Claims:WE CLAIM:

1. A composition for producing ultrahigh strength steel with improved hardenability for hot stamping comprising essentially both Boron and Molybdenum.

2. The composition as claimed in claim 1, comprising C (0.20 -0.30%), Mn (1.20 -1.30%), B (30-40ppm), Cr (0.10-0.20%), Al (0.02-0.04%), S (up to 0.01%), P (up to 0.01%), Si(0.25-0.45%), Ti(0.02- 0.05%), Mo (0.05-0.15%) and balance being Fe.

3. The composition as claimed in claims 1 and 2, in unquenched condition has a yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 5 to 12% and in quenched condition has a yield strength in the range of 1000 MPa to 1100 MPa, ultimate tensile strength in the range of 1600 MPa to 1700 MPa and elongation in the range of 5 to 8%.

4. The composition as claimed in claims 1 to 3, wherein prior strain in the range of 20-60% in the claimed steel would stand to decrease volume fraction of martensite.

5. The composition as claimed in claims 1 to 4, wherein critical cooling rate of boron steel would stand to be reduced from 600C/s to 20-400C/s.

6. The composition as claimed in claims 1 to 5, wherein Molybdenum in the steel in presence of Boron stands to control the microstructure of the steel by suppressing the formation of pearlite and delaying the formation of proeutectoid ferrite composition and promoting fully lath martensitic microstructure.

7. The composition as claimed in claims 1 to 6, when used for manufacturing machineries requiring ultrahigh strength steel particularly in the automobile industry.

8. A process for producing ultrahigh strength steel with improved hardenability for hot stamping characterised in that the alloying composition essentially comprising both Boron and Molybdenum.

9. A process as claimed in claim 8 wherein the alloying composition comprising C (0.20-0.30%), Mn (1.20-1.30%), B (30-40ppm), Cr ( 0.10-0.20%) ,Al ( 0.02-0.04%), S (up to 0.01%), P (up to 0.01%), Si(0.25-0.45%), Ti (0.02-0.0.05%), Mo (0.05-0.15%) and balance being Fe.

10. Ultra high strength steel comprising Boron and Molybdenum produced by the process as claimed in any of the claims 8 & 9.

11. Structural components including from automotive industry made of ultrahigh strength steel whenever produced from the steel composition as claimed in any of the preceding claims.

Dated: this 26th day of November, 2016.

(N. K. Gupta)
Patent Agent
Of NICHE
For SAIL

To,
The Controller of Patents,
The Patent Office, Kolkata.

, Description:COMPOSITION FOR PRODUCING ULTRA HIGH STRENGTH STEEL CONTAINING BORON AND MOLYBDENUM , A PROCESS FOR PRODUCING SAID STEEL AND USE THEREOF

FIELD OF THE INVENTION

This invention relates to steel composition for producing high strength steel with improved hardenability. The invention particularly relates to steel composition for producing ultrahigh strength steel with improved hardenability for hot stamping in which the alloying composition essentially comprising both Boron and Molybdenum. The invention further relates to ultrahigh strength steel produced from the said composition and the steel when used for manufacturing structural components requiring such steel particularly in the automotive industry.

BACKGROUND OF THE INVENTION AND PRIOR ART

With the new safety norms and rise in fuel prices there is a need to manufacture automobile structural components from ultrahigh strength steels. Boron steels with extremely high strength (Y.S=1200 MPa) are the most suitable steels for the automotive industries, which demand cost effectiveness without compromising the mechanical properties. Even though it is well established that optimum combination of hardenability and toughness is obtained by the addition of 15-25 ppm of Boron there is enough room for further improving the real effectiveness of boron in the steel with other alloying element.

There have been several references of reported published patent literature in the steel industry. A few such relevant references are mentioned here.

