A nickel-based low-temperature steel and its production method

Abstract

The present invention belongs to the technical field of nickel-based low-temperature steel, and specifically relates to a nickel-based low-temperature steel and its production method, including rough rolling, medium rolling, pre-finishing rolling and finishing rolling. The temperature of the rolled piece obtained by medium rolling is 1020 - 1050 °C, and after medium rolling, it is subjected to water-cooling through water injection once, and then enters the slow-cooling channel for natural cooling until the temperature of the rolled piece when it enters pre-finishing rolling is 970 - 1020 °C; and after pre-finishing rolling, it is subjected to water-cooling through water injection twice, and then enters the slow-cooling channel for natural cooling until the temperature of the rolled piece when it enters finishing rolling is 790 - 930 °C; and after finishing rolling, it is subjected to water-cooling through water injection three times, and then enters the slow-cooling channel for natural cooling, so that the temperature of the rolled piece on the cooling bed during cooling on the cooling bed is 560 - 600 °C. The present invention has a specific microstructure, reduces the ductile-brittle transition temperature, can reasonably control surface cracks, and at the same time has good low-temperature mechanical properties under extremely cold conditions of -165 °C, and has great market advantages.

Description

Technical Field

[0001] The present invention belongs to the technical field of nickel-based cryogenic steels, and particularly relates to a nickel-based cryogenic steel and a production method thereof. Background Art

[0002] Cryogenic steels refer to alloy steels suitable for applications below 0°C. A typical example where cryogenic steel bars are required is the construction of low-temperature resistant steel bars for liquefied natural gas storage tanks (abbreviated as LNG) in the petrochemical industry. The basic requirements are as follows: (1) High strength, yield strength ≥ 400 MPa or ≥ 500 MPa; (2) The production process is not restricted, and the heat treatment with residual heat quenching process can be adopted for production; (3) There are certain requirements for low-temperature performance, which are specifically agreed upon by the project purchaser and the supplier. The lowest storage temperature is -165°C, and mainly certain requirements for low-temperature mechanical properties are required. However, so far, no public reports on the specific composition of the steel for cryogenic steel bars and its rolling process parameters have been seen.

[0003] At present, there are several Ni-based cryogenic steels in the world, but the Ni content is relatively high, above 2.5%, and the manufacturing cost is extremely high. Among them, the cryogenic steel with a Ni content of 2.5% has a service temperature of -70°C, the cryogenic steel with a Ni content of 3.5% has a service temperature of -110°C, and the cryogenic steel with a Ni content of 5% has a service temperature of -130°C. The existing Ni-based cryogenic steels have relatively low mechanical properties under extremely cold conditions of -165°C and cannot meet the use requirements under extremely cold conditions of -165°C.

[0004] Therefore, there is an urgent need in the art for a cryogenic steel that can be used under extremely cold conditions of -165°C. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defect that the low-temperature mechanical properties of nickel-based cryogenic steels in the prior art need to be further improved, and to provide a nickel-based cryogenic steel and a production method thereof. The Ni content of the nickel-based cryogenic steel can be 1 - 1.5%, which is much lower than the Ni content in mainstream nickel-based cryogenic steels, and has a specific microstructure, reduces the ductile-brittle transition temperature, can reasonably control surface cracks, and at the same time has good low-temperature mechanical properties under extremely cold conditions of -165°C, presenting great market advantages.

[0006] To achieve the above object, in a first aspect, the present invention provides a method for producing nickel-based low-temperature steel, including steelmaking, heating, rolling, and cooling on a cooling bed. The rolling includes rough rolling, medium rolling, pre-finishing rolling, and finishing rolling. The temperature of the rolled piece obtained after medium rolling is 1020 - 1050 °C, and after medium rolling, it is subjected to water-cooling through water once, with a water-cooling rate of 55 - 65 °C / s, and then enters a slow-cooling channel for natural cooling until the temperature of the rolled piece when it enters the pre-finishing rolling is 970 - 1020 °C; and after pre-finishing rolling, it is subjected to water-cooling through water twice, with a water-cooling rate of 160 - 220 °C / s, and then enters a slow-cooling channel for natural cooling until the temperature of the rolled piece when it enters the finishing rolling is 790 - 930 °C; and after finishing rolling, it is subjected to water-cooling through water three times, with a water-cooling rate of 350 - 460 °C / s, and then enters a slow-cooling channel for natural cooling, so that the temperature of the rolled piece on the cooling bed during the cooling on the cooling bed is 560 - 600 °C.

[0007] In some preferred embodiments, the temperature of the rolled piece obtained after pre-finishing rolling is 990 - 1050 °C, and the temperature of the rolled piece obtained after finishing rolling is 850 - 1010 °C.

[0008] In some preferred embodiments, the time for water-cooling through water once is 1 - 3 s, the time for water-cooling through water twice is 1 - 3 s, and the time for water-cooling through water three times is 1 - 3 s.

[0009] In some preferred embodiments, the water flow rate for water-cooling through water once is 85 - 95 m 3 / h, the water flow rate for water-cooling through water twice is 270 - 360 m 3 / h, and the water flow rate for water-cooling through water three times is 650 - 760 m 3 / h.

[0010] In some preferred embodiments, the rough rolling and medium rolling result in a stock shape compression ratio of the rolled piece obtained after medium rolling of more than 3.2.

[0011] In some preferred embodiments, during the rolling, the starting rolling temperature is controlled to be 1000 - 1050 °C, and the finishing rolling temperature is controlled to be 900 - 930 °C.

[0012] In some preferred embodiments, during the rolling, the finishing rolling speed is controlled to be 10 - 10.8 m / s.

[0013] In some preferred embodiments, during the rolling, it is controlled that the rolled piece is only under compressive stress between adjacent rolling mills.

