High-strength and high-toughness steel for ultralow-temperature engineering and preparation method of high-strength and high-toughness steel

Abstract

The invention belongs to the technical field of ultralow-temperature materials, and discloses high-strength and high-toughness steel for ultralow-temperature engineering and a preparation method of the high-strength and high-toughness steel. Through ultra-low carbon, control of the chemical composition proportion of Creq content and Nieq content, and combination of a vacuum induction melting and vacuum consumable remelting duplex smelting process, the grain size grade of the novel steel is more than 6 grade, and the matrix structure is fine lath martensite and film-shaped reversed austenite with the volume fraction of 26%-50%. According to the high-strength and high-toughness steel for ultralow-temperature engineering, the room-temperature yield strength is larger than or equal to 1100 MPa, the room-temperature tensile strength is larger than or equal to 1200 MPa, the ductility is larger than or equal to 20%, the room-temperature V-type impact energy KV2 is larger than or equal to 200 J, the-196 DEG C yield strength is larger than or equal to 1300 MPa, the-196 DEG C tensile strength is larger than or equal to 1800 MPa, the ductility is larger than or equal to 20%, and the-196 DEG C V-type impact energy KV2 is larger than or equal to 60 J.

Description

Technical Field

[0001] The present invention relates to the technical field of ultra-low temperature materials, and in particular to a high-strength and high-toughness steel for ultra-low temperature engineering and a preparation method thereof. Background Art

[0002] With the development of cryogenic engineering, my country's demand for high-performance materials suitable for ultra-low temperature environments is becoming increasingly urgent. Large cryogenic engineering components are subject to long-term service under extreme operating conditions such as ultra-low temperatures, a wide temperature range (77-323K), impact, and heavy loads. These new service conditions place higher demands on materials for high strength, high toughness, high corrosion resistance, and formability. In particular, they must maintain a good balance of strength and toughness even in ultra-low temperature environments (-196°C). Domestic research on ultra-low temperature engineering steels has primarily focused on improving strength.

[0003] The invention patent application with publication number CN103695796A discloses a high-strength and high-toughness stainless steel and a manufacturing method. The composition of the stainless steel is C = 0.13% ~ 0.19%, Cr = 15.0% ~ 16.0%, Ni = 3.0% ~ 4.0%, Mo = 1.4% ~ 1.9%, Cu = 1.0% ~ 2.0%, W = 0.7% ~ 1.2%, V = 0.0% ~ 0.6%, N = 0.05% ~ 0.12%, and the remainder is Fe and unavoidable impurities; its yield strength is 690MPa ~ 1388MPa, and its tensile strength is 1200MPa ~ 1670MPa, but its plasticity is only greater than 10%, and it is impossible to achieve a good strength-plasticity match.

[0004] The invention patent application with publication number CN106906429A discloses an ultra-high-strength martensitic stainless steel and its preparation method. The stainless steel has the following composition (mass percentage, %): C = 0.10% to 0.25%, Cr = 11.0% to 17.0%, Ni = 0.1% to 4.0%, Cu = 0.1% to 0.3%, with the remainder being iron and unavoidable impurity elements; its yield strength is 1300 MPa, its tensile strength is 1600 MPa, and its plasticity is 16%. Although the above solution has high strength, the high carbon content deteriorates its corrosion resistance, ultra-low temperature toughness, and weldability, limiting its application below -196°C.

[0005] Patent application publication number CN109439870A discloses a "method for improving the low-temperature impact energy of 17-4PH maraging stainless steel forgings based on microstructure control." The method involves controlling the forging heating temperature, deformation amount, and deformation direction, and then undergoing two solid solution + aging heat treatments. During the recrystallization process, fine high-temperature ferrite is further eliminated, thereby increasing the low-temperature impact energy. However, its KV2 (-40°C) is only 27J, which is difficult to meet the high toughness requirements at -196°C.

[0006] Patent publication number CN111118258A discloses "a heat treatment method for improving the low-temperature impact toughness of 00Cr12Ni10MoTi maraging stainless steel". Through a complex heat treatment process, the low-temperature impact toughness reaches AKV (-196°C): 90 to 140J. However, its maximum yield strength is only 890MPa, making it difficult to achieve a good strength-toughness match at -196°C.

[0007] Therefore, how to achieve ultra-low temperature engineering steel with high strength and good low-temperature toughness in ultra-low temperature (-196°C) environment is of great theoretical significance and practical value. Based on this, the present invention provides a high-strength and high-toughness steel for ultra-low temperature engineering and a preparation method thereof. Through a reasonable alloy system and organizational design, combined with high-clean smelting and a simple heat treatment process with a wide process window, while ensuring high strength in ultra-low temperature (-196°C) environment, it has good low-temperature toughness, forming an ultra-low temperature environment strength-toughness matching mechanism of "fine lamellar martensite matrix, thin film inversion austenite and high defect density", further realizing the improvement of the mechanical properties of ultra-low temperature materials. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide a high strength and high toughness steel for ultra-low temperature engineering and a preparation method thereof, by ultra-low carbon and controlled Cr eq Content and Ni eq The chemical composition ratio of the content is optimized, and high-cleanliness smelting is carried out by combining vacuum induction melting and vacuum consumable remelting double smelting process to reduce the content of impurity elements C, S, P, O, N, and H, while avoiding the formation of large TiN, TiCN inclusions and MnS inclusions. Combined with forging hot forming, rolling hot forming and appropriate heat treatment process, the grain size grade of high-strength and high-toughness steel for ultra-low temperature engineering is guaranteed to be above level 6, forming a fine lath martensite matrix and a film-like reverse transformation austenite with a volume fraction of 26% to 50%, which significantly improves the strength and toughness of the steel, especially the strength and toughness matching in ultra-low temperature environment.

