A high-strength and high-toughness steel for ultra-low temperature engineering and a preparation method thereof

Abstract

The application belongs to the technical field of ultralow-temperature materials, and discloses a high-strength and high-toughness steel for ultralow-temperature engineering and a preparation method thereof. eq The chemical composition ratio of the steel contains ultralow-carbon and controlled Cr eq content and Ni content, and is combined with a vacuum induction melting and vacuum consumable remelting duplex smelting process, so that the grain size grade of the new steel is above 6, the matrix structure is fine lath martensite, and the volume fraction of thin film reverse transformation austenite is 26% to 50%. The high-strength and high-toughness steel for ultralow-temperature engineering has a room temperature yield strength of no less than 1100 MPa, a room temperature tensile strength of no less than 1200 MPa, an elongation of no less than 20%, a room temperature V-type impact energy KV2 of no less than 200 J, a-196℃ yield strength of no less than 1300 MPa, a-196℃ tensile strength of no less than 1800 MPa, an elongation of no less than 20%, and a-196℃ V-type impact energy KV2 of no less than 60 J.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of ultralow-temperature materials, and particularly relates to a high-strength and high-toughness steel for ultralow-temperature engineering and a preparation method thereof. BACKGROUND

[0002] With the development of the field of low-temperature engineering, the demand for high-performance materials serving in an ultralow-temperature environment is increasingly urgent. Large low-temperature engineering components serve in an ultralow-temperature, alternating wide temperature range (77-323K), impact, and heavy load extreme working condition for a long time, and new service conditions put forward higher requirements for the high strength and high toughness, high corrosion resistance, and processing formability of the materials, especially the requirement that the materials still have good strength and toughness matching in an ultralow-temperature (-196℃) environment. The research on the steel for ultralow-temperature engineering in China mainly focuses on improving the strength.

[0003] The patent application for invention 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 balance is Fe and inevitable impurities. The yield strength is 690MPa-1388MPa, the tensile strength is 1200MPa-1670MPa, but the plasticity is only greater than 10%, and the strength and plasticity matching cannot be good.

[0004] The patent application for invention with publication number CN106906429A discloses an ultrahigh-strength martensitic stainless steel and a preparation method thereof. The composition of the stainless steel is (mass percentage, %) C=0.10%-0.25%, Cr=11.0%-17.0%, Ni=0.1%-4.0%, Cu=0.1%-0.3%, and the balance is iron and inevitable impurity elements. The yield strength is 1300MPa, the tensile strength is 1600MPa, and the plasticity is 16%. The above scheme has relatively high strength, but the high carbon content will deteriorate the corrosion resistance, ultralow-temperature toughness, and welding formability, limiting the application at -196℃.

[0005] The patent with application publication number CN109439870A discloses a method for improving the low-temperature impact energy of 17-4PH martensitic age-hardening stainless steel forgings based on organization control. The reasonable forging heating temperature, deformation amount, and deformation direction are controlled, and then the small high-temperature ferrite is further eliminated in the recrystallization process through two solid solution + aging heat treatments, so as to improve the low-temperature impact energy. However, the KV2(-40℃) is only 27J, which is difficult to meet the requirement of high toughness at -196℃.

[0006] Patent with publication number CN111118258A discloses "a heat treatment method for improving the low-temperature impact toughness of 00Cr12Ni10MoTi martensitic age-hardening stainless steel", which makes the low-temperature impact toughness reach AKV(-196℃): 90~140J through a complex heat treatment process, but the highest yield strength is only 890MPa, and it is difficult to achieve good strength and toughness matching at-196℃.

[0007] Therefore, how to realize high strength and good low-temperature toughness of ultra-low-temperature engineering steel in ultra-low-temperature(-196℃) environment has important theoretical significance and practical value. Based on this, the present application provides an ultra-low-temperature engineering high-strength high-toughness steel and a preparation method thereof, which realizes high strength and good low-temperature toughness in ultra-low-temperature(-196℃) environment through reasonable alloy system and organization design, combined with high-purity melting and simple and wide process window heat treatment process, forms the ultra-low-temperature environment strength and toughness matching mechanism of "fine lath martensite matrix, thin film reverse austenite and high defect density", and further realizes the improvement of the mechanical properties of ultra-low-temperature materials. SUMMARY

