | 11255604 | Oct 2005 | US |
Child | 11744881 | US |