1. Press-formed product and method for producing same – United States Patent9,315,876:There is provided a useful method for producing a press-formed product without causing disadvantages such as hardness variation, which product has favorable formability in a level so as to be able to be produced by deep drawing, and which method is carried out by heating a thin steel sheet to a temperature not lower than an Ac.sub.3 transformation point thereof; and then cooling the thin steel sheet at a rate not lower than a critical cooling rate, during which the thin steel sheet is formed into the press-formed product, wherein the forming is started from a temperature higher than a martensitic transformation start temperature Ms thereof, the cooling rate is kept to be 10.degree.C./sec. or higher during the forming, and the forming is finished in a temperature range not higher than the martensitic transformation start temperature Ms.

2. Method for producing a hot-formed and press-hardened metal component – United
States Patent 9,340,233: The present invention relates to a method for producing a hot-formed and press-hardened metal component for an automobile having at least two regions of different hardness. A hardenable sheet-metal blank is heated to at least an austenizing temperature and a first region of the sheet-metal blank is intermediately cooled at a cooling speed greater than the lower critical cooling speed of the material of the sheet-metal blank. The sheet-metal blank is then hot-formed and press-hardened in a press-hardening tool by quenching the first region from a bainitic structure transformation stage, thereby adjusting a mixed structure of martensite and bainite in the first region.

3. High strength steel sheet excellent in delayed fracture resistance and low temperature toughness, and high strength member manufactured using the same – United States Patent 9,353,424:A high strength steel sheet containing chemical components of, in mass %, C: 0.20 to 0.42%,Si: 0.06 to 0.5%, Mn: 0.2 to 2.2%, Cr: 0.1 to 2.5%, B: 0.0005 to 0.01%, O: 0.0020 to 0.020%,Al: 0.001 to 0.03%, Ti: 0.001 to 0.05%, N: 0.1% or less, P: 0.03% or less, S: 0.02% or less, and the balance: Fe and inevitable impurities. In steel, 5.times.10.sup.3 pieces per mm.sup.2 or more to 1.times.10.sup. 5 pieces per mm.sup.2 or less of Mn oxides having a maximum length of 1 .mu.m or more to 5 .mu.m or less are present, and 1.7.times.10.sup.2 pieces permm.sup.2 or more to 5.times.10.sup.3 pieces per mm.sup.2 or less of Mn--Si composite oxides having a short-axial length of 1 .mu.m or more and a longitudinal length of 10 .mu.m or less are present.

4. Method for hot press formed steel member – United States Patent 9,359,663:
Provided is a method for manufacturing a hot press formed steel member that has high strengths, an excellent balance between strength and ductility, and good deformation properties upon crush on collision (crashworthiness). The manufacturing method is highly efficient and allows a high degree of freedom with respect to the shape to be formed. The method manufactures a steel member by heating a steel sheet having a specific chemical composition and subjecting the steel sheet to at least one time of hot press forming. In the method, the heating temperature is equal to or higher than the Ac.sub.3 transformation temperature, and a starting temperature of the hot press forming is in the range from the heating temperature to martensite start (Ms) temperature. Cooling from [(Ms temperature)-150.degree. C.] down to 80.degree. C. is performed so that a tempering parameter (lamda) specified by Expression (1) is in the range from 7100 to 8030.

5. Lightweight steel door for vehicle and method for manufacturing the same – United
States Patent 9,308,577:A vehicle door assembly comprises an inner panel integrally formed by upper and lower horizontal beams interconnected by front and rear upright beams, and an outer panel bonded to the inner panel. The frame portion comprises opposite front and rear U-shaped side frame members non-detachably attached to each other by upper and lower intermediate frame members so as to form a continuous perimeter. The side frame members are made of a first steel material and the intermediate frame members made of a second steel material, which is different from the first steel material. A method for manufacturing the inner panel comprises the steps: providing U-shaped sheets of the first steel material, providing top and bottom sheets of the second steel material, non-detachably connecting the U-shaped shaped sheets to the top and bottom sheets so as to form a blank, and stamping the blank into the frame portion.

