CN115874111A - Mn-Ni series ultralow temperature steel and preparation method thereof - Google Patents

Abstract

The invention discloses Mn-Ni series ultralow temperature steel and a preparation method thereof. Belongs to the field of low-temperature steel manufacturing, and comprises the chemical components of C, si, mn, P, S, cr, ni, cu, mo, nb and N, and the balance of Fe and inevitable impurities; the method comprises the following specific operation steps: 1. smelting in a converter; 2. rolling; 3. and (4) carrying out laminar cooling after finishing rolling. The Mn-Ni series ultralow-temperature steel provided by the invention has the advantages of shorter production flow, easily controlled smelting process, simple and easily controlled rolling operation, higher automation degree, capability of realizing intelligent control and reducing human subjective factor intervention, stable performance and better repeatability; mn replaces part of expensive alloy element Ni, and the raw material cost is lower. The traditional quenching and tempering processes of the 5Ni steel are eliminated, the production flow is greatly reduced, the process cost is reduced, and the production efficiency and the yield are high.

Description

Mn-Ni series ultralow temperature steel and preparation method thereof

Technical Field

The invention belongs to the field of low-temperature steel manufacturing, and relates to economical low-temperature steel and a preparation method thereof; in particular to Mn-Ni series ultralow temperature steel and a preparation method thereof.

Background

In the prior art, along with the increasing requirements on environmental protection and low carbon emission, the demand of clean energy (liquefied petroleum gas (LPG), liquefied Natural Gas (LNG) and the like) is increased year by year, and a low-temperature storage tank constructed by adopting Ni series low-temperature steel is widely used for storing liquefied clean energy; 5Ni steel, 7Ni steel, 9Ni steel and the like are successively and successfully developed and applied to the construction of low-temperature pressure vessels. Because the alloy element Ni belongs to an expensive raw material, in order to reduce the cost, a new production process route is necessarily explored to reduce the alloy element Ni.

The international invention patent (application number PCT/CN 2021/070624) discloses a method for manufacturing a marine 5Ni steel plate with low remanence and excellent surface quality, which comprises the following chemical components in percentage by weight: 0.07 to 0.10%, si:0.05 to 0.20%, mn:0.60 to 0.80%, ni: 4.90-5.25%, P is less than or equal to 0.0070%, S is less than or equal to 0.0020%, alt: 0.010-0.035%, V:0.010 to 0.015%, nb:0.010 to 0.020%, ca:0.0005 to 0.0030 percent, less than or equal to 0.0012 percent of O, less than or equal to 0.0040 percent of N, less than or equal to 0.00010 percent of H, and the balance of Fe and inevitable impurities. In the production method, casting blanks are peeled, hot rolled at high temperature, water cooling is not performed after rolling, then quenching and tempering are performed for two times, the first quenching temperature is increased, the segregation with high Ni content is improved, and the steel is hoisted by a vacuum chuck, so that good 5Ni steel with high toughness is obtained, and the product has excellent surface quality and low remanence. The patent application provides a production method of a low-temperature 5Ni plate, two times of quenching and tempering heat treatment are needed, the process is complex, the production efficiency is low, and the cost is high.

The Chinese invention patent (application number 202111522319.7) discloses a continuous casting and rolling production line for avoiding ferrite rolling mixed crystal and a control method thereof. The continuous casting and rolling production line for avoiding mixed crystal in ferrite rolling comprises a continuous casting machine, a roughing mill group, an intermediate billet heat preservation device, a finishing mill group, a cooling device after rolling, a high-speed flying shear and a coiling machine group which are sequentially arranged along a rolling line, and the production line can also be provided with a rolled piece heating device, a descaling device, an accident treatment device, an intermediate billet cooling device and the like according to requirements. The intermediate billet heat preservation device is arranged in front of the finishing mill group, the temperature of the intermediate billet reaching the inlet of the intermediate billet heat preservation device is controlled to be below the ferrite phase transition temperature, the heating temperature of the intermediate billet heat preservation device is controlled, the intermediate billet is subjected to heat preservation or slow cooling in the device, the austenite-ferrite phase transition time is prolonged, the phase transition is fully performed, mixed crystals caused by double-phase rolling are avoided, and the uniformity of the structure performance of a ferrite rolling product is improved. The patent application provides a continuous casting and rolling production line facility, and a cooling device after rolling does not design temperature control measures and precision, only controls the rolling temperature to be more than 660 ℃, is suitable for producing hot continuous rolling steel grades, and is not suitable for preparing low-temperature steel of the patent application.

