HOT-ROLLED STEEL AND A METHOD OF MANUFACTURING THE SAME.

Abstract

The invention relates to a hot-rolled steel having a composition comprising the following elements, expressed as a percentage by weight: 15% = nickel = 25%, 6% = cobalt = 12%, 2% = molybdenum = 6%, 0.1% = titanium = 1%, 0.0001% = carbon = 0.03%, 0.002% = phosphorus = 0.02%, 0% = sulfur = 0.005%, 0% = nitrogen = 0.01%, and may contain one or more of the following optional elements: 0% = aluminum = 0.1%, 0% = niobium = 0.1%, 0% = vanadium = 0.3%, 0% = copper = 0.5%, 0% = chromium = 0.5%, 0% = boron = 0.001%, 0% = magnesium = 0.0010%. The remaining composition consists of iron and unavoidable impurities caused by the processing, the microstructure of said steel sheet comprising in area fraction, 20% to 40% of tempered martensite, at least 60% of reverted austenite and intermetallic compounds of molybdenum, titanium and nickel.

Description

HOT-ROLLED STEEL AND A METHOD OF MANUFACTURING THE SAME The present invention relates to hot-rolled steel suitable for use under corrosive environments, particularly under acid corrosion in the oil and gas industry. Today, oil and gas are extracted from deep wells. These deep wells are generally classified as either freshwater or sour. Freshwater wells are mildly corrosive, while sour wells are highly corrosive due to the presence of corrosive agents such as hydrogen sulfide, carbon dioxide, chlorides, and free sulfur. The corrosive conditions in sour wells are exacerbated by high temperatures and pressures. Therefore, extracting oil or gas from these sour wells becomes very difficult. Consequently, for acidic oil and gas environments, materials are selected to meet stringent criteria for acid corrosion resistance while simultaneously possessing excellent mechanical properties. Therefore, intensive research and development efforts are underway to meet corrosion resistance requirements in corrosive and highly toxic environments while simultaneously increasing material strength. Conversely, increased strength hinders steel processing into products such as seamless pipes and conduit pipes due to decreased formability. Consequently, the development of materials with high strength, formability, and adequate corrosion resistance in accordance with standards is necessary. Previous research and development in the field of high-strength, high-formability steel with corrosion resistance has resulted in several methods for steelmaking, some of which are listed herein for a conclusive appreciation of the present invention: Document US20100037994 claims a method for processing an aged martensitic steel workpiece, comprising receiving an aged martensitic steel workpiece having a composition comprising 17 wt% - 19 wt% nickel, 8 wt% - 12 wt% cobalt, 3 wt% - 5 wt% molybdenum, 0.2 wt% - 1.7 wt% titanium, 0.15 wt% - 0.15% by weight of aluminum and a remainder of iron and that has been subjected to thermomechanical processing at an austenite solubilization temperature; and directly aging the aged martensitic steel workpiece at an aging temperature to form precipitates within a microstructure of the aged martensitic steel workpiece, without any intermediate heat treatment between the thermomechanical processing and the direct aging, wherein the thermomechanical processing and the direct aging provide the aged martensitic steel workpiece with an ASTM average grain size of 10. But document US20100037994 does not guarantee corrosion resistance and only claims an economically aging method of processing martensitic steel. EP2840160 provides an aged martensitic steel with excellent fatigue characteristics, including, in terms of mass %: C: < 0.015%, Ni: 12.0 to 20.0%, Mo: 3.0 to 6.0%, Co: 5.0 to 13.0%, Al: 0.01 to 0.3%, Ti: 0.2 to 2.0%, O: < 0.0020%, N: < 0.0020% and zzL / nn / Lznz / E / Yi Zr: from 0.001 to 0.02%, the remainder being Fe and unavoidable impurities. Document EP2840160 provides the required strength but does not provide a steel with resistance to acid corrosion. The purpose of the present invention is to solve these problems by making available a hot-rolled steel that simultaneously has: a tensile strength greater than or equal to 1100 MPa and preferably greater than 1200 MPa, a total elongation greater than or equal to 18% and