Claims:WE CLAIM:

1. A composition of steel for producing ultra high strength steel with improved resistance to hydrogen embrittlement for hot stamping comprising essentially both Boron and Niobium.

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

3. The steel produced from the steel composition as claimed in claims 1 and 2, is high strength with hydrogen embrittlement for hot stamping.

4. The steel as claimed in claim 3, in unquenched condition has a yield strength in the range of 500 Mpa to 550 MPa, ultimate tensile strength in the range of 750 Mpa to 900 MPa and elongation in the range of 10 to 20% and in quenched condition has a ultimate tensile strength in the range of 1400-1500 Mpa.

5. The steel as claimed in claims 3 and 4, has the microstructure depicting the refinement of grain size of ferrite 20-45 ?m.

6. The steel as claimed in claims 3 to 5, which has the hydrogen embrittlement index of 0 – 2 for 96 hrs. hydrogen charging conditions.

7. A process for producing ultra – high strength steel with improved resistance to hydrogen embrittlement for hot stamping comprising/including in the composition essentially both Boron and Niobium.

8. A process as claimed in claim 7, wherein the composition comprising C ( 0.20 to 0.30 %), Mn ( 1.20 to 1.30 %), B ( 30 – 40 ppm), Cr ( 0.10 – 0.20 %), Al (0.02 – 0.04 %), S ( upto 0.01 %), P ( upto 0.01 %), Si ( 0.25 to 0.45 %), Ti ( 0.02 to 0.05 % ), Nb ( 0.01 to 0.05 %) and the balance being Fe.
9. Structural steel components used in the automotive industry made from the steel and the steel composition as claimed in claims 1 to 6.

10. A process for producing structural steel components used in the automotive industry made from the steel and the steel composition as claimed in claims 1 to 6.

, Description:STEEL WITH LOW HYDROGEN EMBRITTLEMENT BEHAVIOUR, A PROCESS THEREOF AND AUTOMOTIVE PARTS PRODUCED THEREFROM

FIELD OF THE INVENTION

This invention relates to steel composition for producing high strength steel with increase in resistance to hydrogen embrittlement behaviour. The invention particularly relates to steel composition for producing ultrahigh strength steel with improved resistance to hydrogen embrittlementfor hot stamping in which the alloying composition essentially comprising both Boron and Niobium. The invention further relates to ultrahigh strength steel produced from the said composition and the steel when used for crash resistance parts in the automotive industry.

BACKGROUND OF THE INVENTION AND PRIOR ART

Advanced high-strength steels (AHSS) are used in the manufacture of light weight, crash worthy cars. Boron manganese steels are one such steel used for the fabrication of thinner automotive components without sacrificing strength, thereby saving weight, increasing fuel efficiency, and decreasing the emitted CO2. However, the issue of hydrogen embrittlement due to corrosion and welding during service is a major drawback which needs to be addressed without compromising crash resistant properties.

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

1. Steel plate for hot stamping - European patent: WO2018134872:
This steel plate for hot stamping has a steel structure in which the total area fraction of bainite, fresh martensite, and tempered martensite is 80% or more and the product of the number density of carbides (carbides/µm2) and the ratio of carbides precipitated within prior austenite grains among the carbides is 0.50 or more.

2. Multiphase ultra-high strength hot rolled steel and the method of manufacture - European patent: US2018127857: A hot rolled, ultra-high strength, complex metallographic structured or multi-phase structured steel that improves formability during stamping or forming process, while possessing one or more of the following properties: excellent castability, rollability and coatability, excellent structural performance, excellent stretch formability, excellent stretch flangeability, excellent dent resistance, excellent durability, excellent impact performance, excellent intrusion and crash resistance without the purposeful addition of boron.

3. Hot stamped steel - United states Patent: 9,976,196; Hot-stamped steel includes: a steel base metal including a tempered portion having hardness corresponding to 85% or less of the highest quenching hardness defined as a Vickers hardness at a depth position spaced away from a surface layer by 1/4 times a sheet thickness in a case of performing water quenching after heating at a temperature equal to or higher than an A.sub.c3 point and retention for 30 minutes; and a Zn coating layer formed on the tempered portion of the base metal. The Zn coating layer includes a solid-solution layer including a solid-solution phase containing Fe and Zn that is solid-soluted in Fe, and a lamella layer including solid-solution phase and a capital gamma phase. In the Zn coating layer, an area ratio of the lamella layer is 30 to 100% and an area ratio of the solid-solution layer is 0 to 70%.