[0009] In order to achieve the above object, the technical solution of the present invention is as follows:

[0010] Disclosed is a high-strength and high-toughness steel for ultra-low temperature engineering, wherein the chemical composition and weight percentages are as follows: C≤0.03%, Si=0.10%-0.50%, Mn=0.10%-0.50%, S≤0.003%, P≤0.012%, Cr=10.00%-12.00%, Ni=7.00%-9.00%, Co=6.50%-7.50%, Mo=2.50%-3.50%, V=0.02%-0.10%, Al=0.08%-0.15%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance being Fe and unavoidable impurities.

[0011] Control Cr eq Content and Ni eq content:

[0012] Cr eq =Cr+Mo+1.5Si+0.5Nb+2Ti;

[0013] Ni eq =Ni+Co+0.5Mn+0.3Cu+30C+30N;

[0014] Among them: Cr eq <18.0%;8%<Ni eq <26.0%;

[0015] The content of each element is expressed in mass percentage.

[0016] The high-strength and high-toughness steel for ultra-low temperature engineering has a grain size grade of 6 or above, a matrix structure of fine lath martensite, and a film-like reversed austenite with a volume fraction of 26% to 50% in the matrix structure.

[0017] Under room temperature stretching, the yield strength of ultra-low temperature engineering high-strength and high-toughness steel is ≥1100MPa, the tensile strength is ≥1200MPa, the elongation is ≥20%, and the room temperature V-type impact energy KV2 is ≥200J;

[0018] When stretched at -196℃, the yield strength of high-strength and high-toughness steel for ultra-low temperature engineering is ≥1300MPa, the tensile strength is ≥1800MPa, the elongation is ≥20%, and the V-type impact energy KV2 at -196℃ is ≥60J.

[0019] A method for preparing high-strength and high-toughness steel for ultra-low temperature engineering comprises the following steps:

[0020] Step (1) adopts vacuum induction melting and vacuum consumable remelting double smelting process, prepares metal raw materials according to chemical composition and weight percentage, casts the obtained smelting liquid to obtain steel ingot, and ensures that the C content of the steel ingot is ≤0.03% after smelting;

[0021] Step (2), forging hot forming or rolling hot forming;

[0022] The forging hot forming process includes: performing high-temperature homogenization annealing on the steel ingot before forging, heating in a furnace-heated manner, a forging heating temperature of 1150° C. to 1200° C., a holding time of 1.5 hours / 100 mm forging effective thickness to 2 hours / 100 mm forging effective thickness, a start forging temperature of 1100° C. to 1150° C., and a final forging temperature of 1000° C. to 1050° C.;

[0023] The hot rolling forming process includes: performing high temperature homogenization annealing on the steel ingot before hot rolling, wherein the hot rolling heating temperature is 1150°C to 1200°C and the holding time is 4 to 8 hours to make the internal structure uniform; the hot rolling start temperature is 1100°C to 1150°C and the final rolling temperature is 950°C to 1050°C;

[0024] Step (3), heat treatment process;

[0025] The steel material after hot forming in step (2) is subjected to solution treatment and aging heat treatment in sequence to obtain the high-strength and high-toughness steel for ultra-low temperature engineering; the solution temperature is 750°C to 850°C, the holding time is 60min to 180min, and the steel material is water-cooled to room temperature, and the water temperature is ensured to be ≤40°C during the cooling process; the aging temperature is 500°C to 600°C, the holding time is 4h to 10h, and the steel material is air-cooled or slowly cooled to room temperature.

[0026] In step (1), ultra-low carbon ultra-pure iron and high-purity alloy are used as raw materials, and electrode rods are prepared by vacuum induction melting, and then steel ingots are obtained by vacuum consumable remelting; high vacuum smelting is adopted throughout the process, and the vacuum degree during vacuum induction melting reaches below 0.1 Pa; during vacuum induction melting, the refining temperature reaches 1550℃~1600℃, the refining time is not less than 40 minutes, and the stirring time is not less than 5 minutes; the vacuum induction melting pouring temperature is 1530℃-1550℃; the vacuum degree during vacuum consumable remelting is maintained at 10 -2 Pa and below, the melting rate is 100Kg / h~260Kg / h.

[0027] The high temperature homogenization annealing before forging has a heating rate of ≤80°C / h below 700°C, a total forging ratio of ≥5, and air cooling or heat preservation and slow cooling to room temperature after forging deformation.

[0028] The rolling hot forming has a pass reduction rate of 10% to 12%, and the hot rolling forming is followed by air cooling or slow cooling to room temperature.

[0029] The high strength and high toughness steel for ultra-low temperature engineering provided by the present invention does not rely on carbon elements to improve strength, but adopts ultra-low carbon composition design, strictly controls C≤0.03%, and avoids the formation of Cr during heat treatment. 23Brittle carbides such as C6 and Cr7C3 are used to prevent grain boundary weakening and loss of low-temperature toughness.