6. Steel plate used for hot stamping forming, forming process of hot stamping andhot-stamped component – Patent No. WO2016131218 (A1):A steel plate used for stamping forming, a forming process of hot stamping and a hot-stamped component. The steel plate used for hot stamping forming comprises 0.18-0.42% ofC, 4-8.5% of Mn and 0.8-3.0% of Si+Al, with balance of Fe and unavoidable impurities, wherein the alloy component of the steel plate meets the requirement that the actual measuring value of martensitic transformation starting temperature after hot stamping forming is no more than 280 °C. The manufacturing process for the hot-stamped component comprises: stamping forming after heating the material to 700-850 °C; then cooling to the temperature which is 150-260 °C lower than the martensitic transformation starting temperature; after tempering heat treatment of heating the hot stamped component to 160-450 °C and maintaining for 1-100,000 s, cooling the tamped component to room temperature. The formed component has yield strength≥1200MPa,tensile strength≥1600 MPa and total elongation percentage≥10%.

7. High strength and easy stamping forming cold-rolled steel sheet and production method thereof – Patent No. CN105714197 (A): The invention provides a high strength and easy stamping forming cold-rolled steel sheet and a production method thereof. The chemical composition wt% consists of: 0.14-0.18% of C, less than or equal to 0.030% of Si, 1.05-1.30% of Mn, less than or equal to 0.020% of P, less than or equal to 0.020% of S, 0.010-0.050% of Al, 0.0003-0.0008% of B, and the balance Fe and inevitable impurities. Specifically, during continuous casting: the target molten steel superheat temperature is controlled at less than or equal to 30DEG C, the casting speed of a casting blank is less than or equal to 1.3m/min, the casting speed is maintained stable, and the casting blank is subjected to hot delivery and hot charging; during hot continuous rolling: the heating section temperature of the heating furnace is 1230-1270DEG C, the initial rolling temperature of finish rolling is 1060-1100DEG C, the finish rolling temperature is 830-900DEG C, and the sheet thickness is 3.5-4.0mm; cover annealing is carried out at 600-700DEG C for 3h; temperature is preserved at 700DEG C for 2h; temperature is preserved at700-670DEG C for 3h; temperature is preserved at 670DEG C for 4h, and temperature is maintained at 670-400DEG C for 4h. The steel sheet involved in the invention has the characteristics of low production cost, wide applicability, a tensile strength of 400-550 MPa, an elongation of more than or equal to 23% and hardness of less than or equal to 75HRB, and has good easy stamping and forming performance.

8. Steel sheets for hot stamping, use, method of production of same, and method of production of high-strength parts – European Patent No. EP2684972 (A1):
The present invention has as its object the provision of steel sheet for hot stamping use which is excellent in part strength after hot stamping and delayed fracture resistance comprised of large C content high strength steel sheet in which effective hydrogen traps are formed in the steel material. The steel sheet of the present invention solves this problem by forming Fe-Mn-based composite oxides in the steel sheet and trapping hydrogen at the interfaces of the composite oxides and matrix steel and in the voids around the composite oxides. Specifically, it provides steel sheet for hot stamping use which is comprised of chemical ingredients which contain, by mass%, C: 0.05 to 0.40%, Si: 0.02% or less, Mn: 0.1 to 3%, S: 0.02% or less, P:0.03% or less, Al: 0.005% or less, Ti: 0.01% or less, N: 0.01% or less, one or both of Cr and Mo in a total of 0.005 to 1%, and O: 0.003 to 0.03% and which have a balance of Fe and unavoidable impurities and which contains average diameter 0.1 to 15 µm Fe-Mn-based composite oxide particles dispersed in the steel sheet or furthermore has crushed voids around the composite oxide particles, a method of production of the same, and a method of production of a hot stamped high strength part.

9. Sheet for hot stamping member and method of producing same – European Patent
No. EP2703511 (A1):A steel sheet for a hot stamping member contains, as a chemical composition, 0.10 mass% to0.35 mass% of C; 0.01 mass% to 1.0 mass% of Si; 0.3 mass% to 2.3 mass% of Mn; 0.01mass% to 0.5 mass% of Al; limited to 0.03 mass% or less ofP; limited to 0.02 mass% or less of S; limited to 0.1 mass% or less of N; and a balance consisting of Fe and unavoidable impurities, in which a standard deviation of diameters of iron carbides which are contained in a region from a surface to a 1/4 thickness position of the steel sheet is less than or equal to 0.8 µm.