4. 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 per mm.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.
5. Galvanized steel sheet for hot stamping and having excellent impact characteristics, and method for manufacturing steel product having different strengths using same – WO2014092376A1: According to the invention, a galvanized steel sheet for hot stamping comprises by weight%: C: 0.05-0.17%; Si: 0.01-0.55%; Mn: 1.0-2.3%; P: 0.04% or less; S: 0.015% or less; Cr: 0.01-0.38%; Mo: 0.001-0.25%; Ti: 0.03-0.1%; Nb: 0.004-0.1%, and the remainder of Fe and inevitable impurities. A plated layer including zinc is formed on the surface of the galvanized steel sheet, and the galvanized steel sheet after hot stamping exhibits a tensile strength of 700-1200 MPa and tensile modulus of at least 12.0%.

6. 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 sheets to the top and bottom sheets so as to form a blank, and stamping the blank into the frame portion.

7. Steel plate used for hot stamping forming, forming process of hot stamping and hot-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% of C, 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 1-100,000s, cooling the stamped component to room temperature. The formed component has yield strength=1200MPa, tensile strength=1600 MPa and total elongation percentage=10%.

8. 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 300C, 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 1230-12700C, the initial rolling temperature of finish rolling is 1060-11000C, the finish rolling temperature is 830-9000C, and the sheet thickness is 3.5 - 4.0mm; cover annealing is carried out at 600-7000C for 3h; temperature is preserved at 7000C for 2h; temperature is preserved at 700-6700C for 3h; temperature is preserved at 6700C for 4h, and temperature is maintained at 670-4000C 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.

9. 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.

10. 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% to 0.35 mass% of C; 0.01 mass% to 1.0 mass% of Si; 0.3 mass% to 2.3 mass% of Mn; 0.01 mass% to 0.5 mass% of Al; limited to 0.03 mass% or less of P ; limited to 0.02 mass% or less of S; limited to 0.1 mass% or less ofN; 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 ¼ thickness position of the steel sheet is less than or equal to 0.8 µm.

11. 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 to 0.01%, 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 invention is to provide for an ultrahigh strength steel resistant to hydrogen embrittlement without compromising crash resistant properties.

The specific object of the invention is to provide for ultrahigh strength steel with improved resistance to hydrogen embrittlement 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 the above object.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:

Fig. 1 illustrates scanning electron micrographs after hot-rolling and air-cooling (a) steel 1 and (b) steel 2 in accordance with the present invention;

Fig. 2 illustrates schematic of hydrogen charging system in accordance with the present invention.

Fig. 3 illustrates Hydrogen embrittlement index of steel 1 and 2 in accordance with the present invention.

Fig. 4 illustrates SEM micrograph shows fracture surfaces after hydrogen charging test at 96 hrs and macroscopic fracture appearance indicating the area of brittle and ductile fracture (a) steel 1 (b) steel 2in accordance with the present invention.

DESCRIPTION OF THE INVENTION

Conventional alloy of 22MnB5 is used in hot stamped crashworthy components as they offer very high strength after hot stamping process. However, 22MnB5 steel exhibits hydrogen embrittlement behaviour, which directly influences the crashworthiness of hot stamped components. The present invention was aimed to reduce the hydrogen embrittlement of boron steel by addition of niobium with boron. A considerable increase in resistance to hydrogen embrittlement behaviour was observed with the addition of niobium due to micro structural refinement.

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

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

In unquenched condition the invented steel has a yield strength in the range of 500 Mpa to 550 MPa, ultimate tensile strength in the range of 750 MPa to 900 MPa and elongation in the range of 10 to 20% and in quenched condition has a ultimate tensile strength in the range of 1400-1500 MPa.

In the invented steel the microstructure depicts the refinement of grains. The average grain size of ferrite was 20-45?m.

According to another embodiment of the invention the hydrogen embrittlement index was 0 – 2 for 96 hrs hydrogen charging conditions.

Two laboratory heats, one alloyed with boron (without niobium) named hereafter as steel 1 and the other containing boron and niobium named hereafter as steel 2, of 25 kg each were made using a high frequency induction melting furnace. The melts were cast into 100 mm × 100 mm square ingots, which were subsequently hot-rolled to plates and sheets in an experimental rolling mill. Ingots were hot-rolled in two stages. In the first stage, the ingots were hot-rolled to 16 mm plates, while in the second stage, the 16 mm plates were hot-rolled to 2 mm sheets after soaking both at 1100oC for 120 and 60 minutes, respectively. The finishing temperature after hot-rolling was in the range of 800 to 8500C.The chemical composition of laboratory heats was determined as shown in Table 1.