preferably greater than 19%, acid corrosion resistance and crack-free steel in accordance with NACE TM0177 standards at least 85% of the yield strength load. In a preferred embodiment, the steel according to the invention may also exhibit a yield strength of 850 MPa or more. In a preferred embodiment, the steel sheets according to the invention can also exhibit a yield strength to tensile strength ratio of 0.6 or more. Preferably, such steel may also have good forming suitability, particularly for rolling, with good weldability and coating capability. Another objective of the present invention is also to make available a method for manufacturing these sheets that is compatible with conventional industrial applications while being resistant to changes in manufacturing parameters. The hot-rolled steel sheet of the present invention can optionally be coated to further improve its corrosion resistance. Nickel is present in steel at levels between 15% and 25%. Nickel is essential for the steel of the present invention to impart strength by forming intermetallic compounds with molybdenum and titanium during preheating before quenching. These intermetallic compounds also act as sites for the formation of reversed austenite. Nickel also plays a crucial role in the formation of reversed austenite during quenching, which imparts elongation to the steel. However, nickel content below 15% will not impart strength due to the decreased formation of intermetallic compounds, whereas when nickel is present at levels above 25%, it will form more than 80% reversed austenite, which is also detrimental to the tensile strength of the steel. A preferred nickel content for the present invention can be maintained between 16% and 24%, and more preferably between 16% and 22%. Cobalt is an essential element for the steel of the present invention and is present in a concentration between 6% and 12%. The purpose of adding cobalt is to aid in the formation of reversed austenite during quenching, thereby imparting elongation to the steel. Additionally, cobalt also helps form molybdenum intermetallic compounds by decreasing the molybdenum ratio required to form a solid solution. However, when cobalt is present in concentrations exceeding 12%, it forms excessive reversed austenite, which is detrimental to the strength of the steel. Conversely, if the cobalt content is less than 6%, it will not decrease the rate of solid solution formation. A preferred cobalt content for the present invention may be between 6% and 11%, and more preferably between 7% and 10%. Molybdenum is an essential element, constituting 2% to 6% of the steel of the present invention. Molybdenum increases the strength of the steel of the present invention by forming intermetallic compounds with nickel and titanium during quenching. Molybdenum is essential for imparting corrosion-resistant properties to the steel of the present invention. However, the addition of molybdenum excessively increases the cost of alloying elements, so for economic reasons its content is limited to 6%. The preferred limit for molybdenum is between 3% and 6%, and more preferably between 3.5% and 5.5%. The titanium content of the steel of the present invention is between 0.1% and 1%. Titanium forms both intermetallic compounds and carbides to impart strength to the steel. If the titanium content is less than 0.1%, the required effect is not achieved. A preferred content for the present invention may be between 0.1% and 0.9%, and more preferably between 0.2% and 0.8%. Carbon is present in steel at levels between 0.0001% and 0.03%. Carbon is a residual element resulting from the manufacturing process. Carbon impurities below 0.0001% are not possible due to process limitations, and carbon levels above 0.03% must be avoided as they reduce the steel's corrosion resistance. The phosphorus content of the steel of the present invention is between 0.002% and 0.02%. Phosphorus reduces spot weldability and hot ductility, particularly due to its tendency to segregate at grain boundaries or co-segregate. For these reasons, its content is limited to 0.02% and preferably less than 0.015%. Sulfur is not an essential element, but it may be present as an impurity in steel. From the perspective of the present invention, the sulfur