[0014] In some preferred embodiments, during the rolling process, the heating process includes three-stage heating, namely a preheating stage, a heating stage, and a soaking stage. The temperature of the preheating stage is below 800°C, the temperature of the heating stage is 1050 - 1200°C, the temperature of the soaking stage is 1030 - 1180°C, and the total heating time is 85 - 95 minutes.

[0015] In some preferred embodiments, the composition of the nickel-based low-temperature steel includes: by mass percentage of elements, C 0.06 - 0.10%, Si 0.10 - 0.40%, Mn 1.30 - 1.60%, P ≤ 0.020%, S ≤ 0.020%, V 0.050 - 0.080%, Ni 1.30 - 1.50%, Cu 0.05 - 0.30%, Al 0.020 - 0.035%, O ≤ 10 ppm, and the balance is Fe and unavoidable impurities.

[0016] In some preferred embodiments, the nickel-based low-temperature steel also satisfies: Q ≤ 0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15.

[0017] In a second aspect, the present invention provides a nickel-based low-temperature steel prepared by the production method described in the first aspect; the obtained nickel-based low-temperature steel has cracks on its surface, the crack depth is below 280 μm, and the microstructure of the nickel-based low-temperature steel satisfies: on its surface, it is tempered sorbite and tempered martensite, and in its core, it is pearlite, ferrite, and granular bainite.

[0018] In some preferred embodiments, by area fraction, at a depth of 1500 μm from the surface of the nickel-based low-temperature steel, the content of tempered sorbite is 80 - 90%, and the content of tempered martensite is 10 - 20%; in the core of the nickel-based low-temperature steel, the content of granular bainite is 5 - 15%, the content of ferrite is 40 - 55%, and the content of pearlite is 40 - 55%.

[0019] In some preferred embodiments, the chemical composition of the nickel-based low-temperature steel includes: by mass percentage of elements, C 0.06 - 0.10%, Si 0.10 - 0.40%, Mn 1.30 - 1.60%, P ≤ 0.020%, S ≤ 0.020%, V 0.050 - 0.080%, Ni 1.30 - 1.50%, Cu 0.05 - 0.30%, Al 0.020 - 0.035%, O ≤ 10 ppm, and the balance is Fe and unavoidable impurities; and it also satisfies: Q ≤ 0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15.

[0020] Preferably, the grain size of the nickel-based low-temperature steel of the present invention is 12-13.

[0021] The low-temperature mechanical properties of the nickel-based low-temperature steel obtained by the present invention tested according to GB / T 13239-91 meet the following requirements:

[0022] In the notch-free low-temperature tensile test at -165°C with a tensile load of 100,000 KN, 41 specimens were tested, and the obtained average yield strength was above 690 MPa, and the obtained average tensile strength was above 890 MPa;

[0023] In the notched low-temperature tensile test at -165°C with a tensile load of 100,000 KN, 41 specimens were tested, and the obtained average yield strength was above 650 MPa, the obtained average tensile strength was above 850 MPa, and the average value of the sensitivity coefficient NSR was greater than 1.10;

[0024] In the tensile test at room temperature with a tensile load of 100,000 KN, the obtained average yield strength was above 570 MPa, and the obtained average tensile strength was above 700 MPa.

[0025] Through the above technical solutions, the present invention adopts a specific on-line cooling method of controlled rolling and controlled cooling in the steel rolling process, organically combines rolling deformation and three-pass water cooling. Among them, the temperature of the rolled piece is reduced through the first-pass water cooling and the second-pass water cooling, the recrystallization activation energy is reduced, and extremely fine and uniformly distributed grains are formed, making preparations for finally obtaining a specific structure. Combined with the third-pass water cooling, the surface of the rolled piece obtained after finish rolling is rapidly cooled to form a suitable shallow surface martensite, avoiding the formation of too deep martensite. In this way, the core part has not had time to cool and the heat is transferred outward, causing the surface martensite to self-temper. The tempered martensite structure is conducted to the surface through the residual heat of the core part of the rolled piece, improving the surface martensite structure by tempering, eliminating the internal stress of the martensite structure obtained by quenching, so that even if there are cracks on the product surface, the low-temperature toughness and strength of the nickel-based low-temperature steel can still be improved to meet the market demand.

[0026] In the prior art, offline water cooling is usually carried out after rolling. Since on-line cooling will cause cracks on the product surface, and the mechanical properties of its products cannot meet the requirements; conventional water cooling usually makes the temperature on the cooling bed 900-930°C, obtaining fine ferrite and pearlite structures, which are difficult to meet the manufacturing requirements of low-temperature steel. And the surface tempered martensite in the prior art is usually obtained by quenching and tempering steps in the water cooling after rolling.

[0027] The present invention adopts the above technical solution for online cooling, which improves the production efficiency, can reasonably control the depth of surface cracks of the product, enables the low-temperature performance of the product to be unaffected, and even improves the low-temperature mechanical properties. It can also achieve self-tempering without additional quenching and tempering steps. The obtained product has a specific microstructure, with fine and evenly distributed grains, ensuring that the material still has good low-temperature mechanical properties and uniformity under extremely cold conditions (such as -165 °C). The grain size of the obtained finished nickel-based low-temperature steel is 12 - 13. Under the same conditions, if water cooling is not adopted before finish rolling and only water cooling is carried out after finish rolling, when the finish rolling temperature is 1050 °C, the grain size of the finished product is only 11 - 11.4. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0029] Figure 1 It is a picture of the surface of the product of Embodiment 1 of the present invention observed with a metallurgical microscope.

[0030] Figure 2 It is a picture of the surface of the product of Embodiment 1 of the present invention observed with a stereoscopic microscope.

[0031] Figure 3 It is a picture of the core of the product of Embodiment 1 of the present invention observed with a metallurgical microscope.