[0030] The mass fraction of Cr in the high-strength and high-toughness steel for ultra-low temperature engineering provided by the present invention is 10.00%~12.00%. On the one hand, the Cr element is the key to ensuring good corrosion resistance of steel, and its content is usually required to be greater than 10% to ensure that corrosion failure does not occur, which is crucial for ultra-low temperature engineering steel serving in a wide temperature range. The Cr element is also a ferrite-forming element, which will reduce the austenite phase area and lower the Ms point. For steels with high Cr content, austenite-forming elements such as Mn and Ni must be added to prevent the formation of partial ferrite structure, thereby deteriorating impact toughness and corrosion resistance. At the same time, the Cr element will generate Cr with the C element. 23 Carbides such as C6 and Cr7C3 reduce the bonding energy with the martensite matrix, thus harming corrosion resistance and toughness.

[0031] The mass fraction of nickel in the ultra-low temperature engineering high-strength and high-toughness steel provided by the present invention is 7.00% to 9.00%. As an austenite stabilizing element, nickel acts synergistically with chromium. As the nickel content in the steel increases, the austenite phase region shifts toward the high chromium content. This means that even when the chromium content in the steel is increased, a single ferrite structure will not form. Ni can promote the formation of reversed austenite during aging, and the nickel-rich austenite will not undergo martensitic transformation at low temperatures, thus having good thermodynamic stability and significantly increasing low-temperature impact toughness.

[0032] The mass fraction of Co in the ultra-low temperature engineering high-strength and high-toughness steel provided by the present invention is 6.50% to 7.50%. The Co element can significantly increase the strength of the ultra-low temperature engineering steel. On the one hand, the Co element can enhance the solid solution strengthening effect and improve the strength of the martensite itself. On the other hand, the Co element and the Mo element have a synergistic effect, that is, the Co element can reduce the solid solubility of Mo in the martensite matrix and promote the precipitation of Mo-containing strengthening phases (Mo-rich phase, Ni3Mo, Fe2Mo) during the aging process. In addition, the Co element can also inhibit the recovery of dislocation substructures in the martensite matrix, enhance the aging strengthening effect, and significantly improve the strength of the high-strength and high-toughness steel.

[0033] The high-strength, high-toughness steel for ultra-low-temperature engineering provided by the present invention has a Mo mass fraction of 2.50% to 3.50%. Mo contributes to the corrosion resistance, strength, and toughness of the high-strength, high-toughness steel for ultra-low-temperature engineering. During the aging process, Mo-rich phases and Fe2Mo hexagonal Laves phases, which are age-strengthening phases, precipitate. Furthermore, the Mo-rich precipitated phases form a unique core-shell structure with η-Ni3Ti, inhibiting the coarsening of Ni3Ti. Simultaneously, the Mo-rich phase also inhibits the coarsening of the Ni(Al, Fe) phase and prevents the precipitation of precipitated phases along the original austenite grain boundaries, thus avoiding intergranular fracture.

[0034] The high-strength, high-toughness steel for ultra-low-temperature engineering provided by the present invention contains Si and Mn by mass fractions of 0.10% to 0.50% and 0.10% to 0.50%, respectively. Silicon and manganese primarily serve as deoxidizers in steel. The steel is produced using vacuum induction melting and vacuum consumable remelting, using ultra-low-carbon, ultra-pure iron and high-purity alloy raw materials, eliminating the need for excessive silicon and manganese additions.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The high strength and high toughness steel for ultra-low temperature engineering of the present invention is obtained by combining the structure design, Cr eq 、Ni eq By optimizing the alloying element ratios and employing a dual vacuum induction melting and vacuum consumable remelting process, along with corresponding thermomechanical treatments, the martensitic matrix and precipitation-strengthening phase were controlled, and reversed austenite was introduced into the martensitic matrix. By manipulating the morphology, size, and distribution of nanoscale precipitates and reversed austenite, and introducing 26%-50% thin-film reversed austenite into a high-dislocation-density martensitic matrix, a remarkable balance of strength and toughness was achieved at ultra-low temperatures.

[0037] (2) Compared with the grain refinement process such as cyclic phase transformation, the present invention achieves grain refinement of high-strength and high-toughness steel for ultra-low temperature engineering and good matching of strength and toughness at ultra-low temperature through reasonable forging and hot rolling process combined with simple one-time solid solution + aging heat treatment. The process is simple and the economic cost is greatly reduced.

[0038] (3) The preparation method of the high-strength and high-toughness steel for ultra-low temperature engineering of the present invention is simple. The high-strength and high-toughness steel for ultra-low temperature engineering can be obtained through a heat treatment process with a wide process window. The process is highly controllable and efficient, and it is easy to realize industrial production.

[0039] (4) The present invention has broad application prospects in aerospace heavy-load rocket engines, cryogenic wind tunnel components, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in Example 1;

[0041] Figure 2 This is a metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in Example 1;

[0042] Figure 3 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in Example 2;

[0043] Figure 4 This is the metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in Example 2;

[0044] Figure 5 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in Example 3;

[0045] Figure 6 This is the metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in Example 3;

[0046] Figure 7 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 1;

[0047] Figure 8 This is the metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 1;

[0048] Figure 9 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 2;

[0049] Figure 10 This is the metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 2;

[0050] Figure 11 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 3;

[0051] Figure 12 This is the metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 3;

[0052] Figure 13 This is the EBSD image of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 4;

[0053] Figure 14 This is the metallographic morphology of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment in comparative example 4. DETAILED DESCRIPTION

[0054] The present invention provides a high-strength and high-toughness steel for ultra-low temperature engineering, which has a grain size grade of 6 or above and a dual-phase structure of lath martensite + thin film reversed austenite, wherein the volume fraction of the thin film reversed austenite is 26% to 50%. Due to the high dislocation density, martensitic transformation does not spontaneously occur at an ultra-low temperature of -196°C, thereby achieving good strength-toughness matching at ultra-low temperatures.