content is preferably as low as possible, but 0.005% or less is preferable for manufacturing cost reasons. Furthermore, if the sulfur content in the steel is higher, it combines to form sulfides and reduces its beneficial effect on the steel of the present invention; therefore, a content below 0.003% is preferred. Nitrogen is limited to 0.01% to prevent aging of the material. Nitrogen forms nitrides that impart strength to the steel of the present invention through precipitation strengthening with vanadium and niobium. However, whenever the presence of nitrogen exceeds 0.01%, a large amount of aluminum nitrides can form, which are detrimental to the present invention. Therefore, the preferred upper limit for nitrogen is 0.005%. Aluminum is not an essential element, but it may be present as a processing impurity in steel. This is because aluminum is added to the molten steel to clean it by removing oxygen and preventing it from forming a gaseous phase. Therefore, it may be present up to 0.1% as a residual element. However, from the perspective of the present invention, the aluminum content is preferably as low as possible. Niobium is an optional element of the present invention. The niobium content may be present in the steel of the present invention between 0% and 0.1% and is added to the steel of the present invention to form carbides or carbonitrides to impart strength to the steel of the present invention by precipitation strengthening. Vanadium is an optional element that constitutes between 0% and 0.3% of the steel of the present invention. Vanadium is effective in improving the strength of the steel by forming carbides, nitrides, or carbonitrides, and the upper limit is 0.3% for economic reasons. These carbides, nitrides, or carbonitrides form during the second and third cooling stages. The preferred limit for vanadium is between 0% and 0.2%. Copper can be added as an optional element in quantities of 0% to 0.5% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% copper is required to achieve this effect. However, when its content exceeds 0.5%, it can degrade surface finishes. Chromium is an optional element of the present invention. The chromium content that may be present in the steel of the present invention is between 0% and 0.5%. Chromium is an element that improves the corrosion resistance of steel, but a chromium content greater than 0.5% leads to central co-segregation after casting. Other elements such as boron or magnesium can be added individually or in combination in the following weight proportions: boron 0.001%, magnesium 0.0010%. Up to the maximum content levels indicated, these elements refine the grain during solidification. The rest of the steel composition consists of iron and unavoidable impurities resulting from processing. The microstructure of steel comprises: Reversed austenite is the matrix phase of the steel of the present invention and is present in at least 60% of the area fraction. The reversed austenite of the present steel is nickel-enriched, meaning it contains a higher amount of nickel compared to the residual austenite. The reversed austenite forms during the tempering of the steel and is simultaneously enriched with nickel. The reversed austenite of the steel of the present invention imparts both elongation and corrosion resistance to acidic environments. Martensite is present in the steel of the present invention at a fraction of 20% to 40% by area. The martensite of the present invention includes both fresh martensite and quenched martensite. Fresh martensite forms during cooling after annealing and is quenched during the tempering stage. Martensite imparts both elongation and strength to the steel of the present invention. Nickel, titanium, and molybdenum intermetallic compounds are present in the steel of the present invention. These intermetallic compounds form both during forging and tempering. The resulting intermetallic compounds are both intergranular and intragranular. The intergranular intermetallic compounds of the present invention are present in both martensite and reversed austenite. These intermetallic compounds of the present invention can be cylindrical or globular in shape. The intermetallic compounds of the steel of the present invention form as Ni3Ti, Ni3Mo, or