[0032] Figure 4 It is a picture of the part with the deepest crack on the surface of the product of Embodiment 1 of the present invention observed with a metallurgical microscope. Detailed Embodiments

[0033] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0034] In the present invention, the temperature refers to the temperature of the surface of the rolled piece.

[0035] In a first aspect, the present invention provides a method for producing a nickel-based low-temperature steel, which includes steelmaking, heating, rolling, and cooling on a cooling bed. The rolling includes rough rolling, medium rolling, pre-finishing rolling, and finishing rolling. The temperature of the rolled piece obtained after the medium rolling is 1020 - 1050 °C, and after the medium rolling, direct water quenching is carried out once, and the direct water cooling rate is 55 - 65 °C / s, and then it enters a slow cooling channel for natural cooling until the temperature of the rolled piece when it enters the pre-finishing rolling is 970 - 1020 °C; and after the pre-finishing rolling, direct water quenching is carried out twice, and the direct water cooling rate is 160 - 220 °C / s, and then it enters a slow cooling channel for natural cooling until the temperature of the rolled piece when it enters the finishing rolling is 790 - 930 °C; and after the finishing rolling, direct water quenching is carried out three times, and the direct water cooling rate is 350 - 460 °C / s, and then it enters a slow cooling channel for natural cooling, so that the temperature of the rolled piece on the cooling bed during the cooling on the cooling bed is 560 - 600 °C.

[0036] In the above technical solution of the present invention, in the first aspect, after the medium rolling, direct water quenching is carried out once. For the medium-rolled piece at a specific temperature, with an appropriate direct water cooling rate, the temperature of the rolled piece when it enters the pre-finishing rolling is controlled at 970 - 1020 °C, so that the surface temperature of the rolled piece is low, which can reduce the surface plasticity of the rolled piece. During the rolling process, the deformation amount can penetrate into the core of the rolled piece, and the grain growth in the core can be prevented by deformation, reducing the grain size difference between the core and the surface of the rolled piece and promoting the performance homogenization; at the same time, the pre-finishing rolling temperature is reduced, reducing the thermal activation energy of grain growth and inhibiting the recrystallization growth process of grains, playing a role in refining grains. After the pre-finishing rolling, direct water quenching is carried out twice, with an appropriate direct water cooling rate, reducing the temperature of the rolled piece when it enters the finishing rolling to 790 - 930 °C, reducing the thermal activation energy of grain growth, and ensuring that small-sized grains are obtained after the finishing rolling. In the second aspect, by using direct water cooling twice during the rolling process, fine grain or sub-grain strengthening can be generated during the rolling process due to the lattice distortion caused by the uniform distribution of Ni. In the third aspect, after the finishing rolling, direct water quenching is carried out three times, so that the surface of the rolled piece is rapidly cooled to form quenched martensite. At this time, the surface and the core are cooled to different degrees. Also, because the grains are small and evenly distributed, it is beneficial for the heat in the core to be transferred to the outside, causing self-tempering of the surface martensite, which is beneficial for the formation of tempered martensite on the product surface, thereby promoting the low-temperature strength and toughness; at the same time, controlling the surface cracks of the product within a reasonable range without affecting the product performance.

[0037] In the primary water-through water cooling, the water-through cooling rate can be, for example, any value among 55, 56, 57, 58, 59, 60, 61, 63, 65 °C / s and any value between adjacent values. In the secondary water-through water cooling, the water-through cooling rate can be, for example, any value among 160, 170, 180, 190, 200, 210, 220 °C / s and any value between adjacent values. In the tertiary water-through water cooling, the water-through cooling rate can be, for example, any value among 350, 360, 370, 380, 400, 420, 440, 460 °C / s and any value between adjacent values.

[0038] In some embodiments, the temperature when entering the pre-finishing rolling is 970 - 1000 °C, and the temperature when entering the finishing rolling is 790 - 820 °C. In some other embodiments, the temperature when entering the pre-finishing rolling is 990 - 1020 °C, and the temperature when entering the finishing rolling is 900 - 930 °C.

[0039] In some preferred embodiments, the temperature of the rolled piece obtained by the pre-finishing rolling is 990 - 1050 °C, and the temperature of the rolled piece obtained by the finishing rolling is 850 - 1010 °C. Specifically, in some specific embodiments, the temperature of the rolled piece obtained by the pre-finishing rolling is 990 - 1020 °C, and the temperature of the rolled piece obtained by the finishing rolling is 850 - 900 °C. In some other specific embodiments, the temperature of the rolled piece obtained by the pre-finishing rolling is 1030 - 1050 °C, and the temperature of the rolled piece obtained by the finishing rolling is 980 - 1010 °C.

[0040] In some preferred embodiments, the time of the primary water-through water cooling is 1 - 3 s, the time of the secondary water-through water cooling is 1 - 3 s, and the time of the tertiary water-through water cooling is 1 - 3 s. Under this preferred scheme, the water cooling time of each stage is appropriate, which can promote the cooling of the core and the surface to different degrees, making it more conducive to subsequent self-tempering with the core temperature to form a more suitable surface tempered martensite structure.

[0041] Preferably, the water-through flow rate of the primary water-through water cooling is 85 - 95 m 3 / h, the water-through flow rate of the secondary water-through water cooling is 270 - 360 m 3 / h, and the water-through flow rate of the tertiary water-through water cooling is 650 - 760 m 3 / h.

[0042] In the present invention, the water-through cooling rate refers to the value of the difference between the temperatures of the rolled piece before and after water-through divided by the water-through time.