[0008] Therefore, the present application provides an ultra-low-temperature engineering high-strength high-toughness steel and a preparation method thereof, which realizes high strength and good low-temperature toughness in ultra-low-temperature(-196℃) environment through reasonable alloy system and organization design, combined with high-purity melting and simple and wide process window heat treatment process, forms the ultra-low-temperature environment strength and toughness matching mechanism of "fine lath martensite matrix, thin film reverse austenite and high defect density", and further realizes the improvement of the mechanical properties of ultra-low-temperature materials. eq The present application provides an ultra-low-temperature engineering high-strength high-toughness steel and a preparation method thereof, which realizes high strength and good low-temperature toughness in ultra-low-temperature(-196℃) environment through reasonable alloy system and organization design, combined with high-purity melting and simple and wide process window heat treatment process, forms the ultra-low-temperature environment strength and toughness matching mechanism of "fine lath martensite matrix, thin film reverse austenite and high defect density", and further realizes the improvement of the mechanical properties of ultra-low-temperature materials. eq The present application provides an ultra-low-temperature engineering high-strength high-toughness steel and a preparation method thereof, which realizes high strength and good low-temperature toughness in ultra-low-temperature(-196℃) environment through reasonable alloy system and organization design, combined with high-purity melting and simple and wide process window heat treatment process, forms the ultra-low-temperature environment strength and toughness matching mechanism of "fine lath martensite matrix, thin film reverse austenite and high defect density", and further realizes the improvement of the mechanical properties of ultra-low-temperature materials.

[0009] In order to achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0010] The application discloses a high-strength and high-toughness steel for super-low temperature engineering, and chemical compositions 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 rest is Fe and inevitable 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] Wherein: Cr eq <18.0%;8%<Ni eq <26.0%;

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

[0016] The high-strength and high-toughness steel for super-low temperature engineering has a grain size grade of more than 6, a fine lath martensite matrix structure, and thin film-shaped reversed austenite with a volume fraction of 26%-50% in the matrix structure.

[0017] When stretched at room temperature, the high-strength and high-toughness steel for super-low temperature engineering has a yield strength of >=1100MPa, a tensile strength of >=1200MPa, an elongation of >=20%, and a room temperature V-type impact energy KV2 of >=200J.

[0018] When stretched at-196 DEG C, the high-strength and high-toughness steel for super-low temperature engineering has a yield strength of >=1300MPa, a tensile strength of >=1800MPa, an elongation of >=20%, and a-196 DEG C V-type impact energy KV2 of >=60J.

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

[0020] In step (1), a vacuum induction melting and vacuum self-consumption remelting double-melting process is adopted, metal raw materials are prepared according to the chemical compositions and weight percentages, the obtained smelting liquid is cast to obtain a steel ingot, and the C content in the steel ingot after smelting is ensured to be <=0.03%;

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

[0022] The forging hot forming: the steel ingot is subjected to pre-forging high-temperature homogenizing annealing, the heating mode is furnace heating, the forging heating temperature is 1150-1200 DEG C, the holding time is 1.5 hours / 100mm effective thickness of the forged piece-2 hours / 100mm effective thickness of the forged piece, the open forging temperature is 1100-1150 DEG C, and the final forging temperature is 1000-1050 DEG C.

[0023] The rolling hot forming: the steel ingot is subjected to pre-rolling high-temperature homogenizing annealing, the hot rolling heating temperature is 1150-1200 DEG C, and the holding time is 4-8 hours to make the internal structure uniform; the hot rolling open rolling temperature is 1100-1150 DEG C, and the final rolling temperature is 950-1050 DEG C.

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

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

[0026] In step (1), ultra-low-carbon ultra-pure iron and high-purity alloy are used as raw materials, an electrode rod is prepared through vacuum induction smelting, and then a steel ingot is obtained through vacuum consumable remelting; high-vacuum smelting is adopted throughout the whole process, and the vacuum degree reaches below 0.1 Pa during the vacuum induction smelting process; during the vacuum induction smelting, the refining temperature reaches 1550-1600 DEG C, the refining time is not less than 40 min, and the stirring time is not less than 5 min; the vacuum induction smelting pouring temperature is 1530-1550 DEG C; during the vacuum consumable remelting process, the vacuum degree is kept at 10 Pa and below, and the melting speed is 100-260 Kg / h. -2 Pa and below, and the melting speed is 100-260 Kg / h.

[0027] The pre-forging high-temperature homogenizing annealing has a heating rate of ≤80 DEG C / h below 700 DEG C, and a total forging ratio of ≥5, and the forged product is air cooled or slowly cooled to room temperature after deformation.