[0055] The high-strength and high-toughness steel for ultra-low temperature engineering described in the present invention has a chemical composition by weight percentage as follows: C≤0.03%, Si=0.10%~0.50%, Mn=0.10%~0.50%, S≤0.003%, P≤0.012%, Cr=10.00%~12.00%, Ni=7.00%~9.00%, Co=6.50%~7.50%, Mo=2.50%~3.50%, V=0.02%~0.10%, Al=0.08%~0.15%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities.

[0056] The ultra-low temperature engineering high strength and high toughness steel of the present invention is to avoid the formation of ferrite phase by controlling the chromium and nickel equivalent, and to form a microstructure of lath martensite plus 26% to 50% film-like reversed austenite after aging. eq and Ni eq The following relationship must be satisfied:

[0057] Cr eq =Cr+Mo+1.5Si+0.5Nb+2Ti;

[0058] Ni eq =Ni+Co+0.5Mn+0.3Cu+30C+30N;

[0059] Cr eq <18.0%;

[0060] 8<Ni eq <26.0%;

[0061] Wherein: the content of each element is expressed in mass percentage.

[0062] Furthermore, the alloy composition of the high-strength and high-toughness steel for ultra-low temperature engineering described in the present invention is designed to use Co and Mo as the main strengthening elements. The synergistic effect of Co-Mo promotes the precipitation of Mo-containing precipitation strengthening phase (Laves phase) during aging; the Ni element promotes the formation of reversed austenite during aging, thereby significantly increasing the low-temperature impact toughness; the Al content is added as a deoxidizer and is controlled at 0.08-0.15%, and other harmful elements C, Si, Mn, S, P, O, and N are all controlled at low levels to improve the purity of the molten steel and avoid the formation of large-sized inclusions, thereby improving the ultra-low temperature toughness of the steel.

[0063] Furthermore, the high-strength and high-toughness steel for ultra-low temperature engineering described in the present invention has a tensile yield strength of ≥1100MPa, a tensile strength of ≥1200MPa, an elongation of ≥20% at room temperature, and a V-type impact energy KV2 of ≥200J; when stretched at -196°C, the yield strength is ≥1300MPa, the tensile strength is ≥1800MPa, the elongation is ≥20%, and the V-type impact energy KV2 at -196°C is ≥60J.

[0064] The method for preparing the above-mentioned high-strength and high-toughness steel for ultra-low temperature engineering comprises the following steps:

[0065] (1) Using vacuum induction melting and vacuum consumable remelting double smelting process, prepare metal raw materials according to chemical composition and weight percentage, cast the obtained smelting liquid to obtain steel ingots, and ensure that the C content of the steel ingot is ≤0.03% after smelting;

[0066] (2) Forging hot forming or rolling hot forming;

[0067] The forging hot forming process includes: performing high-temperature homogenization annealing on the steel ingot before forging, heating in a furnace-heated manner, a forging heating temperature of 1150° C. to 1200° C., a holding time of 1.5 hours / 100 mm forging effective thickness to 2 hours / 100 mm forging effective thickness, a start forging temperature of 1100° C. to 1150° C., and a final forging temperature of 1000° C. to 1050° C.;

[0068] The rolling hot forming process comprises the following steps: subjecting the steel ingot to high temperature homogenization annealing before hot rolling, wherein the hot rolling heating temperature is 1150°C to 1200°C and the holding time is 4 to 8 hours to make the internal structure uniform; the hot rolling start temperature is 1100°C to 1150°C and the final rolling temperature is 950°C to 1050°C.

[0069] (3) Heat treatment process;

[0070] The steel material after hot forming in step (2) is subjected to solution treatment and aging heat treatment in sequence to obtain the high-strength and high-toughness steel for ultra-low temperature engineering; the solution temperature is 750°C to 850°C, the holding time is 60min to 180min, and the steel material is water-cooled to room temperature, and the water temperature is ensured to be ≤40°C during the cooling process; the aging temperature is 500°C to 600°C, the holding time is 4h to 10h, and the steel material is air-cooled or slowly cooled to room temperature.

[0071] Example 1

[0072] In this embodiment, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting and vacuum consumable remelting. The chemical composition and weight percentage of the ultra-low temperature engineering high-strength and high-toughness steel are as follows: C≤0.03%, Si=0.20%, Mn=0.20%, S≤0.003%, P≤0.012%, Cr=10.00%, Ni=7.00%, Co=6.50%, Mo=2.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities. Under this alloy system, Cr eq =12.8、Ni eq =13.6.

[0073] High temperature homogenization annealing treatment and forging hot forming: before forging, heat the steel ingot to 1200℃, the heating method is furnace heating, the heating rate is 70℃ / h below 700℃, and the heating rate is 200℃ / h above 700℃. After reaching the temperature, keep it warm for 6 hours before forging. The starting forging temperature is 1130±10℃, the final forging temperature is 1030±10℃, and air cool to room temperature after forging.

[0074] The forged steel is subjected to solution treatment and aging treatment in sequence. The solution treatment process is: keeping the steel at 850℃ for 120 minutes, then water cooling to room temperature. The water temperature is always ≤40℃ during the cooling process; the aging treatment process is: keeping the solution treated steel at 500℃ for 6 hours, and slowly cooling it to room temperature, that is, 850℃×120min+500℃×6h.