[0043] In the present invention, after the steel rolled piece undergoes corresponding water-through cooling, there is a certain temperature gradient difference between the surface and the core. Through natural cooling in the slow cooling channel, the temperature difference between the surface and the core can be gradually reduced, achieving the homogenization of the surface and core temperatures, reducing the mechanical property differences formed due to the core-surface temperature difference, and improving the uniformity of the low-temperature mechanical properties of the product. Those skilled in the art can select the length of the slow cooling channel and the moving speed of the rolled piece according to the temperature required for the next process; for example, in some embodiments, in the three times of water-through cooling, the water-through length is 20 - 30 m, and the moving speed of the rolled piece is 10 - 10.8 m / s.

[0044] In the present invention, in order to perform corresponding water cooling treatment, those skilled in the art can select several groups of water-through tanks and / or add a controlled cooling system according to actual needs to achieve water-through cooling.

[0045] In some preferred embodiments, the rough rolling and medium rolling make the shape compression ratio of the rolled piece obtained by medium rolling be above 3.2. In this preferred scheme of the present invention, large reduction ratios are adopted in rough rolling and medium rolling, so that the shape compression ratio of the rolled piece reaches above 3.2, which can break large grains and dendrites generated by the long-time heat preservation of the continuous casting billet during heating, ensure that the grain size reaches grade 7 - 8, and make primitive preparations for obtaining fine grains by subsequent controlled rolling and controlled cooling, thereby further promoting the improvement of the low-temperature strength and toughness of the product.

[0046] The compression ratio described in the present invention has the conventional meaning in the art and will not be elaborated here.

[0047] In some preferred embodiments, during the rolling, the starting rolling temperature is controlled to be 1000 - 1050 °C. Under this preferred scheme, the starting rolling temperature is preferably low, which can control the size of the primary grains as small as possible. The smaller the size of the primary grains, the smaller the compression ratio required for subsequent rolling, and the wider the process conditions.

[0048] In some embodiments, the starting rolling temperature is 1020 - 1050 °C. In some other embodiments, the starting rolling temperature is 1000 - 1020 °C.

[0049] Preferably, during the rolling, the finishing rolling temperature is controlled to be 900 - 930 °C.

[0050] In some preferred embodiments, during the rolling, the finishing rolling speed is controlled to be 10 - 10.8 m / s.

[0051] For the rolling passes described in the present invention, those skilled in the art can select according to actual needs, as long as the low-temperature steel with the required profile is obtained through rolling and the above rolling process is satisfied; for example, for different specifications, the number of passes can be 16 - 18 according to the profile control plan. In the rolling of the present invention, a flying shear and its parameters (cutting head length and lead ratio) can be set at an appropriate position according to actual needs to cut the head of the rolled piece. The cutting head length ensures the removal of the "black head" generated by the cooling of the pre-piercing water tank, that is, the part with very low temperature. The cutting head length can be appropriately lengthened or reduced according to the change of the head of the rolled piece on site.

[0052] In some preferred embodiments, during the rolling, the rolled piece is controlled to be only under compressive stress between adjacent rolling mills. Under this preferred scheme, there is a very small unidirectional tensile stress or tension-free rolling between adjacent rolling mills, and the rolled piece is only under compressive stress; this can avoid the elongation of the grains of the rolled piece, reduce the compression ratio of the next pass, resulting in a decrease in grain size and unqualified mechanical properties.

[0053] In some preferred embodiments, during the rolling, the heating process includes: three-stage heating of a preheating section, a heating section, and a soaking section in sequence.

[0054] The temperature of the soaking section in the present invention is not higher than 1200 °C.

[0055] Preferably, the temperature of the preheating section is below 800 °C, the temperature of the heating section is 1050 - 1200 °C, the temperature of the soaking section is 1030 - 1180 °C, and the total heating time is 85 - 95 min. Under this preferred scheme, on the one hand, extending the residence time of the billet in the furnace is more conducive to promoting the full diffusion of Ni element. Ni will cause lattice distortion and increase the energy required for dislocation movement. During the deformation process, more uniform fine grains and sub-grains (fine grain strengthening and sub-grain strengthening) are formed, which is more conducive to improving the low-temperature performance; at the same time, it is more conducive to promoting the uniformity of performance. On the other hand, the temperature of the soaking section is appropriately low to prevent local overheating easily caused by temperature fluctuations in the temperature distribution in the heating furnace, and combined with a long holding time, it will cause an increase in local grain size or the formation of Widmanstatten structure, resulting in a sudden drop in local mechanical properties and affecting the low-temperature mechanical properties.

[0056] In some preferred embodiments, the composition of the nickel-based low-temperature steel includes: by mass percentage of elements, C 0.06 - 0.10%, Si 0.10 - 0.40%, Mn 1.30 - 1.60%, P ≤ 0.020%, S ≤ 0.020%, V 0.050 - 0.080%, Ni 1.30 - 1.50%, Cu 0.05 - 0.30%, Al 0.020 - 0.035%, O ≤ 10 ppm, and the balance is Fe and unavoidable impurities. In this preferred solution of the present invention, the content of elements harmful to low-temperature properties (the content of phosphorus, sulfur, and oxygen) in the molten steel can be strictly controlled, and the elements of manganese, vanadium, nickel, and copper are appropriately increased, which is more conducive to promoting the uniform distribution of low-content Ni elements, more conducive to reducing the ductile-brittle transition temperature, further improving low-temperature toughness and strength, so as to maintain good low-temperature mechanical properties while reducing alloy costs.

[0057] In some preferred embodiments, the composition of the nickel-based low-temperature steel satisfies: Q ≤ 0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15. In this preferred solution, the composition of the nickel-based low-temperature steel of the present invention satisfies the above formula, and in combination with the aforementioned water-cooling treatment, surface tempered martensite can be formed even at a relatively slow cooling rate. In the prior art, the formation of surface tempered martensite usually requires a very fast cooling rate.

