[0001]FIELD OF INVENTION
[0002]The present invention relates to a hot rolled ultra-high strength steel and method of producing thereof. Particularly, the invention relates to hot rolled ultra-high strength steel adaptable to automotive structural applications, defence equipment applications, lifting and excavation equipment applications.
[0003]BACKGROUND
[0004]Motor vehicle fuel consumption and resultant emission is one of the major contributors to air pollution. Light-weight environmental friendly vehicle design is required to address the problems of environmental pollution. Successful light-weight motor vehicles require utilization of advanced high strength and ultra-high strength steel (UHSS) sheets. However, because of its poor formability, the UHSS sheet cannot be applied easily to a wide variety of motor vehicle components. Hence, the ductility and formability required for UHSS sheet becomes increasingly demanding. Therefore addressing the present scenario has necessitated development of a hot rolled steel sheet with high tensile strength coupled with excellent uniform elongation and total elongation for automotive component such as lower suspension, long and cross member and bumpers as well.
[0005]Such steels have been produced by many researchers where major part of strengthening was due to the nano-structured bainitic ferrite sheaves—famously known as ‘nano-bainitic steel’ (Bhadeshia, MSE - A , Volume 481-482, pp. 36-39, 2008; F. G. Caballero, H. K. D. H. Bhadeshia, K. J. A. Mawella, D. G. Jones and P. Brown, MST , Volume 18, pp. 279-284, 2002; C. Garcia-Mateo, F. G. Caballero and H. K. D. Bhadeshia, ISIJ International , Volume 43, pp. 1238-1243, 2003). Though they have produced highest strength ever achieved in any bulk material, production of the steel sheet takes about a week due to slower kinetics at mandatory low temperature during coiling of the rolled sheet. Such a long duration during coiling for commercial production is not viable. The second concern is the limited total elongation which is about 7% at a strength range of 2260 MPa. This limited elongation does not allow the steel to be used in wider areas of application where formability is an important aspect. Another issue is related with the alloy composition where the amount of Carbon in steel typically lies in the range of 0.8-1.0 wt. % along with Ni and Co. High Carbon decreases the weldability of the steel and high alloying makes steel expensive.
[0006]Another group of researchers (F. G. Caballero, M. J. Santofima, C. Capdevila, C. G. Mateo and C. G. De Andres, ISIJ International , Volume 46, pp. 1479-1488, 2006; F. G. Caballero, M. J. Santofima, C. Garcia Mateo, J. Chao and C. Garcia de Andres, Materials and Design , Volume 30, pp. 2077-2083, 2009) have been working since then dealing with reducing the amount of C for good weldability and increasing the total elongation. However, the work has not been considered for the production of such steels through continuous production line and also the steels contain high amount of expensive alloying additions like Ni and Mo in their production.
[0007]In an effort to meet the demand of present day motor vehicle manufacturers, recent work (Ref. US 2014/0102600 A1) attempted to obtain high strength and ductility combination. This work has successfully achieved minimum 1200 MPa tensile strength with 20% total elongation. However, it has high Carbon (>0.3 wt. %) and Silicon (>1.5 wt. %). High amount of Carbon decreases the weldability and high Silicon causes surface scales during the process of hot rolled steel sheets. These problems are yet to be addressed.
[0008]OBJECTIVES OF THE INVENTION
[0009]In view of the foregoing limitations inherent in the prior-art, it is an object of the invention to develop a process for making a hot rolled high strength steel product whose commercial production is viable.
[0010]Another object of the invention is that the product having good weldability and lesser scale severity over the product.
[0011]Another object of the invention is that the total elongation of the hot rolled high strength steel product ≥16%.
[0012]Still another object of the invention is that the tensile strength of the hot rolled high strength steel product ≥1000 MPa.
[0013]SUMMARY OF THE INVENTION
[0014]In one aspect, the invention provides a process for making a hot rolled high strength steel (HRHSS) product comprising steps of casting a steel slab with composition C: 0.18-0.22, Mn: 1.0-2.0, Si: 0.8-1.2, Cr: 0.8-1.2, S: 0.008 max, P: 0.025 max, A): 0.01-0.15, N: 0.005 max, Nb: 0.02-0.035, Mo: 0.08-0.12 rest iron (Fe) and incidental ingredients (all in wt. percentage), hot rolling the steel slab into strip at finish rolling temperature (FRT) of 850-900° C., cooling the hot rolled strip at 40° C./s or more over run out table (ROT) till it reaches to 380-400° C.; and coiling the hot rolled strip and then air cooling to room temperature.