[0075] Figure 1 and Figure 2 The microstructure EBSD map and metallographic morphology map of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in Example 1. Figure 1 and Figure 2 It can be seen that after aging, the high-strength and high-toughness steel for ultra-low temperature engineering is a martensite plus thin-film reversed austenite structure. The EBSD phase distribution diagram statistics show that 28.3% of thin-film reversed austenite is formed after aging treatment, and the grain size is greater than level 6.

[0076] The mechanical property test results of Example 1 are shown in Table 1. The tensile yield strength at room temperature is 1196 MPa, the tensile strength is 1279 MPa, the elongation is 22.4%, and the room temperature V-type impact energy KV2 = 221 J; when stretched at -196°C, the yield strength is 1346 MPa, the tensile strength is 1889 MPa, the elongation is 27.3%, and the V-type impact energy KV2 at -196°C = 71 J.

[0077] The high-strength and high-toughness steel for ultra-low temperature engineering of the present invention can obtain good strength and toughness and ultra-low temperature toughness in a wide range of heat treatment processes, and the heat treatment process of Example 1 in Table 1 is only a typical representative thereof.

[0078] Example 2

[0079] In this embodiment, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting and vacuum consumable remelting. The chemical composition and weight percentage of the ultra-low temperature engineering high-strength and high-toughness steel are as follows: C≤0.03%, Si=0.20%, Mn=0.20%, S≤0.003%, P≤0.012%, Cr=10.00%, Ni=7.00%, Co=6.50%, Mo=2.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities. Under this alloy system, Cr eq =12.8、Ni eq =13.6.

[0080] High temperature homogenization annealing treatment and hot rolling forming: before hot rolling, the steel ingot is heated to 1200℃ and kept at 1200℃ for 4 hours for high temperature homogenization annealing to make its internal structure uniform; after keeping for 4 hours, it is taken out of the furnace for hot rolling forming, the initial hot rolling temperature T=1150±20℃, the final rolling temperature T=1000±20℃, and the temperature is measured using an infrared thermometer. The hot rolling reduction rate is 10%~12%, and it is air-cooled to room temperature after rolling.

[0081] The hot-rolled steel was subjected to solution treatment and aging treatment in sequence. The solution treatment process was as follows: keeping the steel at 850°C for 120 minutes, followed by water cooling to room temperature. The water temperature was always ≤40°C during the cooling process. The aging treatment process was as follows: keeping the solution-treated steel at 500°C for 6 hours, and then slowly cooling it to room temperature, i.e., 850°C×120 minutes+500°C×6 hours.

[0082] Figure 3 and Figure 4 The microstructure EBSD map and metallographic morphology of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in Example 2 are shown in FIG. Figure 3 and Figure 4 It can be seen that after aging, the high-strength and high-toughness steel for ultra-low temperature engineering is a martensite plus thin-film reversed austenite structure. The EBSD phase distribution diagram statistics show that 33.7% of thin-film reversed austenite is formed after aging treatment, and the grain size is greater than level 6.

[0083] The mechanical property test results of Example 2 are shown in Table 1. The tensile yield strength at room temperature is 1187 MPa, the tensile strength is 1298 MPa, the elongation is 22.6%, and the V-type impact energy KV2 at room temperature is 229 J. When stretched at -196°C, the yield strength is 1325 MPa, the tensile strength is 1867 MPa, the elongation is 26.9%, and the V-type impact energy KV2 at -196°C is 72 J.

[0084] The high-strength and high-toughness steel for ultra-low temperature engineering of the present invention can obtain good strength and toughness and ultra-low temperature toughness in a wide range of heat treatment processes, and the heat treatment process of Example 2 in Table 1 is only a typical representative thereof.

[0085] Example 3

[0086] In this embodiment, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting and vacuum consumable remelting. The components and weight percentages of the ultra-low temperature engineering high-strength and high-toughness steel are as follows: C≤0.03%, Si=0.10%, Mn=0.10%, S≤0.003%, P≤0.012%, Cr=12.00%, Ni=9.00%, Co=7.50%, Mo=3.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities. Under this alloy system, Cr eq =15.65、Ni eq =16.55.

[0087] High temperature homogenization annealing treatment and hot rolling forming: before hot rolling, heat the steel ingot furnace to 1200℃, keep it at 1200℃ for 4 hours for high temperature homogenization annealing to make its internal structure uniform; after keeping it at 1200℃ for 4 hours, take it out of the furnace for hot rolling forming, the initial hot rolling temperature is T=1150±20℃, the final rolling temperature is T=1000±20℃, and the temperature is measured using an infrared thermometer. The hot rolling reduction rate is 10%~12%, and it is air-cooled to room temperature after rolling.

[0088] The hot-rolled steel was subjected to solution treatment and aging treatment in sequence. The solution treatment process was as follows: keeping the steel at 750°C for 120 minutes, then water cooling to room temperature. The water temperature was always ≤40°C during the cooling process. The aging treatment process was as follows: keeping the solution-treated steel at 600°C for 6 hours, and then slowly cooling it to room temperature, i.e., 750°C×120 minutes+600°C×6 hours.

[0089] Figure 5 and Figure 6 The microstructure EBSD map and metallographic morphology of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in Example 3 are shown in FIG. Figure 5 and Figure 6It can be seen that after aging, the high-strength and high-toughness steel for ultra-low temperature engineering is a martensite plus thin-film reversed austenite structure. The EBSD phase distribution diagram statistics show that 46.3% of the thin-film reversed austenite is formed after aging treatment, and the grain size is greater than level 6.