[0058] The present invention places no restrictions on the steelmaking process. Generally, the steelmaking process includes: converter smelting, LF refining, RH treatment, and continuous casting, and each process can respectively adopt any existing method in the art, which does not require creative labor and will not be elaborated here. Generally, the continuous casting obtains a square billet, and the size can be selected according to requirements.

[0059] In a second aspect, the present invention provides a nickel-based low-temperature steel prepared by the production method described in the first aspect; the surface of the obtained nickel-based low-temperature steel has cracks with a crack depth of less than 280 μm, and the microstructure of the nickel-based low-temperature steel satisfies: on its surface, it is tempered sorbite tissue and tempered martensite tissue, and in its core, it is pearlite tissue, ferrite tissue, and granular bainite tissue.

[0060] In some preferred embodiments, by area fraction, at a depth of 1500 μm from the surface of the nickel-based low-temperature steel, the content of tempered sorbite tissue is 80 - 90%, and the content of tempered martensite tissue is 10 - 20%; in the core of the nickel-based low-temperature steel, the content of granular bainite tissue is 5 - 15%, the content of ferrite tissue is 40 - 55%, and the content of pearlite tissue is 40 - 55%.

[0061] In some preferred embodiments, the chemical composition of the nickel-based low-temperature steel includes: by mass percentage of elements, C 0.06 - 0.10%, Si 0.10 - 0.40%, Mn 1.30 - 1.60%, P ≤ 0.020%, S ≤ 0.020%, V 0.050 - 0.080%, Ni 1.30 - 1.50%, Cu 0.05 - 0.30%, Al 0.020 - 0.035%, O ≤ 10 ppm, and the balance is Fe and unavoidable impurities; and, it also satisfies: Q ≤ 0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15.

[0062] Preferably in the present invention, the grain size of the nickel-based low-temperature steel is 12 - 13.

[0063] The low-temperature mechanical properties of the nickel-based low-temperature steel obtained in the present invention tested according to GB / T13239 - 91 satisfy:

[0064] In a non-notch low-temperature tensile test at -165°C with a tensile load of 100000 KN, 41 specimens were tested, and the obtained average yield strength was above 690 MPa, and the obtained average tensile strength was above 890 MPa;

[0065] In a notched low-temperature tensile test at -165°C with a tensile load of 100000 KN, 41 specimens were tested, and the obtained average yield strength was above 650 MPa, the obtained average tensile strength was above 850 MPa, and the average value of the sensitivity coefficient NSR was greater than 1.10;

[0066] In a tensile test at room temperature with a tensile load of 100000 KN, the obtained average yield strength was above 570 MPa, and the obtained average tensile strength was above 700 MPa.

[0067] The room-temperature bending and reverse bending properties of the nickel-based low-temperature steel of the present invention comply with national standards.

[0068] The nickel-based low-temperature steel of the present invention can be used in any occasion under low-temperature and extremely cold conditions, such as low-temperature steel for ships and low-temperature steel for liquefied gas.

[0069] The present invention will be described in detail below with specific embodiments.

[0070] Example 1

[0071] The composition of the nickel-based low-temperature steel is as follows: by mass percentage of elements, C is 0.06 - 0.10%, Si is 0.10 - 0.40%, Mn is 1.30 - 1.60%, P ≤ 0.020%, S ≤ 0.020%, V is 0.050 - 0.080%, Ni is 1.30 - 1.50%, Cu is 0.05 - 0.30%, Al is 0.020 - 0.035%, O ≤ 10 ppm, and the balance is Fe and unavoidable impurities. And Q ≤ 0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15.

[0072] The production method is as follows:

[0073] I. Using low-carbon protective slag during steelmaking:

[0074] Converter smelting: The ferronickel required for alloying and scrap steel are added into the furnace together. Nitrogen is blown from the bottom throughout the smelting process. Vanadium-nitrogen alloy is added to make it vanadium micro-alloyed, and slag blocking for tapping is done well.

[0075] LF refining: In the early stage of refining, aluminum pellets, calcium aluminate or silicon carbide, silicon powder, and composite deoxidizer are added for diffusion deoxidation to ensure rapid formation of a reducing slag. In the later stage of refining, silicon carbide or ferrosilicon powder is added to maintain the slag. During the refining process, lime, bauxite or synthetic slag, and fluorite are added to adjust the slag amount and slag fluidity.

[0076] RH treatment: The RH nitrogen increasing process is adopted, and nitrogen circulation is carried out throughout the process. After RH treatment, the nitrogen content is controlled between 70 ppm and 100 ppm. The high-vacuum time is 12 minutes, and the pure degassing time is 8 minutes. After the vacuum treatment, calcium treatment is carried out, and then argon blowing treatment is carried out for more than 10 minutes. It is not allowed to break the slag surface, and the slag surface should fluctuate slightly. Steel water exposure is strictly prohibited.

[0077] Continuous casting: Protective casting is adopted to prevent secondary oxidation. The liquidus temperature of this steel grade is about 1510 °C, the superheat of the tundish is controlled at 30 ± 10 °C, the casting speed is 2 m / min, and mold and final electromagnetic stirring are adopted.

[0078] II. Heating and rolling process

[0079] Heating: The temperature of the preheating section is 700 °C and the time is 40 min, the temperature of the heating section is 1050 °C and the time is 30 min, and the temperature of the soaking section is 1100 °C and the time is 20 min.

[0080] Billet: 170 * 170 mm square billet.

[0081] Rolling:

[0082] It is carried out by the rolling method shown in Table 1 below. Among them, passes 1-6 are rough rolling, passes 7-10 are medium rolling, passes 11-16 are pre-finishing rolling, and passes 17-18 are finishing rolling. During the rolling process, the rolled piece is only under compressive stress between two rolling mills and not under any unidirectional tensile stress. Among them, there is a No. 1 shear after the No. 6 rolling mill, the head cutting length is 50 mm, and the leading rate is 1.4. There is a No. 2 shear after the No. 10 rolling mill, the head cutting length is 100 mm, and the leading rate is 1.35. There is a No. 3 shear before the No. 17 rolling mill, the head cutting length is 800 mm, and the leading rate is 1.15 (the head cutting length ensures the removal of the "black head" generated by the cooling of the pre-through-water water tank, that is, the part with very low temperature).