[0028] The rolling hot forming has a rolling reduction of 10-12%, and the hot formed product is air cooled or slowly cooled to room temperature.

[0029] The high-strength high-toughness steel for ultra-low-temperature engineering provided by the application does not rely on carbon elements to improve the strength, but adopts an ultra-low-carbon component design, strictly controls C ≤0.03%, and avoids the formation of Cr 23C6, Cr7C3, etc. brittle carbides to prevent grain boundary weakening and low-temperature toughness decline.

[0030] The high-strength high-toughness steel for ultra-low temperature engineering provided by the application has a mass fraction of Cr of 10.00% to 12.00%. On the one hand, Cr element is a key element for steel to ensure good corrosion resistance, and its content is usually required to be greater than 10%, so as to ensure that corrosion failure does not occur, which is crucial for the steel for ultra-low temperature engineering serving in a wide temperature range. Cr element is also a ferrite forming element, which can reduce the austenite phase region and lower the Ms point. For the steel with a high Cr content, Mn, Ni and other austenite forming elements must be added to prevent the formation of part of ferrite structure, thereby deteriorating impact toughness and corrosion resistance. Meanwhile, Cr element can form Cr 23 C6, Cr7C3, etc. brittle carbides to prevent grain boundary weakening and low-temperature toughness decline.

[0031] The high-strength high-toughness steel for ultra-low temperature engineering provided by the application has a mass fraction of Ni of 7.00% to 9.00%. Ni element, as an austenite stabilizing element, cooperates with Cr, and with the increase of the Ni content in the steel, the austenite phase region moves to the high-Cr direction, that is, the increase of Cr in the steel will not form single ferrite structure. Ni element can promote the formation of reverse austenite in the aging process, and the austenite rich in Ni will not undergo martensite phase change at low temperature, and has good thermodynamic stability, thereby significantly increasing the low-temperature impact toughness.

[0032] The high-strength high-toughness steel for ultra-low temperature engineering provided by the application has a mass fraction of Co of 6.50% to 7.50%. Co element can greatly improve the strength of the steel for ultra-low temperature engineering. On the one hand, Co element can enhance the solid solution strengthening effect and improve the strength of martensite itself. On the other hand, Co element has a cooperative effect with Mo element, that is, Co element can reduce the solid solubility of Mo in the martensite matrix, promote the precipitation of Mo-containing strengthening phases (Mo-rich phase, Ni3Mo, Fe2Mo) in the aging process. In addition, Co element can also inhibit the recovery of dislocation substructure in the martensite matrix, thereby enhancing the aging strengthening effect, so that the strength of the high-strength high-toughness steel is greatly improved.

[0033] The high-strength high-toughness steel for ultra-low temperature engineering provided by the application has a mass fraction of Mo of 2.50% to 3.50%. Mo element is beneficial to the corrosion resistance, strength and toughness of the high-strength high-toughness steel for ultra-low temperature engineering. On the one hand, Mo-rich phase and Fe2Mo hexagonal Laves phase and other aging strengthening phases will be precipitated in the aging process; on the other hand, the Mo-rich precipitated phase and η-Ni3Ti will form a unique core-shell structure, which can prevent and inhibit the coarsening of Ni3Ti, and the Mo-rich phase can also inhibit the coarsening of Ni (Al, Fe) phase, and prevent the precipitated phase from precipitating along the original austenite grain boundary, thereby avoiding intergranular fracture.

[0034] The high-strength high-toughness steel for ultra-low temperature engineering provided by the application has the mass fraction of Si and Mn being 0.10%-0.50% and 0.10%-0.50% respectively. Silicon and manganese mainly play the role of deoxidation in the steel, the steel of the application adopts vacuum induction melting and vacuum consumable remelting, the raw material adopts ultra-low carbon ultra-pure iron and high-purity alloy raw material, and there is no need to add too much silicon element and manganese element.

[0035] Compared with the prior art, the application has the beneficial effects that:

[0036] (1) The high-strength high-toughness steel for ultra-low temperature engineering has the following advantages: through the combination of organizational design, Cr eq , Ni eq , optimization of alloy element ratio, vacuum induction melting and vacuum consumable remelting duplex smelting process and the corresponding thermal mechanical treatment process, the regulation of martensite matrix and precipitation strengthening phase is realized, and the reverse transformation austenite is introduced into the martensite matrix. By regulating the morphology, size and distribution of nanoscale precipitates and reverse transformation austenite, and by introducing 26%-50% of thin film reverse transformation austenite into the high dislocation density martensite matrix, the good matching of strength and toughness under ultra-low temperature environment is realized.