[0090] The mechanical property test results of Example 3 are shown in Table 1. The tensile yield strength at room temperature is 1127 MPa, the tensile strength is 1221 MPa, the elongation is 24.3%, and the room temperature V-type impact energy KV2 = 233 J; when stretched at -196°C, the yield strength is 1311 MPa, the tensile strength is 1817 MPa, the elongation is 33.2%, and the V-type impact energy KV2 at -196°C = 86 J.

[0091] The high-strength and high-toughness steel for ultra-low temperature engineering of the present invention can obtain good strength and toughness and ultra-low temperature toughness in a wide range of heat treatment processes, and the heat treatment process of Example 3 in Table 1 is only a typical representative thereof.

[0092] Comparative Example 1

[0093] In this comparative example, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting + vacuum consumable remelting. The chemical composition and weight percentage of ultra-low temperature engineering high-strength and high-toughness steel are as follows: C≤0.03%, Si=0.20%, Mn=0.20%, S≤0.005%, P≤0.012%, Cr=16.00%, Ni=7.00%, Co=6.50%, Mo=2.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities, Cr eq =18.8、Ni eq =13.6.

[0094] High temperature homogenization annealing treatment and hot rolling forming: before hot rolling, heat the steel ingot furnace to 1200℃, keep it at 1200℃ for 4 hours for high temperature homogenization annealing to make its internal structure uniform; after keeping it at 1200℃ for 4 hours, take it out of the furnace for hot rolling forming, the initial hot rolling temperature is T=1150±20℃, the final rolling temperature is T=1000±20℃, and the temperature is measured using an infrared thermometer. The hot rolling reduction rate is 10%~12%, and it is air-cooled to room temperature after rolling.

[0095] The hot-rolled steel was subjected to solution treatment and aging treatment in sequence. The solution treatment process was as follows: keeping the steel at 850°C for 120 minutes, followed by water cooling to room temperature. The water temperature was always ≤40°C during the cooling process. The aging treatment process was as follows: keeping the solution-treated steel at 500°C for 6 hours, and then slowly cooling it to room temperature, i.e., 850°C×120 minutes+500°C×6 hours.

[0096] Figure 7 and Figure 8 The microstructure EBSD map and metallographic morphology of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in comparative example 1 are shown in FIG. Figure 7 and Figure 8 It can be seen that after aging, the high-strength and high-toughness steel for ultra-low temperature engineering is a martensite plus film-like austenite structure. The EBSD phase distribution diagram statistics show that 4.80% of film-like reverse transformed austenite is formed after aging treatment, and the grain size is greater than level 6.

[0097] The mechanical property test results of Comparative Example 1 are shown in Table 1. The tensile yield strength at room temperature is 1255 MPa, the tensile strength is 1396 MPa, the elongation is 19.4%, and the V-type impact energy KV2 at room temperature is 122 J. When stretched at -196°C, the yield strength is 1469 MPa, the tensile strength is 1933 MPa, the elongation is 20.3%, and the V-type impact energy KV2 at -196°C is 21 J.

[0098] Compared with Example 2, the excessively high Cr content and the low Ni content, after the same thermal processing and heat treatment process, will lead to a significant decrease in the austenite content, which is basically a full martensite structure with higher strength, but reduced impact toughness at -196°C.

[0099] Comparative Example 2

[0100] In this comparative example, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting and vacuum consumable remelting. The chemical composition and weight percentage of high-strength and high-toughness steel for ultra-low temperature engineering are as follows: C≤0.03%, Si=0.20%, Mn=0.20%, S≤0.003%, P≤0.012%, Cr=16.00%, Ni=10.00%, Co=6.50%, Mo=2.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities, Cr eq =18.8、Ni eq =16.6.

[0101] High temperature homogenization annealing treatment and hot rolling forming: before hot rolling, heat the steel ingot furnace to 1200℃, keep it at 1200℃ for 4 hours for high temperature homogenization annealing to make its internal structure uniform; after keeping it at 1200℃ for 4 hours, take it out of the furnace for hot rolling forming, the initial hot rolling temperature is T=1150±20℃, the final rolling temperature is T=1000±20℃, and the temperature is measured using an infrared thermometer. The hot rolling reduction rate is 10%~12%, and it is air-cooled to room temperature after rolling.

[0102] The hot-rolled steel was subjected to solution treatment and aging treatment in sequence. The solution treatment process was as follows: keeping the steel at 850°C for 120 minutes, followed by water cooling to room temperature. The water temperature was always ≤40°C during the cooling process. The aging treatment process was as follows: keeping the solution-treated steel at 500°C for 6 hours, and then slowly cooling it to room temperature, i.e., 850°C×120 minutes+500°C×6 hours.

[0103] Figure 9 and Figure 10 The microstructure EBSD map and metallographic morphology of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in comparative example 2 are shown in FIG. Figure 9 and Figure 10 It can be seen that after aging, the high-strength and high-toughness steel for ultra-low temperature engineering is a martensite plus film-like austenite structure. The EBSD phase distribution diagram statistics show that 67.3% of austenite is formed after aging treatment, and the grain size is greater than level 6.

[0104] The mechanical property test results of Comparative Example 2 are shown in Table 1. The tensile yield strength at room temperature is 917 MPa, the tensile strength is 984 MPa, the elongation is 24.3%, and the V-type impact energy KV2 at room temperature is 279 J. When stretched at -196°C, the yield strength is 1163 MPa, the tensile strength is 1677 MPa, the elongation is 36.9%, and the V-type impact energy KV2 at -196°C is 103 J.