[0083] The starting rolling temperature: 1020 - 1050 °C, the finishing rolling temperature is 900 - 930 °C, and the finishing rolling speed: 10.8 m / s.

[0084] And for the rough and medium rolling units of No. 1 - 10, large reduction ratios are adopted, so that the shape compression ratio of the material coming out of No. 10 reaches more than 3.2, breaking large grains and dendrites generated by the long-time heat preservation of the broken billet, ensuring that the grain size reaches grades 7 - 8, and making primitive preparations for obtaining fine grains by subsequent controlled rolling and controlled cooling.

[0085] The temperature of the rolled piece of the shape coming out of No. 10 is 1020 - 1050 °C. There is a set of controlled cooling water tanks after the No. 10 rolling mill as the pre-through-water water tank, the water flow rate through the water is 90 m 3 / h, the water cooling rate through the water is 62.5 °C / s, the water cooling time through the water is 2.08 s, and then it enters the slow cooling channel for natural cooling until the temperature of the rolled piece entering the pre-finishing rolling is controlled at 990 - 1020 °C. Since the current size of the rolled piece is a circle with a diameter of 53 mm and the cross-sectional size is relatively large, the low surface temperature can reduce the surface plasticity of the rolled piece. During the rolling process, the deformation amount can penetrate into the core of the rolled piece, and the growth of core grains can be prevented by deformation, reducing the grain size difference between the core and the surface of the rolled piece and making the performance uniform. At the same time, reducing the temperature can also inhibit the recrystallization growth process of grains, playing a role in refining grains.

[0086] There are two groups of through-water tanks as pre-through-water tanks after the No. 16 rolling mill. The purpose is to reduce the temperature of the rolled piece entering the finishing rolling, reduce the thermal activation energy of grain growth, and ensure the acquisition of small-sized grains. The temperature of the rolled piece obtained by the No. 16 rolling mill is 1030 - 1050 °C, the water flow rate through the water is 280 m 3 / h, the water cooling rate through the water is 166.67 °C / s, the water cooling time through the water is 1.68 s, and then it enters the slow cooling channel for natural cooling until the temperature entering the finishing rolling is 900 - 930 °C, reducing the activation energy of grain growth and ensuring that after finishing rolling, a fine grain structure is obtained. There is a 45-meter-long channel after passing through the water. The purpose is to ensure the temperature uniformity between the surface and the core of the steel after cooling.

[0087] The temperature of the rolled piece obtained by the 18th rolling mill is 980 - 1010 °C. There are 3 groups of water-piercing tanks after the 18th rolling mill. The water-piercing flow rate is 740 m 3 / h, the water-piercing length is 25.3 m, the moving speed of the finished product is 10.8 m / s, the cooling time is 1.75 s, the temperature of the finished product entering the first group of water-piercing tanks is about 980 °C (the rolling speed of finish rolling is fast and the rolled piece is in temperature-rising rolling), and the temperature coming out of the third group of tanks is about 180 °C. The overall water-piercing cooling rate is 457.14 °C / s. Then it enters the slow-cooling channel for natural cooling until the temperature on the cooling bed is 560 - 600 °C, and then it is cooled on the cooling bed.

[0088] Table 1 (for deformed steel bars with a diameter of 20 mm) Stock shape / mm

[0089] Mill Number / Pass 1 2 3 4 5 6 7 8 9 Stock Height 126 118 86 87 66 83 51 64 42 Pass System Flat Flat Flat Flat Flat Round Elliptical Round Elliptical Mill Number / Pass 10 11 12 13 14 15 16 17 18 Stock Height 53 33 56 34 31.6 19.5 24.8 15.2 19.1 Pass System Round Flat Box Pre-Cut Split Elliptical Round Elliptical Round

[0090] After the special process treatment of cooling through the above-mentioned water-piercing tanks, due to the relatively short water-piercing time, the core part has come out of the water-piercing tank before it has time to cool, and the core part is still in a high-temperature state. A certain temperature gradient is formed between the core and the surface, and the heat of the core part will be transferred to the surface. The martensite structure obtained by quenching the surface of the rolled piece using the core temperature is self-tempered to form a tempered martensite structure. Without additional quenching and tempering steps, a ring of martensite structure can be obtained on the surface of the finished product.

[0091] The surface and core of the obtained nickel-based low-temperature steel are observed with a metallographic microscope, and the pictures are respectively as Figure 1 、 Figure 3 shown. And the surface of the obtained nickel-based low-temperature steel is observed with a stereomicroscope, and the pictures are respectively as Figure 2 shown. The metallographic picture of the deepest crack part on the surface of the nickel-based low-temperature steel is as Figure 4 shown, and the deepest depth of the crack is 277.11 μm.

[0092] From Figure 1 the metallographic microscope observation pictures, it can be seen that the structure of the product surface is tempered martensite structure + tempered sorbite structure. From Figure 2 the stereomicroscope observation pictures, it can be clearly seen that there is a closed ring of tempered martensite on the surface of the product. The surface contains tempered martensite structure, which can improve the strength of the product. The martensite after tempering treatment not only retains the characteristic of improving strength but also improves the toughness of the finished product. From Figure 3 it can be known that the core of the product is pearlite structure, ferrite structure and a small amount of granular bainite structure. The above-mentioned organizational structures of the core and surface of the product are beneficial to improving the low-temperature strength and toughness of the product.