Disclosure of Invention

The purpose of the invention is as follows: because the Ni content of the 5Ni low-temperature steel commonly used at present is about 5.0 percent, the Ni content of a noble element is reduced, and the production cost is reduced; the invention aims to provide economical low-temperature steel, which adopts Mn to replace part of Ni, adopts a controlled roll forming technology and controls a laminar cooling system after final rolling, avoids quenching and tempering processes, reduces the alloy cost and the production process cost and has obvious economic benefit. The produced low-temperature steel has good mechanical property, welding property and corrosion resistance. The yield strength ReL of the prepared low-temperature steel is more than or equal to 395MPa, and the tensile strength Rm is as follows: 560-695MPa, and the impact energy of V-shaped notch is more than or equal to 125J at-135 ℃. The HS environmental corrosion rate is less than or equal to 0.3 mm/year. Can be widely applied to the construction of low-temperature chemical storage tanks such as liquefied LPG, liquid ammonia, liquefied ethylene and the like.

The technical scheme is as follows: the invention provides a preparation method of economical low-Ni low-temperature steel, which has the advantages of high production efficiency, easily controlled production process, good low-temperature toughness, good welding performance and high-speed (HS) corrosion resistance, and relatively low cost;

the Mn-Ni series ultralow temperature steel comprises the following chemical components in percentage by weight: 0.056 to 0.086 percent of C, 0.05 to 0.095 percent of Si, 1.95 to 2.25 percent of Mn, less than or equal to 0.007 percent of P, less than or equal to 0.003 percent of S, less than or equal to 0.012 percent of Cr, ni:1.85 to 2.06%, cu 0.46 to 0.88%, mo: 0.011-0.018%, nb 0.076-0.098%, N not more than 0.0020%, and Fe and inevitable impurities in balance.

Further, the preparation method of the Mn-Ni series ultralow temperature steel comprises the following specific operation steps:

step (1), smelting in a converter;

step (2), rolling;

and (3) carrying out laminar cooling after finish rolling.

Further, in the step (1), the specific process of converter smelting is as follows:

1. loading molten iron subjected to pre-S removal treatment into a top-bottom combined blown converter, blowing oxygen, raising the temperature, oxidizing and removing C, and sampling and inspecting;

2. refining in an LF electric furnace outside a furnace, adding MnFe, siFe, nbFe, niFe and MoFe alloy materials for smelting, and adding high-quality lime to remove the S content until the S content is less than or equal to 0.003%;

3. sampling and inspecting the content of the alloy elements, and adjusting the content of each alloy element;

4. RH vacuum refining, wherein the treatment time under ultimate vacuum is more than or equal to 16 minutes to remove harmful impurities in the steel;

5. sampling and checking the N content, and controlling the N content to be below 0.0020%;

6. continuous casting; the conditions of the continuous casting are as follows:

casting temperature: hoisting the molten steel to a continuous casting platform at the temperature of 1529-1541 ℃;

the cross section of the continuous casting rectangular billet is 260 multiplied by 350mm or the slab size is 230 multiplied by 1860mm;

the plate blank is used for rolling a steel plate with the thickness less than or equal to 60 mm.

