Patent Application
20240026480
The present disclosure generally relates to a high-strength low-alloy steel having a yield strength of at least 490 MPa. The disclosure includes a process for manufacturing a low carbon Ti-microalloyed of steel for the development of cold-rolled and batch-annealed high-strength high-formable steels. In some embodiments, the process includes a clean steel practice combined with hot rolling and batch annealing utilized to retain precipitate strengthening in the final fully processed steel and microstructural attributes that results in increased forming characteristics.
Development of high strength low alloy steels (HSLA) has redefined steel usage in various industries, for example, construction, machinery, automotive, agriculture and transportation. The HSLA steel provides engineering as well as economic benefits such as, weight reduction, improved and excellent weldability, reduced cost of engineering constructions, safety of components, increased payloads for transportation and enhanced fuel economy for passenger cars. The automotive industry has benefitted through advent of higher strength HSLA steels as this steel has provided for increased vehicular safety, enhanced fuel economy, and minimized CO2 emissions. Automotive manufacturers are now leaning towards use of greener HSLA steels in an effort to minimize CO2 emission. Companies are requesting that steel manufacturers provide carbon-neutral or carbon-minimal HSLA steels and the global steel industries are gearing towards innovating steel processing ideas and technologies that provide sustainable solutions to green steel production. In this evolving perspective, mini steel mills employing continuous strip production (CSP) technology have significant advantage in reducing CO2 emissions over discrete integrated steel mills that runs on blast furnace ironmaking technology. Moreover, CSP mills integrated with advanced steel processing technologies are competing with thick slab casting discrete steel mills not only in production throughput but also in offering advanced high strength grades of steels with lean alloying and least cost of production.
HSLA sheet steels with a yield strength of at least 490 MPa are increasingly demanded for many stamped parts as well as tubular forms of autobody applications. High yield strength HSLA steels have not been widely developed. In the past, the addition of vanadium (V) and/or niobium (Nb) microalloyed HSLA steels was investigated. A solid solution was strengthened as well as microalloy V, Nb strengthened sheet steels and mostly with production through thick slab casters and with heavy alloying. Furthermore, these developments also received less attention because of the emergence of dual-phase (DP) steels offering high tensile strengths and strain hardening capacity. However, DP steels are heavily alloyed and the ferrite yield strength is lower due to pre-yielding when compared with similar-tensile strength HSLA steels.
The present disclosure includes a high strength low alloy steel sheet product produced by a continuous strip process. The sheet product comprises from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, and from 0.075 to 0.12% weight percent Titanium and an Fe balance. Wherein the steel sheet product has been subjected to cold rolling and batch annealing to form a steel sheet product having a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 70%.
The present disclosure includes a method for producing a high strength low alloy steel sheet product comprising from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, and from 0.075 to 0.12% weight percent Titanium and an Fe balance, a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%, comprising the steps of: continuously casting a steel slab approximately 55 mm-85 mm thick; maintaining a temperature of the steel slab; hot rolling the steel slab to a steel sheet at a finishing temperature of 875-950C; cooling the steel slab; coiling the steel sheet at a temperature of 600-675C; cold rolling the steel sheet to 40-75%; and hydrogen batch annealing the steel sheet at temperature of 600-650C to achieve a fully recrystallized ferritic microstructure with nano precipitates of TiC.
The present disclosure includes a high strength low alloy steel sheet product produced by a continuous strip process. The sheet product comprising from 0.045 to 0.06 weight percent Carbon, from 0.75 to 1.2 weight percent Manganese, from 0.02 to 0.04 weight percent Aluminum, from 0.075 to 0.12% weight percent Titanium, 0.04 weight percent Niobium and an Fe balance. Wherein the steel sheet product has been subjected to cold rolling and batch annealing to form a steel sheet product having a yield strength of at least 500 MPa, a substantially ferritic microstructure with nano TiC precipitates and a hole expansion ratio of at least 60%.
Mechanical property requirements for HSLA 490 sheet steels are listed in Table 1 as required by various common sheet steel OEM specifications. Table 1 shows the tensile property requirements of HSLA 490 annealed sheet steels as required by various specifications.