[0105] Compared with Example 2, an excessively high Ni content, after the same thermal processing and heat treatment process, will lead to a significant increase in the austenite content, resulting in a martensite + austenite dual-phase structure. It has a good twip effect during the stretching process at -196°C, and the elongation is greatly increased compared to room temperature. However, an excessively high austenite content causes a decrease in strength.

[0106] Comparative Example 3

[0107] In this comparative example, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting + vacuum consumable remelting. The composition and weight percentage of high-strength and high-toughness steel for ultra-low temperature engineering are as follows: C≤0.03%, Si=0.20%, Mn=0.20%, S≤0.003%, P≤0.012%, Cr=10.00%, Ni=7.00%, Co=6.50%, Mo=2.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities. Cr in this alloy system eq =12.8、Ni eq =13.6.

[0108] High temperature homogenization annealing treatment and hot rolling forming: before hot rolling, heat the steel ingot furnace to 1200℃, keep it at 1200℃ for 4 hours for high temperature homogenization annealing to make its internal structure uniform; after keeping it at 1200℃ for 4 hours, take it out of the furnace for hot rolling forming, the initial hot rolling temperature is T=1150±20℃, the final rolling temperature is T=1000±20℃, and the temperature is measured using an infrared thermometer. The hot rolling reduction rate is 10%~12%, and it is air-cooled to room temperature after rolling.

[0109] The hot-rolled steel was subjected to solution treatment and aging treatment in sequence. The solution treatment process was as follows: keeping the steel at 900°C for 120 minutes, followed by water cooling to room temperature. The water temperature was always ≤40°C during the cooling process. The aging treatment process was as follows: keeping the solution-treated steel at 500°C for 6 hours, followed by slow cooling to room temperature, i.e., 900°C×120 minutes+500°C×6 hours.

[0110] Figure 11 and Figure 12 The microstructure EBSD map and metallographic morphology of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in comparative example 3 are shown in FIG. Figure 11 and Figure 12 It can be seen that after aging, the ultra-low temperature engineering high-strength and high-toughness steel is a martensite plus film-like austenite structure. The EBSD phase distribution diagram shows that only 10.5% of reverse transformed austenite is formed after aging treatment. This is because the high solution temperature leads to the growth of austenite grains and the reduction of dislocation density. The low dislocation density will reduce the phase transformation resistance of martensite phase transformation during austenite quenching, thereby reducing the austenite content.

[0111] The mechanical property test results of Comparative Example 3 are shown in Table 1. The tensile yield strength at room temperature is 1053 MPa, the tensile strength is 1118 MPa, the elongation is 18.9%, and the V-type impact energy KV2 at room temperature is 187 J. When stretched at -196°C, the yield strength is 1231 MPa, the tensile strength is 1737 MPa, the elongation is 19.3%, and the V-type impact energy KV2 at -196°C is 22 J.

[0112] Compared with Example 2, an excessively high solution temperature will cause the austenite to coarsen and the dislocation density to decrease, thereby significantly reducing the austenite content after aging. The excessively large austenite grain size and low austenite content will lead to a significant decrease in impact performance at an ultra-low temperature of -196°C.

[0113] Comparative Example 4

[0114] In this embodiment, ultra-low carbon ultra-pure iron and high-purity alloy raw materials are used to obtain steel ingots after vacuum induction melting + vacuum consumable remelting. The components and weight percentages of the ultra-low temperature engineering high-strength and high-toughness steel are as follows: C≤0.03%, Si=0.20%, Mn=0.20%, S≤0.003%, P≤0.012%, Cr=10.00%, Ni=7.00%, Co=6.50%, Mo=2.50%, V=0.10%, Al=0.08%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities. Under this alloy system, Cr eq =12.8、Ni eq =13.6.

[0115] High temperature homogenization annealing treatment and hot rolling forming: before hot rolling, heat the steel ingot furnace to 1200℃, keep it at 1200℃ for 4 hours for high temperature homogenization annealing to make its internal structure uniform; after keeping it at 1200℃ for 4 hours, take it out of the furnace for hot rolling forming, the initial hot rolling temperature is T=1150±20℃, the final rolling temperature is T=1000±20℃, and the temperature is measured using an infrared thermometer. The hot rolling reduction rate is 10%~12%, and it is air-cooled to room temperature after rolling.

[0116] The hot-rolled steel was subjected to solution treatment and aging treatment in sequence. The solution treatment process was as follows: keeping the steel at 700°C for 120 minutes, followed by water cooling to room temperature. The water temperature was always ≤40°C during the cooling process. The aging treatment process was as follows: keeping the solution-treated steel at 600°C for 6 hours, followed by slow cooling to room temperature, i.e., 700°C×120 minutes+600°C×6 hours.

[0117] Figure 13 and Figure 14 The microstructure EBSD map and metallographic morphology of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment in comparative example 4 are shown in FIG. Figure 13 and Figure 14 It can be seen that after aging, the high-strength and high-toughness steel for ultra-low temperature engineering is a martensite plus austenite structure. The EBSD phase distribution diagram shows that 18.3% of reversed austenite is formed after aging treatment. In addition, it can be observed that a large amount of block austenite is formed under this process. The block austenite is large in size and twins are observed in the austenite.

[0118] The mechanical property test results of Comparative Example 4 are shown in Table 1. The tensile yield strength at room temperature is 1098 MPa, the tensile strength is 1185 MPa, the elongation is 20.9%, and the V-type impact energy KV2 at room temperature is 204 J; when stretched at -196°C, the yield strength is 1276 MPa, the tensile strength is 1783 MPa, the elongation is 23.2%, and the V-type impact energy KV2 at -196°C is 51 J.