Further, in the step (2), the specific conditions of the rolling are as follows:

heating the casting blank at a temperature: 1160-1285 ℃, soaking temperature: 1165 to 1265 ℃, and soaking for more than or equal to 68 minutes;

the finishing rolling initial rolling temperature: the finishing temperature is less than or equal to 916 ℃, and the finishing temperature is as follows: 798-816 ℃;

final rolling pass reduction: 28-36%; thickness of blank before finish rolling: 76-126mm.

Further, in the step (3), in the finish post-rolling laminar cooling,

controlling the cooling rate of the finish rolling by a laminar cooling system, and controlling the temperature of the steel material returning to red at 516-616 ℃ after the steel material exits from the laminar cooling device;

the cooling rate of the steel entering the laminar cooling device to the complete outlet laminar cooling device is controlled to be 4.1-6.6 ℃/s;

the device comprises steel (1), a roller way (6), a temperature measuring device (2) connected with each other through a line, a water nozzle (3), a computer (4) and an electronic intelligent system (5);

the specific operation flow is as follows: placing steel (1) on a roller way (6) for cooling, wherein in the cooling and advancing process of the steel (1) on the roller way (6), temperature measuring devices (2) are arranged at the head, the middle part and the tail part of the steel, the water flow of a water nozzle (3) is controlled by a computer (4), the temperature measuring devices (2) are controlled by an electronic intelligent system (5), the cooling rate is calculated by the intelligent control system and is compared with a set value, a deviation value is fed back to the water nozzle (3) to control the computer (4), the water flow of the water nozzle (3) is accurately controlled, and the cooling rate of the steel is controlled within a set range; then air-cooled to room temperature.

The principle of the alloy elements mainly controlled in the invention is illustrated as follows:

c (carbon): on one hand, through a proper preparation process, the strength of the steel is controlled to be in an expected range, the strength of carbon atoms in a solid solution state is strongly improved, and the low-temperature toughness and the plasticity are remarkably reduced, the application emphasizes that the impact toughness at the temperature of-135 ℃ is more than or equal to 125J, the welding performance and the HS corrosion resistance are considered, so the content of the C is set to be in a lower level of 0.056-0.086%, the yield strength ReL of the low-temperature steel produced by the preparation process of the application is more than or equal to 395MPa, and the tensile strength Rm:560-695MPa. The carbon content is more than 0.086%, carbide is easily generated, and the corrosion resistance and the welding performance are reduced, and if the carbon content is less than 0.056%, the yield strength is difficult to reach 395MPa.

Mn (manganese): the alloy element Mn in the structural steel is an austenite stabilizing element, mn replaces part of expensive alloy element Ni, and the application aims to stabilize a high-temperature austenite structure, inhibit high-temperature transformation products (ferrite and/or pearlite and the like) and obtain medium and low-temperature transformation products (refined ferrite and bainite structures) under the combined action of Mn and Ni. Therefore, the Mn content is set to 1.95 to 2.25%. If the content is more than 2.25%, mn segregation tends to occur to deteriorate low-temperature toughness and weldability, and if the content is less than 1.95%, the effect of stabilizing the high-temperature austenite structure is hardly obtained.

P (phosphorus): p in the structural steel belongs to harmful elements, the lower the control is, the lower the low-temperature toughness, the welding performance and the like can be improved, but the production cost is increased, and generally on the premise of ensuring the performance, the P in the application can be controlled to be below 0.007%.

S (sulfur): the residual harmful element S in the steel obviously deteriorates the low-temperature toughness, the welding performance and the HS corrosion resistance, the lower the content is, the better the content is, but if the content is too low, the production cost is increased more, so the S is preferably controlled to be less than or equal to 0.003 percent.

Si (silicon): the alloy element Si is in a replacement solid solution state in the structural steel, the yield strength of the steel is obviously increased, and the low-temperature toughness of the steel is reduced, so the Si content is controlled to be 0.05-0.095%, the low-temperature toughness is obviously reduced when the Si content is higher than 0.095%, and the Si content is preferably more than or equal to 0.05% in order to ensure the yield strength.