In some cases, a Ti-only alloy approach was utilized because very fine sized TiC can be precipitated in hot rolled coils giving a significant increase in ferrite yield strength. Ti is effective in scavenging nitrogen from steel and tying up with most of the carbon to form nano sized precipitates provided hot rolling parameters are properly controlled. The extent of precipitation strengthening can be significant and is approximated by a σppt=B. (wt. % alloy). In some embodiments, B is an average 1500 MPa/wt. % Ti. Carbon was maintained below 0.06 wt. % as excessive carbon to that required for TiC formation will cause Fe-carbide precipitation during coiling and may deteriorate drawability and stretchability. Solid solution elements such as Mn, Si are significant ferrite strengtheners as outlined in Eq [1], as set forth below. In some embodiments, Mn additions were kept low as an excess of Mn introduces mobile dislocations in ferrite during rapid transformations after hot rolling through its influence on Ar3 temperature and such mobile dislocations interfere with yield strength through pre-yielding. Mn also adversely influences rm value in annealed sheet steel as shown in
Total strengthening will thus be additive of ferrite grain size and precipitation strengthening as given below in Eq. [1] as deduced from a treatise of strengthening mechanisms in ferrite.
σy(MPα)=88+37mn'83Si+2900Nfree+17d−1/2+σppt+σd Eq. [1]
In some embodiments, d is grain diameter in mm, a σppt is the precipitation strengthening, σd is dislocation strengthening.
The present disclosure describes a low carbon, lean Ti-bearing alloy to develop ferritic HSLA 490 sheet steel in cold rolled-batch annealed condition. Strengthening in final sheet steel was obtained primarily through ferrite grain size and TiC precipitation strengthening with moderate solid solution strengthening. Precipitation of TiC nano-precipitates in hot rolled steel was induced through control of the finishing deformation temperature, cooling rate and coiling temperature. A batch annealing cycle appropriate of the current steel was evolved through pre-simulations to determine a full recrystallization annealing temperature. Actual mill production of sheet steels of various thicknesses revealed HSLA 490 steels could be successfully developed with excellent elongation and strain hardening index using a lean Ti-only HSLA approach. Nb addition of up to 0.04wt. % to the chosen alloy resulted in similar properties but didn't enhance tensile or formability properties over those of Ti-steel. TEM studies of microstructure and precipitation revealed fully recrystallized very fine ferritic microstructure with homogenously distributed nano TiC precipitates of 1.5-8 nm in size with an average 3.16 nm. Precipitate size or distribution were not affected by the recrystallization anneal temperature. Hole expansion ratio values of more than 90% were obtained for 1.2 mm thick Ti-HSLA sheet steels. Nano-sized TiC precipitates, fine recrystallized grain size and excellent internal cleanliness contributed to the hole expansion ratio values. Formability as indicated through stretchability indicator, hole expansion ratio far exceeds that reported by similar strength sheet steels.
HSLA steels with yield strength of at least 490 MPa having excellent stretch forming capability will be described herein. In some embodiments, a predominantly ferritic Ti-microalloyed HSLA steel may include a higher yield strength as well as formability through nano-scale precipitation in ferrite. The nano scale precipitates when precipitated within ferrite not only increases strain hardening of ferrite but also resists recrystallization during annealing.
In some embodiments, a very low carbon C—Mn based ferritic steel with Ti-only microalloying was selected to develop a family of HSLA sheet steels with a minimum yield strength of 490 MPa meeting SAE J2340 CR 490X specifications. Batch annealing was utilized in place of a conventional continuous annealing approach. Batch annealing provides results in higher total elongation and higher drawability compared to continuous annealing, batch annealing can be performed at lower temperatures, i.e. just high enough to complete recrystallization of cold deformed structure without disturbing the precipitate state, low temperature annealing prevents precipitation coarsening and grain growth is avoided, batch annealing also retains excellent flatness of thin gauge sheet steels as compared to continuous annealing because of prevention of heat buckling. Batch annealing with optimized cycle may result in better cross width property uniformity compared with continuous annealing.
Strength development is based primarily on precipitation strengthening and control of ferrite grain size. Details of alloy design, hot rolling processing approach and annealing parameter optimization through studies of microstructure and precipitation evolution at each stage of processing are described herein.
Chemistry of the HSLA 490 steel is shown in Table 2. A clone chemistry with small Nb addition (<0.04 wt. %) was also chosen to examine partially recrystallized austenite grains on the final ferrite grain size and properties. Carbon was restricted to less than 0.06 wt. % (i) to minimize grain boundary iron carbide formation and allow only sufficient amounts for micro-alloy carbide formation, (ii) to control Ti:C stoichiometry for nano scale precipitation of TiC precipitates. Table 2 shows the chemistry of steels selected in the current study (wt. % max).