10. Steel sheet for hot stamping, and hot stamping formed part using the same – Japanese Patent No. JP2016108644 (A): PROBLEM TO BE SOLVED: To provide a steel sheet for hot stamping that can, while Effectively securing the hardenability improving effect of the B addition even without the Addition of Ti, improve the bendability after the processing.
SOLUTION: The inventive steel sheet for hot stamping contains C: 0.1 to 0.4%, Si: 0% or more and 2.0% or less, Mn: 0.5 to 3.0%, P: 0.015% or less, S: 0.01% or less, B: 0.0003 to0.01%, N: 0.05% or less, and Al in relation to Si content, in which Ti, Zr, Hf and Ta among the unavoidable impurities are each suppressed to 0.005% or less, as well as the nitride-based inclusions having circle equivalent diameter of 1 μm or more satisfy less than 0.10 per 1 mm.

OBJECT OF THE INVENTION

The principal object of the present invention is to provide for an ultrahigh strength steel with far improved properties.

The specific object of the invention is to provide for an ultrahigh strength steel with far improved properties with particular reference to its use in producing structural steel components in the industries where such high quality steels are needed, particularly in the automotive industry. The present invention disclosed herein satisfies the above objectives.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:

Fig. 1 illustrates Optical photomicrograph of Mo-B steel in accordance with the present invention;

Fig. 2 illustrates Schematic illustration of dilatometry tests schedule in accordance with the present invention.

Fig. 3 illustrates Dilatometric curves representing the phase transformation (a) 10C/s (b) 100C/s, (c) 200C/s, (d) 300C/s for Mo-B steel.(Fs-ferrite start; Ff-Ferrite finish; Bs-Bainite start; Bf-bainite finish; Ms-martensite start; Mf-martensite finish) in accordance with the present invention.

Fig. 4 illustrates Optical microstructure Mo-B steel (a) 1 0C/s (b) 10 0C/s, (c) 200C/s & (d) 300C/s in accordance with the present invention.

Fig. 5 illustrates TEM micrographs for Mo-B steel at cooling rates (a) presence of carbides inside lath indicates lower bainite presence of carbides inside lath indicates lower bainite at a cooling rate 200C/s (b) presence of martensitic needles at cooling rate of 300C/s in accordance with the present invention.

Fig. 6 illustrates Schematic illustration of hot compression test (Isothermal forming)in accordance with the present invention.

Fig. 7 illustrates Schematic illustration of hot compression test schedule (Simultaneous forming)in accordance with the present invention.

Fig. 8 illustrates Optical microstructure of Mo-B steel (a) undeformed and (b) deformed (50%) during isothermal treatment at 8000C followed by cooling at the rate of 30 0C/s in accordance with the present invention.

Fig. 9 illustrates Bright Field image of TEM micrograph showing strain-induced martensite (ε), Bainitic ferrite(αB) for Mo-B steel deformed (50%) during isothermal treatment at 8000C followed by cooling at the rate of 300C/s in accordance with the present invention.

DESCRIPTION OF THE INVENTION

It has surprisingly been found that relative effectiveness of different properties of boron steel is vastly improved when Molybdenum is additionally added as an alloying element in the composition. In the present invention the synergistic effect of boron with alloying element Molybdenum on hardenability of steel and the effect of hot sheet metal forming process parameters on properties of the developed quenchable boron alloyed steels has been clearly established.

According to the invention there is provided a steel composition for producing ultra high strength steel with improved hardenability for hot stamping comprising essentially both Boron and Molybdenum.

In one embodiment, the invented steel composition comprising C (0.20-0.30%),
Mn (1.20-1.30%), B (30-40 ppm), Cr (0.10-0.20%), Al (0.02-0.04%), S (upto 0.01%),
P (up to 0.01%), Si (0.25-0.45%), Ti (0.02- 0.05%),Mo (0.05-0.15%) and balance being Fe.