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

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

[0039] (4) The application has wide application prospects in aerospace heavy rocket engine, low-temperature wind tunnel parts and the like. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 It is the EBSD graph of the microstructure of the high-strength high-toughness steel for ultra-low temperature engineering after aging heat treatment of example 1.

[0041] Figure 2 It is the metallographic morphology graph of the high-strength high-toughness steel for ultra-low temperature engineering after aging heat treatment of example 1.

[0042] Figure 3 It is the EBSD graph of the microstructure of the high-strength high-toughness steel for ultra-low temperature engineering after aging heat treatment of example 2.

[0043] Figure 4 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Example 2;

[0044] Figure 5 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Example 3;

[0045] Figure 6 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Example 3;

[0046] Figure 7 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 1;

[0047] Figure 8 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 1;

[0048] Figure 9 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 2;

[0049] Figure 10 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 2;

[0050] Figure 11 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 3;

[0051] Figure 12 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 3;

[0052] Figure 13 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 4;

[0053] Figure 14 Microstructure EBSD map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging heat treatment of Comparative Example 4. DETAILED DESCRIPTION

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

[0055] The high-strength and high-toughness steel for ultra-low temperature engineering has a chemical composition by weight percentage of 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 of Fe and inevitable impurities.

[0056] The high-strength and high-toughness steel for ultra-low temperature engineering forms a microstructure of lath martensite plus 26%-50% thin film reverse transformation austenite after aging by controlling the chromium-nickel equivalent, avoiding the formation of ferrite phase, and Cr eq and Ni eq satisfying the following relationship:

[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 in mass percentage.

[0062] Further, the high-strength and high-toughness steel for ultra-low temperature engineering has an alloy composition design taking Co and Mo as main strengthening elements, promotes the precipitation of Mo-containing precipitation strengthening phase (Laves phase) during aging by the synergistic effect of Co-Mo, promotes the formation of reverse austenite during aging by the element Ni, thereby significantly increasing the low-temperature impact toughness, and the content of Al as a deoxidizer is controlled at 0.08-0.15%, and other harmful elements C, Si, Mn, S, P, O and N are controlled at a low level, thereby improving the purity of the molten steel, avoiding the formation of large-size inclusions, and thereby improving the ultra-low temperature toughness of the steel.

[0063] Further, the high-strength and high-toughness steel for ultra-low temperature engineering has a tensile yield strength of ≥1100 MPa, a tensile strength of ≥1200 MPa, an elongation of ≥20%, and a room temperature V-type impact energy KV2 of ≥200 J at room temperature; and has a tensile yield strength of ≥1300 MPa, a tensile strength of ≥1800 MPa, an elongation of ≥20%, and a V-type impact energy KV2 of ≥60 J at -196 ℃.

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

[0065] (1) A vacuum induction melting and vacuum consumable remelting double-melting process is used, and metal raw materials are prepared according to the chemical composition and the weight percentage, and the obtained smelting liquid is cast to obtain a steel ingot, so as to ensure that C in the steel ingot after smelting is ≤0.03%;

[0066] (2) hot forming by forging or rolling;

[0067] The hot forming by forging comprises the following steps: before forging, the steel ingot is subjected to high-temperature homogenizing annealing, the heating mode is furnace heating, the forging heating temperature is 1150-1200 ℃, the holding time is 1.5 hours / 100 mm effective thickness of the forged piece-2 hours / 100 mm effective thickness of the forged piece, the open forging temperature is 1100-1150 ℃, and the final forging temperature is 1000-1050 ℃.

[0068] The hot forming by rolling comprises the following steps: before hot rolling, the steel ingot is subjected to high-temperature homogenizing annealing, the hot rolling heating temperature is 1150-1200 ℃, and the holding time is 4-8 hours to make the internal structure uniform; the open rolling temperature is 1100-1150 ℃, and the final rolling temperature is 950-1050 ℃.

[0069] (3) a heat treatment process;

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

[0071] Example 1

[0072] In this embodiment, ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used to obtain a steel ingot after vacuum induction melting and vacuum consumable remelting. The chemical composition of the ultra-low temperature engineering high-strength and high-toughness steel is 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 the alloy system, Cr eq =12.8, Ni eq =13.6.