The steel was manufactured utilized an electric arc furnace ensuring least slag carryover and low residuals through suitable choice of scrap mix. Deoxidation practice was aimed at low total dissolved oxygen so that Ti is not lost as TiO2. Calcium treatment of oxide inclusions was optimized to modify alumina as well as sulfide inclusions. Low nitrogen content Fe-alloys were used to result in low nitrogen in the melt. Lower nitrogen helps to control the amount of titanium lost to nitrogen and thus aids in enhancing the Ti:C stoichiometry. The heats were cast at continuous caster with suitable mold powder to result in good surface quality of hot rolled coils. The steels were cast in to slabs of 55-65 mm thickness and fed continuously to 6-stand hot strip processing mill through a 290 meter long tunnel furnace maintained at temperature to eject slabs at exit temperatures of 1120-1130° C. Thin slabs (≤65 mm), fast casting speeds (5-5.5m/min) and low soaking temperature (1125° C.) helped achieving finer starting austenite grains than in a thick slab casting unit.
The slabs were hot rolled to 2.2-3.3 mm thickness using a six-pass reduction schedule and finish hot deformation temperature of 900-925° C. The hot rolled strip was immediately accelerated cooled to a coiling temperature of 600-675° C. A cooling rate of more than 30° C./s was employed using super reinforced laminar cooling to result in non-polygonal ferrite grains with substructures if possible. Hot rolled samples were collected from 600 cm inside of the outer lap of coils after cooling to evaluate mechanical properties and precipitation.
The hot rolled coils were cold rolled to 0.85-1.2 mm thickness (60-65% cold reduction) and then annealed in a batch annealing furnace. The annealing cycle to be used for these cold worked coils were initially determined using laboratory simulations of batch annealing. Strip samples prepared from cold rolled steels of both types were subjected to a programmed batch annealing cycle at various temperatures in a box furnace and tested after furnace cooling for mechanical properties and microstructure.
Samples from batch annealed coils were collected from both head and tail ends for various property evaluations such as hardness, tensile properties and microstructures. Tensile samples across the width were also tested to check cross width property variation.
Full thickness section along rolling direction was examined in optical microscope (Leica DMI5000-M) as well as in scanning electron microscope (Hitachi SU3500) for microstructural features and cleanliness study. Metallographic samples were mechanically polished to 1 μm diamond paste suspension followed by surface treatment in a Hitachi IM4000 ion milling system for orientation imaging mapping. An area of 128 mm×100 mm with a step size of 0.3 μm was chosen for EBSD analysis for orientation imaging and texture evaluation in a Hitachi SU3500 SEM.
Hole expansion ratio tests were done on annealed sheet samples from both steels using an Interlaken SP400 test equipment at AMT-Fadi LLC. For the hole expansion tests, five square coupons of 100 mm×100 mm size were cut from each of three locations across the width-quarter width, center and three-quarter width locations for examining property homogeneity. Holes of 10 mm diameter (do) was punched at center of each coupon for HER tests.
The test coupons were clamped between a holder and die with a clamping force of 100 kN. A clearance of 12±1% of nominal sheet thickness was adopted conforming to ISO 16630:2009(E) specification. A conical punch with 60° angle was pierced through the hole at a speed of 0.25 mm/s and the crack appearance during piercing was monitored using digital imaging system. The piercing was done at least after 30 mins after punching the hole. Diameter of the holes after crack appearance was measured and hole expansion ratio, λ was calculated as
where do and df are initial and final diameter of the hole respectively.
Precipitation in hot rolled samples as well as final fully processed sheet samples were studied in Talos L120C and Talos 200X electron microscope equipped with EDS X-ray spectrometer and electron-energy-loss spectroscopy (EELS). Samples of dimension 10 mm×10 mm were used to prepare TEM foils and replicas using a precision cutter from the center areas of the sample parallel to rolling direction. For foils preparation, samples were thinned by careful mechanical grinding and polishing down to a thickness of around 80 mm. 3 mm diameter discs were punched from thinned sheet followed electro-polishing in electrolyte of 10% perchloric acid in methanol at −40C and 16V. Additionally, extraction replicas were prepared from the same samples for fine precipitation analysis.