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

Steel C Mn Si B Cr Ti Nb S P
Steel 1 0.20 –
0.30% 1.20-
1.30 0.25-
0.45 30-
40 ppm 0.10-
0.20
0.02-
0.05 - 0.01
max 0.01
max
Steel 2 0.20 –
0.30% 1.20-
1.30 0.25-
0.45 30-
40 ppm 0.10-
0.20
0.02-
0.05 0.01-
0.05 0.01
max 0.01
max

For scanning electron microscopy, specimens of steels 1 and 2 were cut from 2 mm sheets and ground successively from 100 to 1200 grit water proof silicon carbide paper and finally polished by using alumina suspension of particle sizes 1.0 ?m and 0.3 ?m. A scanning electron microscope was used for micro structural examination after etching the specimens in 2% Nital solution (98% alcohol + 2% nitric acid). The effect of difference in elemental compositions can be clearly noticed from their microstructure shown in Fig. 1. Figure 1a and 1b showed the initial light micrographs of steels 1 and 2 respectively. The observed microstructure of steel 1 with ferrite and coarse pearlite whereas the composition of steel 2, enabled formation of ferrite and fine pearlite after cooling.

Tensile test specimens were prepared as per ASTM. A 370 standard with gauge length of 50 mm from 2 mm hot-rolled sheets of steels 1 and 2. Tensile tests were performed as per the above standard with cross head speed of 2 mm/minute, which was equivalent to strain rate 6.66 × 10-4 /s for 50 mm gauge length. The tensile and hardness values are reported in Table 2.

Table 2. Mechanical properties of as-rolled steels and quenched steels

Steel Condition Hardness
(VHN) Yield
Strength
(MPa) Ultimate
Tensile
Strength
(MPa) Ultimate Tensile Strength(WQ)
(MPa) %
Elongation
Steel 1 As-rolled
&Air-cooled 150-200 400-500 600-700 1000-1100 20-25
Steel 2 As-rolled &
Air-cooled 200-270 500-600 800-900 1400-1500 20-25

The hydrogen susceptibility was studied for steels 1 and 2 with the electrochemical cathodic hydrogen charging at 0.1 NaOH solutions. The specimens were 24, 48, 72 and 96 hrs in corrosion measurement system. The schematic set up of hydrogen charging set up is shown in Fig.2. The hydrogen embrittlement behaviour after the charging is shown in Fig 3. The test results revealed that Nb micro alloying significantly reduced the hydrogen diffusivity in press hardened steel after charging under same conditions.

Analysis of the fracture surface reveals a ductile dimple-type pattern for all the steels when there is no charging. Increasing charging time resulted in brittle fracture appearance of steel 1. On the contrary, the fracture surface of steel 2 still showed ductile appearance after charging at 0.3 A/m2 and 24 hrs. SEM micrographs shows fracture surfaces after hydrogen charging test at 96 hrs and macroscopic fracture appearance indicating the area of brittle and ductile fracture of steel 1 and steel 2 are shown in Fig 4.

Hydrogen generally segregates around grain boundaries of prior austenite and gets a trapped. Hence, by refining the grain a larger total grain boundary area will be created. These grain boundaries can act as hydrogen trapping site.

From the above results it can be concluded that grain refinement is one possible mechanism responsible for reduced hydrogen embrittlement of Nb micro alloyed hot stamped steel. Nb refined the microstructure which is related to the retardation of recrystallisation by solute Nb. Nb being in solid solution at the end of hot rolling also has the effect of retarding the austenite – to – ferrite phase transformation. This effect resulted in a fine- grained microstructure. Grain refinement created a larger total grain boundary area which acted as hydrogen trapping site in the invented steel.

The inventive aspects of the invention is broadly highlighted in –
I) Refinement of microstructure due to addition of niobium.
II) Decrease in hydrogen embrittlement index due to refinement of grain size.

From the above disclosure of the invention it is apparent that several other embodiments to those described which can be carried out by a person skilled in the art and the same are be included within the ambit of the present

The invention includes structural components and the process for producing thereof used in automobile industry made of ultrahigh strength steel resistant to hydrogen embrittlement whenever produced from the invented steel composition.