Cr (chromium): in order to ensure the welding performance of the application, the Cr element is controlled at a lower level of Cr less than or equal to 0.012%. Because Cr is a strong hardenability element, if the cooling speed is high in the welding process, a martensite structure is easily formed, and a welding joint is cracked due to volume expansion; on the other hand, in the welding process, cr and C are easy to form various compounds CrxCy and coarsen, so that the low-temperature toughness of the welding joint is severely deteriorated.

Nb (niobium) can obviously reduce the recrystallization temperature of austenite due to an alloy element Nb, and the low-temperature steel is prepared by controlling the finish rolling process and the cooling rate, so that 0.076-0.098% of Nb is added, on one hand, the solid-dissolved Nb can obviously reduce the recrystallization temperature of austenite, the recrystallization behavior can not occur in the initial pass of the finish rolling process, and the phenomenon of mixed crystals caused by large crystal grains is avoided; on the other hand, the formed Nb (CN) fine particles can inhibit the grain boundary migration in the subsequent preparation process, refine ferrite grains and further improve the low-temperature toughness. If the content exceeds 0.098%, the production cost is increased, and if the content is less than 0.076%, the effect of grain refinement is not significant.

Ni (nickel): the low-temperature steel and ultralow-temperature steel are usually intentionally added with a certain content of alloy element Ni, and the added Ni atoms and Fe atoms form mutual replacement and dissolution, so that the friction force of an iron-based body-centered cubic lattice structure is reduced, dislocation is easy to move, the buffer capacity under the action of external load is increased, and the low-temperature toughness of steel is improved; in addition, solid-dissolved Ni can remarkably stabilize austenite structures, a certain amount of austenite structures can be formed in the preparation process of the present patent application and are reserved to room temperature, the austenite structures are distributed in a ferrite matrix, and the austenite structures have excellent low-temperature toughness, so that the low-temperature toughness of steel is improved, and the content range of Ni is set to be 1.85-2.06% and lower than 1.85% because the austenite structures adopt alloy element Mn to replace part of Ni, the effect of improving the low-temperature toughness is not remarkable and higher than 2.06%, so that the production cost is increased.

Mo (molybdenum): the invention applies that the steel is delivered in a controlled rolling and controlled cooling speed state after final rolling, has higher control requirement on the cooling rate after the final rolling, can inhibit atomic diffusion by adding the alloy element Mo in the steel, has very fine MoC precipitated phase particles generated by reaction with C atoms in a short-distance diffusion process, has the function of strengthening a matrix, and provides a nucleation source for the formation of ferrite, namely, the nucleation rate is promoted, the grain boundary mobility is inhibited, so that crystal grains are refined, the invention has good comprehensive mechanical property and welding property, mo belongs to a noble alloy element, and the production cost is obviously increased due to excessive addition. Therefore, the addition amount of the Mo alloy element is controlled within the range of 0.011-0.018%.

The steel grade provided by the patent application can be applied to the construction of LPG storage tanks and the construction of low-temperature chemical storage tanks such as liquid ammonia, liquefied ethylene and the like, so that higher requirements are put on corrosion resistance of steel materials such as HS and the like, and therefore, a proper amount of alloy element Cu is added and is controlled to be in a range of 0.46-0.88%, if the alloy element Cu is higher than 0.88%, hot cracks are easily generated in the preparation process, and the yield is reduced, and if the alloy element Cu is lower than 0.46%, the corrosion resistance is difficult to achieve the expectation.

Nitrogen (N): in the present application, N is controlled to 0.0020% or less, so that aging is likely to occur in N atoms dissolved in the matrix, which reduces the low-temperature toughness of the steel, and the resulting N-containing precipitate phase also reduces the low-temperature toughness of the steel. If the residual N content is controlled to a lower level, the production cost is significantly increased.