Ni3(Ti,Mo) intermetallic compounds. The intermetallic compound of the steel of the present invention imparts strength and corrosion resistance, especially against acidic environments. In addition to the microstructure mentioned above, the microstructure of hot-rolled steel sheet is free of microstructural components such as ferrite, bainite, pearlite, and cementite, although trace amounts of these compounds may be present. Trace amounts of iron-iron intermetallic compounds, such as iron-molybdenum and iron-nickel, may also be present, but their presence does not significantly influence the steel's properties in use. The steel of the present invention can be formed into a seamless tubular product or a steel sheet, or even into a structural or operational part for use in the oil and gas industry or any other industry operating in an acidic environment. In a preferred embodiment for the purposes of illustrating the invention, a steel sheet according to the invention can be produced by the following method. A preferred method consists of providing a semi-finished steel casting with a chemical composition according to the invention. The casting can be made in ingots, billets, bars, or continuously in the form of thin slabs or thin strips, i.e., with a thickness ranging from approximately 220 mm for slabs to several tens of millimeters for thin strips. For example, a slab with the chemical composition described above is manufactured by continuous casting, where the slab optionally undergoes direct soft reduction during the continuous casting process to prevent center segregation. The slab produced by the continuous casting process can be used directly at high temperatures after continuous casting or can be cooled to room temperature first and then reheated for hot rolling. The temperature of the slab undergoing hot rolling is preferably at least 1150°C and should be below 1300°C. If the slab temperature falls below 1150°C, an excessive load is placed on the rolling mill. Therefore, the slab temperature is preferably high enough to allow hot rolling to be completed within the 100% austenitic range. Reheating to temperatures above 1275°C results in a loss of productivity and is also industrially costly. Therefore, the preferred reheating temperature is between 1150°C and 1275°C. The hot rolling finishing temperature for the present invention is between 800°C and 975°C and preferably between 800°C and 950°C. Then cool the hot-rolled steel strip obtained in this way from the hot-rolling finishing temperature to a temperature range between 10°C and Ms. The preferred temperature range for cooling the hot-rolled steel strip is between 15°C and Ms-20°C. Subsequently, the hot-rolled steel strip is heated to an annealing temperature range between Ae3 and Ae3 + 350°C. The hot-rolled steel strip is held at the annealing temperature for more than 30 minutes. In a preferred embodiment, the annealing temperature ranges are between Ae3 + 20°C and Ae3 + 350°C and more preferably between Ae3 + 40°C and Ae3 + 300°C. The hot-rolled steel strip is then cooled at a rate between 1°C / s and 100°C / s. In a preferred embodiment, the cooling rate after holding at the annealing temperature is between 1°C / s and 80°C / s, and more preferably between 1°C / s and 50°C / s. The hot-rolled steel strip is cooled to a temperature range between 10°C and Ms after annealing, and preferably between 15°C and Ms-20°C. During this cooling stage, fresh martensite forms, and the cooling rate above 1°C / s ensures that the hot-rolled strip is fully martensitic in its natural state. The hot-rolled steel strip is then heated to the tempering temperature range at a heating rate between 0.1°C / s and 100°C / s, preferably between 0.1°C / s and 50°C / s, or even between 0.1°C / s and 30°C / s. During this heating, as well as during tempering, nickel, titanium, and molybdenum intermetallic compounds are formed. The intermetallic compounds formed during this heating and tempering are both intragranular and intergranular, forming as Ni3Ti, Ni3Mo, or Ni3(Ti,Mo) intermetallic compounds. The tempering temperature range is between 575°C and 700°C, where the steel is tempered for a period of between 30 minutes and 72 hours. In a preferred embodiment, the quenching temperature