[0093] According to the area method, the microstructure of the product was tested. It can be known that, in terms of area fraction, at a depth of 1500 μm from the product surface, the content of tempered sorbite structure is 90%, and the content of tempered martensite structure is 10%; in the core part, the content of granular bainite structure is 10%, the content of ferrite structure is 45%, and the content of pearlite structure is 45%. The grain size of the nickel-based low-temperature steel obtained by the test is 12 - 12.5.

[0094] And its tensile properties were measured according to GB / T13239 - 91. In the unnotched tensile test at - 165 °C with a tensile load of 100000 KN, 41 specimens were tested, and the obtained average yield strength was 690 MPa, and the average tensile strength was 890 MPa.

[0095] In the notched low-temperature tensile test at - 165 °C with a tensile load of 100000 KN, 41 specimens were tested, and the obtained average yield strength was 710 MPa, the average tensile strength was 850 MPa, and the average value of the sensitivity coefficient NSR was 1.22.

[0096] In the tensile test at room temperature with a tensile load of 100000 KN, the obtained average yield strength was 570 MPa, and the average tensile strength was 700 MPa. Its bending and reverse bending properties at room temperature conform to the national standard.

[0097] Example 2

[0098] The low-temperature steel was produced by referring to the method of Example 1, except that the starting rolling temperature was reduced, and the temperature entering the finishing mill was also reduced. The starting rolling temperature was 1000 - 1020 °C. There was a set of controlled cooling water tanks after the No. 10 rolling mill as pre-water-piercing water tanks, the water-piercing flow rate was 90 m 3 / h, the water-piercing cooling rate was 62.5 °C / s, the water-piercing water cooling time was 2.08 s, and then it entered the slow cooling channel for natural cooling until the temperature entering the pre-finishing mill was 970 - 1000 °C, and the temperature obtained by pre-finishing was 990 - 1020 °C; there were two sets of water-piercing water tanks after the No. 16 rolling mill as pre-water-piercing water tanks. The purpose was to reduce the temperature of the rolled piece entering the finishing mill, reduce the thermal activation energy of grain growth, and ensure the acquisition of small-sized grains. The water-piercing flow rate was 350 m 3 / h, the water-piercing cooling rate was 202.3 °C / s, the water-piercing cooling time was 1.68 s, and the temperature obtained after water-piercing was 650 - 680 °C. Then it entered the slow cooling channel for natural cooling until the temperature entering the finishing mill was 790 - 820 °C, and the temperature obtained by finishing was 850 - 900 °C. There were three sets of water-piercing water tanks after the No. 18 rolling mill, the water-piercing flow rate was 650 m 3 / h, the water-piercing cooling rate was 354.28 °C / s, the cooling time was 1.75 s, and the obtained temperature was 230 - 260 °C. Then it entered the slow cooling channel for natural cooling until the temperature on the cooling bed was 560 - 580 °C.

[0099] The obtained nickel-based low-temperature steel has the same microstructure distribution as that in Example 1, and the specific tissue content is similar to that in Example 1. The deepest crack depth of the product is 233 μm. Correspondingly, the grain size of the nickel-based low-temperature steel is measured to be 12.5 - 13.

[0100] Correspondingly, in the unnotched tensile test at -165°C with a tensile load of 100,000 KN, 41 specimens are tested, and the obtained average yield strength is 720 MPa, and the average tensile strength is 935 MPa.

[0101] In the notched low-temperature tensile test at -165°C with a tensile load of 100,000 KN, 41 specimens are tested, the obtained average yield strength is 655 MPa, the obtained average tensile strength is 890 MPa, and the average value of the sensitivity coefficient NSR is 1.24.

[0102] In the tensile test at room temperature with a tensile load of 100,000 KN, 41 specimens are tested, the obtained average yield strength is 585 MPa, and the average tensile strength is 720 MPa.

[0103] The yield strength of the product in this example is too high and can be used as a higher-grade steel bar. Its bending and reverse bending properties at room temperature meet the national standards.

[0104] Comparative Example 1

[0105] The low-temperature steel is produced by referring to the method in Example 1, except that in the water cooling after pre-finishing rolling, the water-through cooling speed is different; specifically, the temperature of the rolled piece entering pre-finishing rolling is controlled at the same temperature as in Example 1; there are two groups of water-through tanks after the 16th rolling mill as pre-water-through tanks, the water-through flow rate is 240 m 3 / h, the water-through cooling speed is 148.8 °C / s, the water-through cooling time is 1.68 s, and then it enters the slow-cooling channel for natural cooling until the temperature entering finishing rolling is 930 - 950 °C.

[0106] The obtained nickel-based low-temperature steel has the same microstructure distribution as that in Example 1, and the specific tissue content is similar to that in Example 1. The deepest surface crack depth of the product is 290 μm. Correspondingly, the grain size of the nickel-based low-temperature steel is measured to be 11.5 - 12.

[0107] Correspondingly, in the unnotched tensile test at -165°C with a tensile load of 100,000 KN, 41 specimens are tested, the obtained average yield strength is 695 MPa, and the average tensile strength is 915 MPa.

[0108] In the notched low-temperature tensile test at -165°C with a tensile load of 100000 KN, 41 specimens were tested. The obtained average yield strength was 645 MPa, the obtained average tensile strength was 835 MPa, and the mean value of the sensitivity coefficient NSR was 1.19.

[0109] In the tensile test at room temperature with a tensile load of 100000 KN, [number] specimens were tested. The obtained average yield strength was 520 MPa, and the average tensile strength was 665 MPa.

[0110] The room-temperature bending and reverse bending properties of the product in this embodiment meet the national standards. However, the mechanical property margin is relatively low, and when the process parameters fluctuate, the mechanical properties may not meet the national standard requirements.