Aiming at the problems that the content of Ni in the conventional 5Ni low-temperature steel is about 5.0 percent, and the content of a noble element Ni is reduced and the production cost is reduced, the invention provides the economical low-temperature steel, mn is adopted to replace part of Ni, and a rolling forming control technology and a laminar cooling system after finish rolling are adopted, so that a quenching and tempering process are omitted, the alloy cost is reduced, the production process cost is reduced, and the economic benefit is obvious. The produced low-temperature steel has good mechanical property, welding property and corrosion resistance; the yield strength ReL of the prepared low-temperature steel is more than or equal to 395Mpa, and the tensile strength Rm is as follows: 560-695MPa, and the impact energy of V-shaped notch is more than or equal to 125J at-135 ℃. The corrosion rate of HS environment is less than or equal to 0.3 mm/year. Can be widely applied to the construction of low-temperature chemical storage tanks such as liquefied LPG, liquid ammonia, liquefied ethylene and the like.

Has the beneficial effects that: compared with the prior art, the Mn-Ni series ultralow-temperature steel provided by the invention has the advantages that the production flow is shorter, the smelting process is easy to control, the rolling operation is simple and easy to master, the automation degree is higher, the intelligent control can be realized, the intervention of artificial subjective factors is reduced, and the manufactured ultralow-temperature steel has stable performance and better repeatability; mn replaces part of expensive alloy element Ni, and the raw material cost is lower. The traditional quenching and tempering processes of the 5Ni steel are eliminated, the production flow is greatly reduced, the process cost is reduced, and the production efficiency and the yield are high.

Drawings

FIG. 1 is a flow chart of the laminar cooling after finish rolling in the example of the present invention;

wherein, 1 is steel, 2 is temperature measuring device, 3 is the water nozzle, 4 is the computer, 5 is electronic intelligent system, 6 is the roll table.

Detailed Description

The invention is further described with reference to the following figures and examples.

The Mn-Ni series ultralow temperature steel comprises the following chemical element components in percentage by weight: 0.056 to 0.086 percent of C, 0.05 to 0.095 percent of Si, 1.95 to 2.25 percent of Mn, less than or equal to 0.007 percent of P, less than or equal to 0.003 percent of S, less than or equal to 0.012 percent of Cr, ni:1.85 to 2.06%, 0.46 to 0.88% of Cu, mo: 0.011-0.018%, nb 0.076-0.098%, N not more than 0.0020%, and the balance Fe and inevitable impurities.

Further, the weight percentage of C is preferably 0.057-0.075%.

Further, the weight percentage of Ni is preferably 1.90 to 2.05%.

Further, the weight percentage of Cu is preferably 0.47 to 0.77%.

Further, the weight percentage of Nb is preferably 0.11 to 0.16%.

Further, the weight percentage of Mo is preferably 0.012-0.016%.

The preparation method of the Mn-Ni series ultralow temperature steel comprises the following steps:

1) And smelting in a converter:

loading molten iron subjected to pre-S removal treatment into a top-bottom combined blown converter, blowing oxygen, raising the temperature, oxidizing and removing C, sampling and checking, and controlling the content of C to be below 0.045%; adding scrap steel and Cu blocks, adding CaO, feO and the like to remove P, and controlling P to be below 0.007%;

refining in an LF electric furnace outside a furnace, adding alloy materials such as MnFe, siFe, nbFe, niFe, moFe and the like, smelting, and adding high-quality lime to remove the S content until the S content is less than or equal to 0.003%;

sampling and inspecting the content of the alloy elements, and adjusting the content of each alloy element to be within a required range;