The steel composition of the invention in unquenched condition has a yield strength in the range of 500 MPa to 550 MPa, ultimate tensile strength in the range of 600 MPa to 650 MPa and elongation in the range of 5 to 12% and in quenched condition has a yield strength in the range of 1000 MPa to 1100 MPa, ultimate tensile strength in the range of 1600 MPa to 1700 MPa and elongation in the range of 5 to 8%.

In the invented steel composition prior strain in the range of 20-60% in the claimed steel would stand to decrease volume fraction of martensite.

In the invented steel composition critical cooling rate of boron steel would stand to be reduced from 600C/s to 20-400C/s. Molybdenum in the steel in presence of Boron stands to control the microstructure of the steel by suppressing the formation of pearlite and delaying the formation of proeutectoid ferrite composition and promoting fully lath martensitic microstructure.

According to another embodiment of the invention there is provided a process for producing steel with improved hardenability for hot stamping characterised in that the alloying composition essentially comprising both Boron and Molybdenum. A specific embodiment of the alloying composition of elements in the C (0.20-0.30%), Mn ( 1.20-1.30%), B (30-40ppm), Cr ( 0.10-0.20%), Al ( 0.02-0.04%), S (up to 0.01%), P( up to 0.01%), Si (0.25-0.45%),Ti ( 0.02-0.0.05%), Mo ( 0.05-0.15%) and balance being Fe.
The invention also includes ultra high strength steel comprising Boron and Molybdenum produced by the above process.

The invention further includes structural components including from automobile industry made of ultrahigh strength steel whenever produced from the invented steel composition.

A laboratory heat of 25 kg, alloyed with both boron and molybdenum was made using a high frequency induction melting furnace. It was cast into 100mm × 100 mm square ingots, which were subsequently hot rolled to plates and sheets in experimental rolling mill. Ingot was hot rolled in two stages. In the first stage, the ingot was hot rolled to 16 mm plates, while in the second stage, the 16mm plates was hot rolled to 2 mm sheets after soaking at 1100deg.C for 120 and 60 minutes respectively. The finishing temperature after hot rolling was in the range of 800 to 850 deg. C. The chemical composition of laboratory heat was determined and results are shown in Table 1.

Table 1: Chemical composition of experimental steels (in wt.%)

STEEL C Mn Si B Cr Ti Al Mo S P
Mo-B 0.22 1.28 0.39 0.0035 0.18 0.05 0.05 0.09 0.018 0.013

Fig. 1 of the accompanying drawings shows the light micrograph of Mo-B steel. The observed microstructure shows the presence of upper bainite and acicular ferrite after air cooling of the hot rolled sheets of 2mm thickness. Mo suppresses the formation of pearlite and delays the formation of proeutectoid ferrite.

Table 2: Mechanical properties

STEELS YIELD STRENGTH( MPa) ULTIMATE TENSILE STRENGTH ( MPa) % ELONGATION
Mo-B( As rolled) 497 631 11
Mo-B (Quenched) 1081 1656 6

Table 2 shows the tensile properties, e.g., yield strength, ultimate tensile strength and total percentage elongation at failure (EL) obtained from the stress-strain curves of Mo-B steel. The steel was found to conform to advance high strength steel in terms of yield strength and tensile properties (Table-2). Dilatometer experiments were performed in Gleeble-3500C using 85mm long cylindrical sample with diameter 10 mm to derive the phase transformation characteristic and critical cooling rates. The steel sample was first soaked at AC3 temperatures (900deg.C) in the simulator for 120s, presuming that this will be adequate for chemical and structural homogenization. Subsequently, the sample was cooled to room temperature by employing cooling rates ranging from 1 to 30deg.C/s. The schematic sketch of the schedule is shown in Fig. 2 of the accompanying drawings.