range is between 575°C and 675°C and more preferably between 590°C and 660°C. During quenching retention, the martensite is reverted to austenite to form reverted austenite.The reversed austenite formed during tempering is enriched with nickel because, in the tempering temperature range of the present invention, part of the intermetallic formed during heating dissolves and enriches the austenite with nickel, and this nickel-enriched reversed austenite is stable at room temperature. Then, the hot-rolled steel strip is cooled to room temperature to obtain hot-rolled steel. Examples The following proofs, examples, figurative illustrations, and tables presented herein are of a non-restrictive nature and should be considered for illustrative purposes only, and will show the advantageous features of the present invention. The steels of different compositions are listed in Table 1, where the steel was produced according to the process parameters stipulated in Table 2. Table 3 then presents the microstructures of the steel obtained during the tests, and Table 4 summarizes the results of the property evaluations. Table 1 zzL / nn / Lznz / B / Yi Steel sample C Ni Co Mo Al Ti VPSN Nb Cu Cr 1 0.0029 17.530 8.76 4.86 0.0354 0.5217 0.0177 0.0042 0.006 0.0016 0.0141 0.0309 0.0530 2 0.0052 18.043 8.98 5245 0.01 0.507 0.067 0.0042 0.0045 0.0015 0 0 0 3 0.0024 13.986 9.05 4.86 0.0380 0.4580 0.0740 0.0038 0.0041 0.0015 0.277 0.0350 0 Underlined values: not in accordance with the invention. Table 2 Table 2 brings together the process parameters implemented in steels from Table 1. Ms for all steel samples is calculated according to the following formula: Ms = 764.2 - 302.6 of C - 30.6 of Mn -16.6 of Ni - 8.9 of Cr + 2.4 of Mo -11.3 of Cu + 8.58 of Co + 7.4 of W-14.5 of Si, where the contents of the elements are expressed as a percentage by weight, considering that Ae3 is calculated in (°C) according to the following formula: Ae3 = 955-350 C - 25 Mn + 51 Si + 106 Nb + 100 Ti + 68 Al - 11 Cr - 33 zzL / nn / Lznz / E / Yi Ni - 16 of Cu + 67 of Mo, where the contents of the elements are expressed as a percentage by weight. Table 2: Steel Sample Tests Reheating Temperature (Ό) Finishing Temperature (l-RfC) Cooling Temperature (l-RfC) Annealing Temperature (fC) Annealing Time (s) Cooling Rate (f(^) Cooling Temperature (fC) Heating Rate (fCfc) Tempering Temperature (CQ) Tempering Time (Ae3 Ms) 1 11 1200 850 20 1020 1800 30 20 15 600 86400 756 558 1 I2 1200 850 20 800 1800 30 20 15 650 3600 756 558 2 I3 1200 850 20 850 1800 30 20 15 650 3600 761 552 1 R1 1200 850 20 800 1800 30 20 15 550 1 756 558 2 R2 1200 850 20 850 1800 30 20 15 500 300 761 552 3 R3 1200 850 20 850 1800 30 20 15 500 300 894 620 I = according to the invention; R = reference; underlined values: not according to the invention. Table 3 Table 3 exemplifies the results of the tests carried out according to the standards in different microscopes such as scanning electron microscope to determine the microstructures of both the inventive and reference steels. The results are stipulated in this document: Steel Sample Tests Reversed Austenite (%) Martensite (%) Intermetallic Compounds 1 11 64 36 Si 1 I2 75 25 Si 2 I3 70 30 Si 1 R1 3 97 Si 2 R2 3 97 Si 3 R3 3 97 Si I = according to the invention; R = reference; underlined values: not according to the invention. Table 4 illustrates the mechanical properties of both the inventive steel and the reference steels. To determine tensile strength, yield strength, and total elongation, tensile tests are performed according to NBN EN ISO 6892-1 on an A25 type specimen, and the corrosion resistance test is performed according to NACE TM0316 by method B with a load of at least 85% of the yield strength. The results of the various mechanical tests carried out in accordance with the standards are compiled. Table 4 zzL / nn / Lznz / B / YL Steel Sample Tests Tensile Strength (MPa) Yield Strength (MPa) Total Elongation (%) Acid Corrosion Resistance (%) 1 11 1312 1009 19 No crack - good 1 I2 1204 899 22.8 No crack - good 2 I3 1273 997 24 No crack - good 1 R1 1477 1407 13.5 Crack - not good 2 R2 1550 1442 13.1 Crack - not good 3 R3 1416 1352 16.8 Crack - not good I = according to the invention; R = reference; underlined values: not according to the invention.