[0119] Compared with Example 3, under the same conditions of smelting and hot forming processes, an excessively low solution temperature will result in insufficient solution of the alloying elements. At the same time, the austenite content of the new steel decreases after aging, and some larger blocky austenite is generated, resulting in a decrease in strength and impact strength at an ultra-low temperature of -196°C.

[0120] Table 1 Mechanical properties of high strength and high toughness steel for ultra-low temperature engineering in Examples and Comparative Examples

[0121]

[0122] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A high strength and high toughness steel for ultra-low temperature engineering, characterized in that: The chemical composition and weight percentage of the high-strength and high-toughness steel for ultra-low temperature engineering are: C≤0.03%, Si=0.10%~0.50%, Mn=0.10%~0.50%, S≤0.003%, P≤0.012%, Cr=10.00%~12.00%, Ni=7.00%~9.00%, Co=6.50%~7.50%, Mo=2.50%~3.50%, V=0.02%~0.10%, Al=0.08%~0.15%, O≤10ppm, N≤40ppm, H≤1.5ppm, and the balance is Fe and unavoidable impurities.

2. The high strength and high toughness steel for ultra-low temperature engineering according to claim 1, characterized in that: Control Cr eq Content and Ni eq content: Cr eq =Cr+Mo+1.5Si+0.5Nb+2Ti; Ni eq =Ni+Co+0.5Mn+0.3Cu+30C+30N; Among them: Cr eq <18.0%;8%<Ni eq <26.0%; The content of each element is expressed in mass percentage.

3. The high strength and high toughness steel for ultra-low temperature engineering according to claim 1, characterized in that: The high-strength and high-toughness steel for ultra-low temperature engineering has a grain size grade of 6 or above, a matrix structure of fine lath martensite, and a film-like reversed austenite with a volume fraction of 26% to 50% in the matrix structure.

4. The high strength and high toughness steel for ultra-low temperature engineering according to claim 1, characterized in that: When stretched at room temperature, the yield strength of high-strength and high-toughness steel for ultra-low temperature engineering shall be ≥1100MPa, the tensile strength shall be ≥1200MPa, the elongation shall be ≥20%, and the room temperature V-type impact energy KV2 shall be ≥200J; When stretched at -196℃, the yield strength of high-strength and high-toughness steel for ultra-low temperature engineering is ≥1300MPa, the tensile strength is ≥1800MPa, the elongation is ≥20%, and the V-type impact energy KV2 at -196℃ is ≥60J.

5. A method for preparing the high-strength and high-toughness steel for ultra-low temperature engineering according to any one of claims 1 to 4, characterized in that: The steps include: Step (1) adopts vacuum induction melting and vacuum consumable remelting double smelting process, prepares metal raw materials according to chemical composition and weight percentage, casts the obtained smelting liquid to obtain steel ingot, and ensures that the C content of the steel ingot is ≤0.03% after smelting; Step (2), forging hot forming or rolling hot forming; The forging hot forming process includes: performing high-temperature homogenization annealing on the steel ingot before forging, heating in a furnace-heated manner, a forging heating temperature of 1150° C. to 1200° C., a holding time of 1.5 hours / 100 mm forging effective thickness to 2 hours / 100 mm forging effective thickness, a start forging temperature of 1100° C. to 1150° C., and a final forging temperature of 1000° C. to 1050° C.; The hot rolling forming process includes: performing high temperature homogenization annealing on the steel ingot before hot rolling, wherein the hot rolling heating temperature is 1150°C to 1200°C and the holding time is 4 to 8 hours to make the internal structure uniform; the hot rolling start temperature is 1100°C to 1150°C and the final rolling temperature is 950°C to 1050°C; Step (3), heat treatment process; The steel material after hot forming in step (2) is subjected to solution treatment and aging heat treatment in sequence to obtain the high-strength and high-toughness steel for ultra-low temperature engineering; the solution temperature is 750°C to 850°C, the holding time is 60min to 180min, and the steel material is water-cooled to room temperature, and the water temperature is ensured to be ≤40°C during the cooling process; the aging temperature is 500°C to 600°C, the holding time is 4h to 10h, and the steel material is air-cooled or slowly cooled to room temperature.

6. The preparation method according to claim 5, characterized in that In step (1), ultra-low carbon ultra-pure iron and high-purity alloy are used as raw materials, and electrode rods are prepared by vacuum induction melting, and then steel ingots are obtained by vacuum consumable remelting; high vacuum smelting is adopted throughout the process, and the vacuum degree during vacuum induction melting reaches below 0.1 Pa; during vacuum induction melting, the refining temperature reaches 1550℃~1600℃, the refining time is not less than 40 minutes, and the stirring time is not less than 5 minutes; the vacuum induction melting pouring temperature is 1530℃-1550℃; the vacuum degree during vacuum consumable remelting is maintained at 10 -2 Pa and below, the melting rate is 100Kg / h~260Kg / h.

7. The preparation method according to claim 5, characterized in that The high temperature homogenization annealing before forging has a heating rate of ≤80°C / h below 700°C, a total forging ratio of ≥5, and air cooling or heat preservation and slow cooling to room temperature after forging deformation.

8. The preparation method according to claim 5, characterized in that The rolling hot forming has a pass reduction rate of 10% to 12%, and the hot rolling forming is followed by air cooling or slow cooling to room temperature.

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