[0073] High-temperature homogenization annealing treatment and forging hot forming: the steel ingot is heated to 1200°C before forging, the heating mode is furnace heating, the heating rate is 70°C / h below 700°C, and the heating rate is 200°C / h above 700°C, and the steel ingot is kept at temperature for 6h before forging, the forging starting temperature is 1130±10°C, and the final forging temperature is 1030±10°C. After the end of forging, air cooling to room temperature.

[0074] The forged steel is sequentially subjected to solid solution treatment and aging treatment. The solid solution treatment process is: keeping at 850°C for 120min, and then water cooling to room temperature, the water temperature is always ≤40°C during the cooling process; the aging treatment process is: keeping the steel after solid solution treatment at 500°C for 6h, and then slowly cooling to room temperature with asbestos, i.e. 850°C×120min+500°C×6h.

[0075] Figure 1 and Figure 2 The EBSD and metallographic morphology maps of the ultra-low temperature engineering high-strength and high-toughness steel after aging treatment of Example 1 are shown in FIGS. 1 and 2. Figure 1 and Figure 2 It can be seen from FIGS. 1 and 2 that the ultra-low temperature engineering high-strength and high-toughness steel after aging treatment is a structure of martensite and thin film reverse transformation austenite, and the thin film reverse transformation austenite accounts for 28.3% after aging treatment, and the grain size is greater than 6 levels.

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

[0077] The high-strength and high-toughness steel for ultra-low temperature engineering can obtain good strength, 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.

[0078] Example 2

[0079] In this embodiment, the ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used to obtain the ingot after vacuum induction melting and vacuum consumable remelting, the chemical composition of the high-strength and high-toughness steel for ultra-low temperature engineering and the weight percentage 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 inevitable impurities, and the Cr eq =12.8, Ni eq =13.6 under the alloy system.

[0080] High-temperature homogenizing annealing treatment and rolling hot forming, the ingot is heated to 1200℃ before hot rolling, high-temperature homogenizing annealing is carried out at 1200℃ for 4h to make the internal organization uniform, and after 4h of heat preservation, hot rolling is carried out, the initial rolling temperature T=1150±20℃, the final rolling temperature T=1000±20℃, the temperature is measured by using an infrared thermometer, and the rolling pass reduction rate is 10%-12%, and the rolled material is air cooled to room temperature.

[0081] The hot-rolled steel is sequentially subjected to solid solution treatment and aging treatment, the solid solution treatment process is: 850℃ for 120min, and then water cooling to room temperature, and the water temperature is always ≤40℃ during the cooling process; the aging treatment process is: the solid solution treated steel is aged at 500℃ for 6h, and the asbestos is slowly cooled to room temperature, that is, 850℃×120min+500℃×6h.

[0082] Figure 3 and Figure 4 The microstructure EBSD map and metallographic morphology map of the high-strength and high-toughness steel for ultra-low temperature engineering after aging treatment of Example 2 can be seen from Figure 3 and Figure 4 It can be seen that the high-strength and high-toughness steel for ultra-low temperature engineering after aging treatment is a martensite plus thin film reverse transformation austenite organization, and the EBSD phase distribution graph statistics shows that 33.7% of thin film reverse transformation austenite is formed after aging treatment, and the grain size is greater than 6 levels.

[0083] The mechanical property detection results of the embodiment 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%, the room temperature V-type impact energy KV2 is 229 J; the tensile yield strength at-196 ℃ is 1325 MPa, the tensile strength is 1867 MPa, the elongation is 26.9%, and the V-type impact energy KV2 at-196 ℃ is 72 J.

[0084] The high-strength and high-toughness steel for ultra-low temperature engineering 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 the embodiment 2 in Table 1 is only a typical representative.

[0085] Embodiment 3

[0086] In the embodiment, the ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used, and the ingot is obtained after vacuum induction melting and vacuum consumable remelting, the composition and weight percentage of the high-strength and high-toughness steel for ultra-low temperature engineering 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≤10 ppm, N≤40 ppm, H≤1.5 ppm, and the balance is Fe and inevitable impurities, and the Cr eq =15.65, Ni eq =16.55 under the alloy system.

[0087] The high-temperature homogenization annealing treatment and rolling hot forming are as follows: the ingot furnace is heated to 1200 ℃, high-temperature homogenization annealing is carried out at 1200 ℃ for 4 h to make the internal organization uniform, the ingot is taken out after 4 h of heat preservation for hot rolling forming, the initial rolling temperature T is 1150±20 ℃, the final rolling temperature T is 1000±20 ℃, the temperature is measured by using an infrared thermometer, the rolling pass reduction rate is 10%~12%, and the rolled steel is air cooled to room temperature.