Tensile properties of batch annealed sheet steels of both steel types are shown in Table 3. Table 3 shows that both the sheet steels met minimum yield strength of 490 MPa in fully annealed condition. Excellent total elongation values were obtained. Ti-steel sheets showed slightly higher elongation values compared to Ti—Nb steel. Tensile properties were also outstandingly uniform from head to tail of the coils as well as across the width of the coils as indicated in
Cleanliness of the steels were evaluated through inclusion mapping in scanning electron microscope with EDS. Ternary diagram of oxide and sulfide inclusions were plotted and inclusion area fraction of various prominent inclusion types were measured.
Table 4 shows a summary of area fraction of various inclusions present in both steels as evaluated through SEM-EDS microanalysis.
In general, both coarse and fine nano-sized precipitates were seen in the hot rolled and annealed specimens. Most of the coarse particles in hot rolled sample were 10-80 nm in size with an average size of 31.2±1.7 nm and seen in low magnification in thin foil samples in
Chemical compositions of the ppts were analyzed using replica samples using EELS and EDS. EELS was performed by Digital Micrograph program from Gatan/Ametek and EDS was performed by Velox from Thermo-fisher Scientific. A majority of precipitates were less than 20 nm were Ti and Nb containing as shown in
Precipitate analysis in fully processed annealed sheet samples of Ti—Nb steel revealed similar size, distribution of precipitates. Coarse precipitates of 20-50 nm in sizes were Ti and Nb rich. Fine precipitates of 3-11 nm were found distributed homogenously in ferrite. Ti was the main element detected in the fine particles as seen in
These particles were mostly characterized as Ti, S bearing as shown in HAADF image and EDS mapping in
Precipitate analysis in fully processed annealed sheet samples of Ti-steel revealed very fine nano-sized precipitates of 3-11 nm in size distributed homogenously in ferrite. Ti was the main element detected in the fine particles as seen in
Formability Evaluation: Hole Expansion Ratio Results
Table 5 lists the hole expansion ratio (HER) values obtained for both the Ti-steel and Ti—Nb-steel in fully processed condition. HER data from various locations across the width were obtained and an average of five samples from each location are summarized. As indicated from the results, outstanding HER values were obtained for both the steels. The values are very uniform across the width of the coils. The HER values are indicative of excellent edge ductility or stretchability of the steel so developed and is expected to perform well during stamping operations. Table 5 shows the hole expansion ratio testing results performed on annealed HSLA sheet steels.
Both Ti-only and Ti—Nb bearing HSLA steels of the selected lean alloy compositions successfully yielded a yield strength minimum of 490 MPa in cold rolled batch annealed condition. Nb microalloying did influence the austenite grain size after finishing deformation as could be seen from elongated ferrite grain structure (
Both steels represented similar softening behavior with temperature. Optical micrographs as well as transmission electron micrographs of annealed samples did indicate full recrystallization of ferrite grains. (
Grain size distribution as estimated from grain boundary mapping is plotted in
The annealing textures of both Ti-steel and Ti—Nb steel are represented in
Precipitation studies revealed significant nano-sized TiC precipitates (1.5 nm to 8 nm) in both steels strongly contributing towards the yield strength increment. The precipitates that formed after hot rolling remained mostly intact in final batch annealed steels as the size and distribution of fine precipitate did not change during recrystallization annealing. In Ti-steel most precipitates were finer than 10 nm in size and lesser precipitates coarser than 10 nm were found. In contrast, Ti—Nb steel showed relatively coarser (Ti,Nb)C precipitates.
Hole expansion ratio data obtained in the current batch annealed steel indicated superior stretchability or edge ductility suitable for most stamping operations. In general, both sheet steels indicated high HER values because of internal cleanliness, finer ferrite grain size and nano precipitates. Higher HER values obtained for Ti-alloyed HSLA steel was possibly due to absence of grain boundary coarser (Ti,Nb)C precipitates and relatively finer annealed ferrite grain structure than Nb added steel.
In some embodiments, hole expansion ratio values and tensile properties of the steel provides steel that can be utilized for autobody stamping applications requiring high stretch ductility.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.
Any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.
In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances. In this application and the appended claims, the articles “a,” “an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.
As used herein, “including,” “containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases, or method steps. As used herein, “consisting of” is understood in the context of this application to exclude the presence of any unspecified element, material, phase, or method step. As used herein, “consisting essentially of” is understood in the context of this application to include the specified elements, materials, phases, or method steps, where applicable, and to also include any unspecified elements, materials, phases, or method steps that do not materially affect the basic or novel characteristics of the disclosure.
This application claims the benefit of U.S. Provisional Patent Application No. 63/341,811 filed May 13, 2022, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63341811 | May 2022 | US |