[0015]In one aspect, the invention provides a hot rolled high strength steel (HRHSS) product comprising composition of C: 0.18-0.22, Mn: 1.0-2.0, Si: 0.8-1.2, Cr: 0.8-1.2, S: 0.008 max, P: 0.025 max, Al: 0.01-0.15, N: 0.005 max, Nb: 0.02-0.035, Mo: 0.08-0.12 rest iron (Fe) and incidental ingredients (all in wt. percentage), tensile strength 1000-1200 MPa and total elongation of 16-17%.
[0016]BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. 1 illustrates various steps of a process for making a newly developed hot rolled high strength steel (HRHSS) product in accordance with an embodiment of the invention. FIG. 2 shows tensile stress—strain plot of the newly developed hot rolled high strength steel (HRHSS) product in accordance with an embodiment of the invention. FIG. 3 shows optical microstructure (Nital etched) of the newly developed HRHSS product in accordance with an embodiment of the invention. FIG. 4 shows an Optical microstructure (Le pera etched) of the newly developed HRHSS product in accordance with an embodiment of the invention: FIGS. 5 a & 5 b show photograph of the newly developed HRHSS product taken at higher magnification using Scanning Electron Microscopy (SEM) at lower magnification: 5000× and at higher magnification: 25000× respectively in accordance with an embodiment of the invention. FIG. 6 shows XRD profile of the newly developed HRHSS product which contains about 20% retained austenite by volume in accordance with an embodiment of the invention. FIG. 7 shows TEM Image of the newly developed HRHSS product showing thin sheaves of bainite in accordance with an embodiment of the invention.
[0017]DETAILED DESCRIPTION OF THE INVENTION
[0018]Various embodiments of the invention provide a process for making a hot rolled high strength steel (HRHSS) product, the process comprising steps of: casting a steel slab with composition C: 0.18-0.22, Mn: 1.0-2.0, Si: 0.8-1.2, Cr: 0.8-1.2, S: 0.008 max, P: 0.025 max, Al: 0.01-0.15, N: 0.005 max, Nb: 0.02-0.035, Mo: 0.08-0.12 rest iron (Fe) and incidental ingredients (all in wt. percentage); hot rolling the steel slab into strip at finish rolling temperature (FRT) of 850-900° C.; cooling the hot rolled strip at 40° C./s or more over run out table (ROT) till it reaches to 380-400° C.; and coiling the hot rolled strip and then air cooling to room temperature.
[0019]Another embodiment of the invention provides a hot rolled high strength steel (HRHSS) product comprising: composition of C: 0.18-0.22, Mn: 1.0-2.0, Si: 0.8-1.2, Cr: 0.8-1.2, S: 0.008 max, P: 0.025 max, Al: 0.01-0.15, N: 0.005 max, Nb: 0.02-0.035, Mo: 0.08-0.12 rest iron (Fe) and incidental ingredients (all in wt. percentage), tensile strength 1000-1200 MPa and total elongation of 16-17%.
[0020]Shown in FIG. 1 are various steps of a process ( 100 ) for making a hot rolled high strength steel (HRHSS) product.
[0021]At step ( 104 ) a steel slab is casted. The composition and preferable composition of the steel slab is shown in Table 1.
[0022]TABLE 1 Composition Preferable Elements (in wt. %) composition Carbon (C) 0.18-0.22 0.22 Manganese (Mn) 1.0-2.0 1.48 Silicon (Si) 0.8-1.2 1.0 Chromium (Cr) 0.8-1.2 0.95 Sulphur (S) 0.008 max <0.004 Phosphorus (P) 0.025 max 0.02 Aluminium (Al) 0.01-0.15 0.14 Nitrogen (N) 0.005 max 0.005 Niobium (Nb) 0.02-0.035 0.035 Molybdenum (Mo) 0.08-0.12 0.1 Iron & incidental ingredients Rest Rest
[0023]C: (0.18-0.22 wt. %) Adequate amount of carbon is necessary to ensure that the desired strength levels are reached. Carbon also increases stability of retained austenite which is essential to achieve enhanced ductility. For ensuring both strength and ductility are maximized, carbon content is kept preferably at 0.22%. Also at this range of Carbon, the weldability of the steel is good.