[0088] The hot-rolled steel is sequentially subjected to solid solution treatment and aging treatment, the solid solution treatment process is: 750 ℃ for 120 min, and then water cooling to room temperature, the water temperature is always ≤40 ℃ during the cooling process; the aging treatment process is: the steel after solid solution treatment is aged at 600 ℃ for 6 h, and then slowly cooled to room temperature by using asbestos, that is, 750 ℃×120 min+600 ℃×6 h.

[0089] Figure 5 and Figure 6 The EBSD graph and the metallographic morphology graph of the high-strength and high-toughness steel for ultra-low temperature engineering after aging treatment of the embodiment 3 are shown in Figures 1 and 2. Figure 5 and Figure 6It can be seen that the high-strength and high-toughness steel for ultra-low temperature engineering after aging is a structure of martensite plus thin film reversed austenite, and the thin film reversed austenite is 46.3% after aging treatment according to the EBSD phase distribution statistics, and the grain size is greater than 6 levels.

[0090] The mechanical property detection 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%, the room temperature V-type impact energy KV2 is 233 J; the tensile 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℃ is 86 J.

[0091] The high-strength and high-toughness steel for ultra-low temperature engineering 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.

[0092] Comparative Example 1

[0093] In the present comparative example, ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used to obtain a steel ingot after vacuum induction melting + vacuum consumable remelting, the chemical composition of the high-strength and high-toughness steel for ultra-low temperature engineering and the weight percentage 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, the balance being Fe and unavoidable impurities, Cr eq =18.8, Ni eq =13.6.

[0094] High-temperature homogenization annealing treatment and rolling hot forming, the ingot furnace is heated to 1200℃ before hot rolling, high-temperature homogenization annealing is carried out at 1200℃ for 4h to make the internal organization uniform; after 4h of heat preservation, hot rolling forming is carried out, the initial rolling temperature T=1150±20℃, the final rolling temperature T=1000±20℃, the temperature is measured by using an infrared thermometer, and the rolling pass reduction rate is 10%-12%, and the rolled steel is air cooled to room temperature.

[0095] The hot-rolled steel is sequentially subjected to solid solution treatment and aging treatment, the solid solution treatment process is: 850℃ for 120min, and then water cooling to room temperature, the water temperature is always ≤40℃ during the cooling process; the aging treatment process is: the steel after solid solution treatment is aged at 500℃ for 6h, and then slowly cooled to room temperature by using asbestos, that is, 850℃×120min+500℃×6h.

[0096] Figure 7 and Figure 8 EBSD and metallographic morphology of the microstructure of the high-strength and high-toughness steel for ultra-low temperature engineering after aging treatment of Comparative Example 1. From Figure 7 and Figure 8 It can be seen that the high-strength and high-toughness steel for ultra-low temperature engineering after aging treatment is a structure of martensite plus thin film austenite, and the thin film reverse austenite is 4.80% after EBSD phase distribution statistics, and the grain size is greater than 6 levels.

[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%, the room temperature V-type impact energy KV2 is 122 J; the tensile yield strength at -196℃ is 1469 MPa, the tensile strength is 1933 MPa, the elongation is 20.3%, and the V-type impact energy KV2 at -196℃ is 21 J.

[0098] Compared with Example 2, the excessively high Cr content and the lower Ni content will cause the austenite content to decrease significantly after the same heat treatment and heat treatment process, and the structure is basically full martensite, the strength is higher, but the impact toughness at -196℃ is reduced.

[0099] Comparative Example 2

[0100] In this comparative example, ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used to obtain a steel ingot after vacuum induction melting and vacuum consumable remelting, the chemical composition of the high-strength and high-toughness steel for ultra-low temperature engineering and the weight percentage 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, the balance being Fe and unavoidable impurities, Cr eq =18.8, Ni eq =16.6.

[0101] High-temperature homogenizing annealing and hot rolling forming, the ingot furnace is heated to 1200℃ before hot rolling, high-temperature homogenizing annealing is carried out at 1200℃ for 4h to make the internal structure uniform; after 4h of heat preservation, hot rolling forming is carried out, the initial rolling temperature T=1150±20℃, the final rolling temperature T=1000±20℃, the temperature is measured by an infrared thermometer, the hot rolling pass reduction rate is 10%~12%, and the rolling is air cooled to room temperature.