[0111] From the above embodiments and comparative examples, it can be seen that by adopting the embodiment scheme of the present invention, better low-temperature and room-temperature mechanical properties of the finished product can be obtained, meeting the national standard requirements. Further, from Example 1 and Example 2, it can be seen that by adopting the preferred cooling method of the present invention, the low-temperature mechanical properties of the obtained finished product are better.

[0112] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A production method of nickel-based low-temperature steel, including steelmaking, heating, rolling and cooling on the cooling bed, characterized in that, the rolling includes rough rolling, medium rolling, pre-finishing rolling and finishing rolling, and the temperature of the rolled piece obtained by the medium rolling is 1020 - 1050 °C, and after the medium rolling, it is subjected to water-cooling through water once, and the water-cooling speed is 55 - 65 °C / s, and then it enters the slow-cooling channel for natural cooling until the temperature of the rolled piece when it enters the pre-finishing rolling is 970 - 1020 °C; and after the pre-finishing rolling, it is subjected to water-cooling through water twice, and the water-cooling speed is 160 - 220 °C / s, and then it enters the slow-cooling channel for natural cooling until the temperature of the rolled piece when it enters the finishing rolling is 790 - 930 °C; and after the finishing rolling, it is subjected to water-cooling through water three times, and the water-cooling speed is 350 - 460 °C / s, and then it enters the slow-cooling channel for natural cooling, so that the temperature of the rolled piece on the cooling bed during the cooling on the cooling bed is 560 - 600 °C; the composition of the nickel-based low-temperature steel includes: by mass percentage of elements, C 0.06 - 0.10%, Si 0.10 - 0.40%, Mn 1.30 - 1.60%, P ≤ 0.020%, S ≤ 0.020%, V 0.050 - 0.080%, Ni 1.30 - 1.50%, Cu 0.05 - 0.30%, Al 0.020 - 0.035%, O ≤ 10 ppm, and the balance is Fe and unavoidable impurities.

2. The production method according to claim 1, characterized in that, the temperature of the rolled piece obtained by the pre-finishing rolling is 990 - 1050 °C, and the temperature of the rolled piece obtained by the finishing rolling is 850 - 1010 °C.

3. The production method according to claim 1, characterized in that, The water flow rate of the first water-through water cooling is 85 - 95 m 3 / h, the water flow rate of the second water-through water cooling is 270 - 360 m 3 / h, and the water flow rate of the third water-through water cooling is 650 - 760 m 3 / h; and / or, the time of the first water-cooling through water is 1 - 3 s, the time of the second water-cooling through water is 1 - 3 s, and the time of the third water-cooling through water is 1 - 3 s.

4. The production method according to claim 1, characterized in that, the rough rolling and medium rolling make the shape compression ratio of the rolled piece obtained by the medium rolling be above 3.

2.

5. The production method according to any one of claims 1 - 4, characterized in that, during the rolling, control the starting rolling temperature to be 1000 - 1050 °C, and the final rolling temperature to be 900 - 930 °C; and / or, control the final rolling speed to be 10 - 10.8 m / s; and / or, control that the rolled piece is only under compressive stress between adjacent rolling mills.

6. The production method according to any one of claims 1 - 4, characterized in that, the heating process includes: three-stage heating of a preheating section, a heating section and a soaking section in sequence, the temperature of the preheating section is below 800 °C, the temperature of the heating section is 1050 - 1200 °C, the temperature of the soaking section is 1030 - 1180 °C, and the total heating time is 85 - 95 min.

7. The production method according to any one of claims 1 - 4, characterized in that, the composition of the nickel-based low-temperature steel also satisfies: Q ≤ 0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15.

8. A nickel-based low-temperature steel, It is characterized in that it is prepared by the production method described in any one of claims 1-7; the surface of the obtained nickel-based cryogenic steel has cracks with a crack depth of less than 280 μm, and the microstructure of the nickel-based cryogenic steel satisfies: the surface is tempered sorbite and tempered martensite, and the core is pearlite, ferrite and granular bainite.

9. The nickel-based cryogenic steel according to claim 8 It is characterized in that the chemical composition of the nickel-based cryogenic steel includes: by mass percentage of elements, C 0.06-0.10%, Si 0.10-0.40%, Mn 1.30-1.60%, P≤0.020%, S≤0.020%, V 0.050-0.080%, Ni 1.30-1.50%, Cu 0.05-0.30%, Al 0.020-0.035%, O≤10 ppm, and the balance is Fe and inevitable impurities; and it also satisfies: Q≤0.48%, where Q = C + Mn / 6 + (Cr + V + Mo) / 5 + (Cu + Ni) / 15; and / or, the grain size of the nickel-based cryogenic steel is 12-13; and / or, by area fraction, at a depth of 1500 μm from the surface of the nickel-based cryogenic steel, the content of tempered sorbite is 80-90%, and the content of tempered martensite is 10-20%; in the core of the nickel-based cryogenic steel, the content of granular bainite is 5-15%, the content of ferrite is 40-55%, and the content of pearlite is 40-55%.

10. The nickel-based cryogenic steel according to claim 8 or 9 It is characterized in that the low-temperature mechanical properties of the obtained nickel-based cryogenic steel tested according to GB / T13239-91 satisfy: in the unnotched low-temperature tensile test at -165°C with a tensile load of 100000 KN, 41 specimens are tested, and the obtained average yield strength is above 690 MPa, and the obtained average tensile strength is above 890 MPa; in the notched low-temperature tensile test at -165°C with a tensile load of 100000 KN, 41 specimens are tested, and the obtained average yield strength is above 650 MPa, the obtained average tensile strength is above 850 MPa, and the average value of the sensitivity coefficient NSR is greater than 1.10; in the tensile test at room temperature with a tensile load of 100000 KN, the obtained average yield strength is above 570 MPa, and the obtained average tensile strength is above 700 MPa.

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