[0024]Mn: (1.0-2.0 wt. %) Manganese is necessary to stabilize austenite and obtain optimum amount of retained austenite. The amount of Mn needs to be 1.0% or more, preferably 1.3% or more, more preferably 1.48% or more. An excess beyond 2.0% however gives rise to an adverse effects such as a casting crack and hence Mn is preferably controlled to 1.48 wt. %.
[0025]Si: (0.8-1.2 wt. %) Silicon is a ferrite stabilizer. It also restricts carbide precipitation during isothermal holding resulting in a larger amount of retained austenite. However, addition of Si leads to surface scale problems during rolling and therefore should be limited to the range mentioned and more preferably at 1.0 wt. %.
[0026]Al: (0.01-0.15 wt. %) Aluminum is added because, to an even stronger degree than Si, it is a ferrite stabilizer. Al also suppresses the precipitation of carbon from the retained austenite during the bainitic transformation step, which results in a higher amount of retained austenite. Unlike Si, Al has no detrimental effect on galvanisability. Preferably, amount of Al should be maintained at 0.14% as higher amount of Al results in problems during casting. Furthermore weldability can deteriorate due to the presence of Al-oxides in the welded area.
[0027]P: (0.025 wt. % maximum) Phosphorus content should be restricted to 0.025% maximum and preferably at 0.02%.
[0028]S: (0.008 wt. % maximum) The S-content has to be limited otherwise it will result in a very high inclusion level that can deteriorate the formability. Preferably the Sulphur is kept at <0.004 wt. %.
[0029]N: (0.005 wt. % maximum) The N content has to be restricted upto 0.005 wt. % maximum, otherwise too much AlN and/or TiN precipitates can form which are detrimental to formability. Preferably the Nitrogen is kept at 0.005 wt. %.
[0030]Nb: (0.02-0.035 wt. %) Niobium is added in order to increase the strength of the steel by grain refinement. It also plays a role in increasing the amount of austenite retained in the final microstructure. Preferably, the niobium is kept at 0.035 wt. % to avoid an increase in cost or extra processing difficulties (e.g. rolling forces).
[0031]Mo: (0.08-0.12 wt. %) Molybdenum is added to avoid formation of polygonal ferrite and formation of pearlite. Mo also enhances formation of bainite. However, excessive addition of Mo increases the cost of steel processing and hence it is preferably restricted to 0.1 wt. %.
[0032]Cr: (0.8-1.2 wt. %) Chromium, similar to Mo, avoids formation of polygonal ferrite and pearlite. It is an economical alloying element addition in UHSS steels. However, excessive addition of Cr will form complex carbides of Cr, hence it is preferably kept at 0.95 wt. %.
[0033]The steel slab before being hot rolled is soaked at temperature about 1250 Deg. C. Steel is held at this temperature for sufficient time for the formation of homogenous structure and composition throughout its mass. The soaking time depends on the thickness of the work piece and the steel composition. Higher temperatures and longer soaking times are required for larger cross sections.
[0034]At step ( 108 ) the steel slab is hot rolled into strip at finish rolling temperature (FRT) of 850-900° C. The temperature is above ferrite transformation start temperature.
[0035]At step ( 112 ) the hot rolled strip is cooled at 40° C./s or more over run out table (ROT) till it reaches to 380-400°. It is to avoid formation of diffusional phase transformation product like ferrite and pearlite.
[0036]At step ( 116 ) the hot rolled strip is coiled and air cooled at room temperature. This step allows austenite to bainite transformation during the bainite transformation carbon gets rejected to neighboring austenite phase. The enriched austenite becomes stable at room temperature.
[0037]Following are the properties of the HRHSS product obtained:
[0038]Yield stress=600-650 MPa Tensile strength=1000-1200 MPa Total elongation=16-17% with uniform elongation ≥9% Strain hardening exponent (“n”)=0.15-0.16
[0039]The HRHSS product obtained has the bainitic ferrite as the predominant phase and retained austenite as secondary phase. Some amount of unavoidable martensite is also present in the steel. The microstructural characteristics of the hot rolled steel sheet produced according to the present invention are described below.