[0102] The hot-rolled steel material is sequentially subjected to solid solution treatment and aging treatment, the solid solution treatment process is: 850℃ for 120min, then water cooling to room temperature, the water temperature is always ≤40℃ during the cooling process; the aging treatment process is: the steel material after solid solution treatment is kept at 500℃ for 6h, and then slowly cooled to room temperature by asbestos, that is, 850℃×120min+500℃×6h.

[0103] Figure 9 and Figure 10 The EBSD 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 the following figures. Figure 9 and Figure 10 It can be seen that the high-strength and high-toughness steel for ultra-low temperature engineering after aging treatment is a structure of martensite plus thin film austenite, and the grain size is greater than 6 levels according to the EBSD phase distribution statistics.

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

[0105] Compared with Example 2, the excessively high Ni content will cause the austenite content to increase significantly after the same hot working and heat treatment process, and the steel is a dual-phase structure of martensite + austenite, which has a good twip effect during the tensile process at-196℃, and the elongation is greatly increased at room temperature. However, the excessively high austenite content causes the strength to decrease.

[0106] Comparative Example 3

[0107] In this comparative example, ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used to obtain a steel ingot after vacuum induction melting + vacuum consumable remelting, the composition and weight percentage of the high-strength and high-toughness steel for ultra-low temperature engineering are: 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.

[0108] High temperature homogenization annealing treatment and hot rolling forming, the ingot furnace is heated to 1200℃ before hot rolling, high temperature homogenization annealing is carried out at 1200℃ for 4h to make the internal organization uniform; after 4h of heat preservation, hot rolling forming is carried out, the initial rolling temperature T = 1150±20℃, the final rolling temperature T = 1000±20℃, the temperature is measured by using infrared thermometer, the hot rolling pass reduction rate is 10%~12%, and the air cooling is carried out to room temperature after rolling.

[0109] The hot-rolled steel is sequentially subjected to solid solution treatment and aging treatment, the solid solution treatment process is: 900℃ for 120min, and then water cooling to room temperature, the water temperature is always ≤40℃ during the cooling process; the aging treatment process is: the steel after solid solution treatment is aged at 500℃ for 6h, and the asbestos is slowly cooled to room temperature, that is, 900℃×120min+500℃×6h.

[0110] Figure 11 and Figure 12 The microstructure EBSD map and metallographic morphology map of the high strength and high toughness steel for ultra-low temperature engineering after aging treatment of comparative example 3 are shown in the following figure. Figure 11 and Figure 12 It can be seen that the high strength and high toughness steel for ultra-low temperature engineering after aging treatment is a structure of martensite and thin film austenite, and it is known from the EBSD phase distribution graph statistics that only 10.5% of the reversed austenite is formed after aging treatment, which is due to the high solid solution temperature, leading to the growth of austenite grains and the reduction of dislocation density, and the low dislocation density will reduce the phase transition resistance of martensite phase change during the austenite quenching process, thereby reducing the content of austenite.

[0111] The mechanical property test results of comparative example 3 are shown in table 1, the tensile yield strength at room temperature is 1053MPa, the tensile strength is 1118MPa, the elongation is 18.9%, the room temperature V-type impact energy KV2=187J; the tensile yield strength at-196℃ is 1231MPa, the tensile strength is 1737MPa, the elongation is 19.3%, and the V-type impact energy KV2 at-196℃ is 22J.

[0112] Compared with example 2, the too high solid solution temperature will cause the austenite to be coarsened and the dislocation density to be reduced, so that the content of austenite after aging is significantly reduced, and the large austenite grain size and low austenite content lead to a large decrease in impact performance at-196℃ ultra-low temperature.

[0113] Comparative example 4

[0114] In this embodiment, ultra-low carbon and ultra-pure iron and high-purity alloy raw materials are used to obtain a steel ingot after vacuum induction melting + vacuum consumable remelting. The composition of the ultra-low temperature engineering high-strength and high-toughness steel is 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. In this alloy system, Cr eq =12.8, Ni eq =13.6.

[0115] High-temperature homogenization annealing treatment and hot rolling forming are performed. Before hot rolling, the steel ingot furnace is heated to 1200°C, and high-temperature homogenization annealing is performed at 1200°C for 4h to make the internal organization uniform. After 4h of heat preservation, the furnace is discharged for hot rolling forming. The initial rolling temperature T=1150±20°C, and the final rolling temperature T=1000±20°C. An infrared thermometer is used for temperature measurement. The hot rolling pass reduction rate is 10%~12%, and the rolled material is air cooled to room temperature.