Fig. 3 of the drawings shows the dilatometric curves and corresponding microstructures of Mo-B steel for cooling rates ranging from 1deg.C/s to 30deg.C/s. The phase transformation temperature was identified by considering the deviation of the curve from expected thermal expansion. The cooling part of the dilatometric curves for Mo-B steel in Fig. 3a of the drawings showstransformations at 614deg.C for sample cooled at slower rates of 1°C/s and microstructure of the same dilated sample in Fig. 4 (a) of the drawings confirmed the presence of ferrite and pearlite phase. Fig. 3b of the drawings shows change in slope at 390deg.C for sample cooled at the rate of 10°C/s and corresponding microstructure in Fig. 4(b) of the drawings confirm incidence of bainite. Fig. 3c of the drawings shows change in slope at 372deg.C for sample cooled at the rate of 20°C/s and corresponding microstructure in Fig. 4(c) shows the presence of bainite. Fig. 3d of the drawings shows change in slope at 350 deg.C and the corresponding microstructure revealed fully martensitic structure for cooling rate 30deg.C/s as shown in Fig. 4 (d) of the drawings. Therefore, dilatometry tests revealed Mo-B steel had fully lath martensitic structure desirable for hot stamped products at a cooling rate of 30deg.C/s. Fig. 5 (a) and (b) of the drawings show TEM micrograph at cooling rates (a) 20deg.C/s (b) 30deg.C/s for Mo-Bsteel. Presence of lower bainite for cooling rate at 20deg.C/s and martensitic needles shown in Fig. 5(a) and (b) of the drawings for cooling rate at 30deg.C/s was clearly evident for Mo-B steel.

In order to study the influence of deformation parameter, especially prior strain on transformation behaviour Mo-B steel, hot compression tests of both isothermal and simultaneous forming were performed using Gleeble-3500C. The schematics of the both the processes are provided in Fig. 6 and 7 of the drawings, respectively. For the isothermal forming, specimens were heated at a rate of 10deg.C/s, soaked at950deg.C for 5 min and then cooled to the compression temperature (800deg.C) at cooling rate of 50deg.C/s.

The specimen was then isothermally deformed up to 50% with a strain rate of 10/s followed by quenching at a cooling rate of 30deg.C/s to ambient temperature. In the case of simultaneous forming, the specimens were heated at a rate of 10deg.C/s, soaked at 950deg.C for 5 min and cooled to initial deformation temperature (800deg.C) at a cooling rate of 50deg.C/s. Subsequently, specimens were simultaneously quenched up to 600deg.C and deformed up to 50 % with a strain rate of 10/s. Finally, specimens were quenched at a cooling rate of 30degC/s to ambient temperature. Optical microstructure at undeformed conditions of Mo-B steel is shown in Fig. 8(a). On the other hand, Fig. 8(b) of the drawings shows the optical images of these steels after isothermally deformed up to 50% at800deg.C followed by fast cooling at the rate of 30deg./s. On comparing these micrographs in both conditions it is inferred that the deformation conditions had promoted bainite formation along with refined martensitic needles for Mo-B steels. Formation of bainite along with martensite in Mo-B steel is due to large number of point and line defects produced when it is heavily deformed. These defects impede martensitic transformation but in isothermal forming most of the defects get recovered by dyanamic recovery. The defects which has not recovered during dyanamic recovery has acted as source of nucleation for bainite. Bright field image of TEM in Mo-B steel confirmed the presence bainite and strain induced martensite in Fig. 9 of the drawings. Mo addition of (0.05 to 0.15%) suppressed the C diffusion towards the gamma –grain boundary by participation of Mo carbonitride. The formation of Fe23(C B)6 is thereby suppressed and gamma to alpha transformation is retarded.

From the above results it can be concluded that Mo-B synergistically improved quenchability of steel even in the lower cooling rate to ensure the good and stable product properties of hot-stamped sheet.

From the above disclosure of the invention it is apparent that several embodiments of the invention are possible which can be carried out by a person skilled in the art and the same are included within the ambit of the invention claimed herein.