[0040]Bainitic Ferrite [75-80% by vol.]: The bainitic ferrite present in the microstructure is essentially with carbide or carbide free bainite with high dislocation density. It has lath morphology. The higher dislocation density results in higher strength but at the same time ductility is reduced.
[0041]Retained Austenite [15-20% by vol.]: Retained austenite is the most important constituent of the microstructure of the HRHSS product developed. On deformation, retained austenite transforms to martensite, resulting in a continuously increasing strain hardening exponent which delays the onset of necking and ensures enhanced ductility (the TRIP effect). For effective TRIP, the amount of retained austenite should be at least 10% and preferably 12% or higher. But a very high volume fraction may lead to a degradation of local deformability and hence the retained austenite is maintained less than or equal to 20%.
[0042]Martensite: <5% (including 0% by vol.): The HRHSS product produced may contain some martensite, which may be left present during the manufacturing process ( 100 ). The HRHSS product possesses bainitic sheaves with thickness less than 200 nm. Strength of the steel depends on thickness of bainite sheaves lesser the thickness, higher is the strength.
[0043]EXAMPLES
[0044]The above mentioned process for making HRHSS product can be validated by the following examples. The following examples should not be construed to limit the scope of invention.
[0045]A 25 kg heat was made for processing. Its composition is given in Table 1 (preferable composition). Subsequently, the heat was forged to 25 mm thickness and cooled to room temperature in an open atmosphere. The steel then soaked at 1250° C. for 30 min. before rolling. To ensure the completion of rolling within the austenite range, the finish rolling temperature was kept at finishing rolling temperature of 850° C. During rolling, thickness of the strip was reduced to 4 to 6 mm after two passes. The rolled sheets were then cooled at 40 deg. C. per sec and held in a salt bath maintained at the temperature of 380-400° C. for one hour and then naturally cooled to room temperature to simulate the coiling process.
[0046]After the samples were cooled down to the room temperature, samples were cut for different characterization experiments (microstructural and mechanical). No additional heat treatment or process was carried out after cooling to room temperature.
[0047]The optical (both Nital and Le pera etched) and SEM microstructures are presented in FIGS. 3, 4, 5 a and 5 b which consist of bainitic ferrite, retained austenite and/or martensite. Tensile test samples with 50 mm gauge length were cut according to ASTM E8 standard. Typical tensile test plot is given in FIG. 2 . Mechanical properties of the newly developed steel are given in Table 2.
[0048]TABLE 2 Uni- Strain form Total hard- Sam- Thick- Gauge Tensile elon- elon- ening ple Width, ness, length, YS Strength gation gation coeff. No mm mm mm (MPa) (MPa) (%) (%) (n) 2_1 12.58 5.05 50.44 618 1115 9 16 0.15 2_2 12.62 5.33 49.63 625 1114 10 17 0.16
[0049]It is evident from the figure and table that newly developed steel has minimum 1100 MPa tensile strength, 9% uniform elongation and minimum 16% total elongation. The newly developed steel also has high strain hardening co-efficient i.e., 0.15.
[0050]The volume fraction and the lattice parameter of retained austenite were calculated from the X-ray diffraction (XRD) data by a method described by B. D. Cullity, 1978, D. J. Dyson and B. Holmes, 1970. Samples were cut from tensile test sample (after completing the test) from gauge and grips are for XRD analysis. XRD plot is shown in FIG. 6 . The curves with the peaks 111, 200, 220 and 311 indicate the presence of retained austenite and the same has been quantified. The curve with the peaks 110, 200 and 211 indicate the presence of bainite ferrite.
[0051]Quantitative results are given in Table 3.
[0052]TABLE 3 Grip area of Tensile sample Gauge area of Tensile sample C in C in Sample Volume fraction Austenite., Volume fraction Austenite., No of Austenite(f y ) wt. % of Austenite(f y ) wt. % 2_1 0.20 0.81 0.10 0.70 2_2 0.17 0.66 0.07 0.62
[0053]It can be noticed that retained austenite in the newly developed steel is as high as 20% by volume.
[0054]It's found that the thickness of bainite sheaves is less than 200 nm. High magnification Transmission Electron Microscopy images are shown in FIGS. 7 a and 7 b.
[0055]Advantages
[0056]The production of the HRHSS is commercial viable. The product has good weldability and lesser scale severity. The total elongation of the product obtained is ≥15%. The tensile strength of the product is ≥1000 MPa.