[0116] The hot-rolled steel material is sequentially subjected to solid solution treatment and aging treatment. The solid solution treatment process is: 700°C for 120min, and then water cooling to room temperature, and the water temperature is always ≤40°C during the cooling process. The aging treatment process is: the solid solution treated steel material is aged at 600°C for 6h, and then slowly cooled to room temperature with asbestos, i.e. 700°C×120min+600°C×6h.

[0117] Figure 13 Figure 14 Fig. 2 is an EBSD phase distribution map and metallographic morphology map of the ultra-low temperature engineering high-strength and high-toughness steel after aging treatment according to the comparative example 4. It can be seen from Figure 13 Figure 14 that the ultra-low temperature engineering high-strength and high-toughness steel after aging treatment is a martensite plus austenite organization. It can be known from the EBSD phase distribution map that 18.3% of the reverse transformation austenite is formed after aging treatment. In addition, a large amount of blocky austenite is formed under this process, and the size of the blocky austenite is large, and the existence of twinning in the austenite is observed.

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

[0119] ​​Compared with Example 3, under the same conditions of smelting and hot forming process, too low solid solution temperature can result in insufficient solid solution of alloying elements, and the austenite content of the new type steel after aging is reduced, part of large blocky austenite is generated, so that the strength and impact at-196℃ are reduced.

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

[0121]

[0122] The above merely describes the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can make equivalent replacements or changes according to the technical solution and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A high-strength high-toughness steel for cryogenic engineering, characterized in that, The chemical composition of the high-strength and high-toughness steel for ultra-low temperature engineering in percentage by weight is: 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 of Fe and inevitable impurities.

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

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

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

5. A method of manufacturing the high-strength high-toughness steel for ultra- low-temperature engineering according to any one of claims 1 to 4, characterized in that, The method comprises the following steps: Step (1), a vacuum induction melting and vacuum consumable remelting double melting process is adopted, metal raw materials are prepared according to the chemical composition and percentage by weight, the obtained smelting liquid is cast to obtain a steel ingot, and the C in the steel ingot after smelting is ensured to be ≤0.03%; Step (2), hot forming by forging or rolling; The hot forming by forging comprises the following steps: high-temperature homogenization annealing of the steel ingot before forging, furnace heating, a forging heating temperature of 1150℃-1200℃, a holding time of 1.5 hours / 100mm effective thickness of the forged piece-2 hours / 100mm effective thickness of the forged piece, a starting forging temperature of 1100℃-1150℃, and a final forging temperature of 1000℃-1050℃; The hot forming by rolling comprises the following steps: high-temperature homogenization annealing of the steel ingot before hot rolling, a hot rolling heating temperature of 1150℃-1200℃, a holding time of 4-8 hours to make the internal structure uniform, a starting rolling temperature of 1100℃-1150℃, and a final rolling temperature of 950℃-1050℃; Step (3), a heat treatment process; The steel material after the hot forming of step (2) is subjected to solid solution treatment and aging heat treatment in sequence to obtain the high-strength and high-toughness steel for ultra-low temperature engineering; the solid solution temperature is 750℃-850℃, the holding time is 60min-180min, water cooling to room temperature, and the water temperature during the cooling process is ensured to be ≤40℃; the aging temperature is 500℃-600℃, the holding time is 4h-10h, and air cooling or slow cooling to room temperature.

6. The production method according to claim 5, characterized by, In step (1), ultra-low carbon and ultra-pure iron and high-purity alloy are used as raw materials, the electrode rod is prepared by vacuum induction melting, and then the ingot is obtained by vacuum consumable remelting; high vacuum melting is used throughout the process, the vacuum degree reaches below 0.1 Pa during vacuum induction melting; during vacuum induction melting, the refining temperature reaches 1550-1600°C, 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°C; during vacuum consumable remelting, the vacuum degree is kept at 10 -2 Pa and below, and the melting speed is 100-260 Kg / 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℃ / h below 700℃, and a total forging ratio of ≥5, and the forged piece is air cooled or slowly cooled to room temperature after deformation.

8. The preparation method according to claim 5, characterized in that The rolling hot forming pass reduction rate is 10%~12%, and the rolling hot forming is air cooled or slowly cooled to room temperature.

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