Claims

1. A hot-rolled steel having a composition comprising the following elements, expressed as a percentage by weight: 15% < nickel < 25% 6% < cobalt < 12% 2% < molybdenum < 6% 0.1% < titanium < 1% 0.0001% < carbon < 0.03% 0.002% < phosphorus < 0.02% 0% < sulfur < 0.005%. 0% < nitrogen < 0.01% and may contain one or more of the following optional elements: 0% < aluminum < 0.1%, 0% < niobium < 0.1%, 0% < vanadium < 0.3%, 0% < copper < 0.5%, 0% < chromium < 0.5%, 0% < boron < 0.001%, 0% < magnesium < 0.0010%. The remaining composition consists of iron and unavoidable impurities caused by processing. The microstructure of said steel sheet comprises, in area fraction, 20% to 40% quenched martensite, at least 60% reverted austenite, and intermetallic compounds of molybdenum, titanium, and nickel.

2. Hot-rolled steel according to claim 1, wherein the composition includes from 16% to 24% nickel.

3. Hot-rolled steel according to claim 1 or 2, wherein the composition includes from 16% to 22% nickel.

4. Hot-rolled steel according to any of claims 1 to 3, wherein the composition includes from 6% to 11% cobalt.

5. Hot-rolled steel according to any of claims 1 to 4, wherein the composition includes 7% to 10% cobalt.

6. Hot-rolled steel according to any of claims 1 to 5, wherein the composition includes 3% to 6% molybdenum.

7. Hot-rolled steel according to any of claims 1 to 6, wherein the composition includes from 3.5% to 5.5% molybdenum.

8. Hot-rolled steel according to any of claims 1 to 7, wherein the composition includes from 0.1% to 0.9% titanium. zzL / nn / Lznz / E / Yi 9. Hot-rolled steel according to any of claims 1 to 8, wherein the composition includes from 0.2% to 0.8% titanium.

10. Hot-rolled steel according to any of claims 1 to 9, wherein the intermetallic compounds of molybdenum, titanium and nickel are at least one or more of Ni3Ti, Ni3Mo or Ni3(Ti,Mo).

11. Hot-rolled steel according to any of claims 1 to 10, wherein the intermetallic compounds of molybdenum, titanium, and nickel include intergranular and intragranular intermetallic compounds.

12. Hot-rolled steel according to any of claims 1 to 11, wherein said steel has a tensile strength of 1100 MPa or more and a total elongation of 18% or more.

13. Hot-rolled steel according to any of claims 1 to 12, wherein said steel has a tensile strength of 1200 MPa or more and a total elongation of 19% or more.

14. A method for producing a hot-rolled steel comprising the following successive steps: providing a steel composition according to any one of claims 1 to 9; reheating said semi-finished product to a temperature between 1150°C and 1300°C; rolling said semi-finished product in the austenitic range wherein the hot-rolling finish temperature is between 800°C and 975°C to obtain a hot-rolled steel strip; then cooling said hot-rolled steel strip to a temperature range between 10°C and Ms; subsequently reheating the hot-rolled steel strip to an annealing temperature between Ae3 and Ae3 + 350°C, holding it at that temperature for more than 30 minutes and cooling it at a rate between 1°C / s and 100°C / s within the temperature range between 10°C and Ms;Next, reheat the hot-rolled steel strip to a quenching temperature range between 575°C and 700°C with a heating rate between 0.1°C / s and 100°C / s and hold the hot-rolled steel strip in the quenching temperature range for a duration between 30 minutes and 72 hours; then cool the hot-rolled steel strip to room temperature to obtain hot-rolled steel.

15. A method according to claim 14, wherein the reheating temperature of the semi-finished product is between 1150°C and 1275°C.

16. A method according to claim 14 or 15, wherein the hot rolling finishing temperature is between 800°C and 950°C. zzL / nn / Lznz / E / Yi 17. A method according to any of claims 14 to 16, wherein the cooling temperature range for the hot-rolled strip after completion of hot rolling is between 15°C and Ms-20°C.

18. A method according to any of claims 14 to 17, wherein the annealing temperature range is between Ae3 + 20°C and Ae3 + 350°C.

19. A method according to claim 18, wherein the annealing temperature range is between Ae3 + 40°C and Ae3 + 300°C.

20. A method according to any of claims 14 to 19, wherein the cooling rate after annealing is between 1°C / s and 80°C / s.

21. A method according to claim 20, wherein the cooling rate after annealing is between 1°C / s and 50°C / s.

22. A method according to any of claims 14 to 21, wherein the cooling temperature range after annealing is between 15°C and Ms-20°C.

23. A method according to any of claims 14 to 22, wherein the tempering temperature range is between 575°C and 675°C.

24. A method according to claim 23, wherein the tempering temperature range is between 590°C and 660°C.

25. A method according to any of claims 14 to 24, wherein the heating rate for tempering is between 0.1°C / s and 50°C / s.

26. A method according to claim 25, wherein the heating rate for tempering is between 0.1°C / s and 30°C / s.

27. The use of a steel according to any of claims 1 to 14 or of a steel produced according to the method of claims 14 to 26, for the manufacture of structural or operational parts for oil and gas wells.

28. A seamless tube, pipe or part obtained according to claim 27. zzL / nn / Lznz / B / YL