RH vacuum refining, wherein the treatment time under ultimate vacuum is more than or equal to 16 minutes to remove harmful impurities such as N, H, O and the like in steel and large particles;

sampling and checking the N content, and controlling the N content to be below 0.0020%;

continuous casting temperature: hoisting the molten steel to a continuous casting platform at the temperature of 1529-1541 ℃; the cross section of the continuous casting rectangular billet is 260 multiplied by 350mm or the slab size is 230 multiplied by 1860mm; the rectangular slab is used for rolling sections such as flat-bulb steel, L-shaped steel, T-shaped steel and the like, and the slab is used for rolling a steel plate with the thickness of less than or equal to 60mm;

2) And rolling:

heating the casting blank at a temperature: 1160-1285 ℃, soaking temperature: 1165 to 1265 ℃, and soaking for more than or equal to 68 minutes;

the finishing rolling initial rolling temperature: the finishing temperature is less than or equal to 916 ℃, and the finishing temperature is as follows: 798-816 ℃;

final rolling pass reduction: 28 to 36 percent;

thickness of blank before finish rolling: 76-126mm;

3) And laminar cooling after finish rolling:

controlling the cooling rate of the final rolling by a laminar cooling system, and controlling the temperature of the red returning of the steel after the steel exits from the laminar cooling device to be 516-616 ℃; the steel enters a laminar cooling device to completely exit the laminar cooling device, and the cooling rate is controlled to be 4.1-6.6 ℃/s;

as shown in figure 1, in the cooling and advancing process of a roller bed 6, the head, the middle part and the tail part of a steel material 1 are provided with temperature measuring devices 2, the water flow of a water nozzle 3 is controlled by a computer 4, the temperature measuring devices 2 are controlled by an electronic intelligent system 5, the cooling rate is calculated by the intelligent control system and is compared with a set value, a deviation value is fed back to a water nozzle control computer, the water flow of the water nozzle is accurately controlled, and the cooling rate of the steel material is controlled within a set range; then air-cooled to room temperature.

The invention is in the intelligent laminar cooling control system and cooling system after the final rolling, and the water spray nozzle 3 controls the design and application of the computer 4, the temperature measuring device 2 and the electronic intelligent control system 5, aiming at avoiding the temperature fluctuation of steel in the laminar cooling process caused by artificial subjective factors, thereby causing abnormal microstructure and even mixed crystal, causing great fluctuation of mechanical properties, and even failing to meet the technical requirements;

controlling the cooling speed at 4.1-6.6 ℃/s and the temperature of the re-reddening at 516-616 ℃, aiming at obtaining a small amount of tempered bainite + tempered martensite structure with uniform structure; if the temperature is higher than 6.6 ℃/s, supersaturated martensite is easy to occur, so that the residual internal stress is larger, the steel deforms more and even damages a laminar cooling facility in the cooling process, and the surface quality cannot meet the technical requirement; if the cooling rate is lower than 4.1 ℃/s, partial ferrite structure is easy to precipitate, eutectoid ferrite is easy to recrystallize and coarsen after the residual temperature tempering, the mixed crystal phenomenon occurs, and the impact toughness at minus 110 ℃ is deteriorated.

According to the mass percentages of the chemical element components and the requirements of the production method, five embodiments are prepared, namely embodiment 1, embodiment 2, embodiment 3, embodiment 4 and embodiment 5; in order to verify the influence of the chemical components and the mass percentage content as well as the process parameters such as the rolling start temperature of final rolling, the reduction rate of the final rolling pass, the intelligent control cooling rate after the final rolling, the temperature of red returning and the like on the performance parameters, three comparative examples, namely comparative example 1, comparative example 2 and comparative example 3, are prepared, namely 8 batches of steel are prepared; wherein, the chemical components of the comparative example 1 are not in the range of the invention, the process parameters of the preparation process are in the range of the invention, the chemical components of the comparative example 2 are in the range of the invention, the process parameters of the preparation process are not in the range of the invention, and the chemical components of the comparative example 3 and the process parameters of the preparation process are not in the range of the invention; the chemical element composition weight percentages of the five examples and the three comparative examples are shown in table 1, wherein the balance is Fe and unavoidable impurities. The process control parameters and steel properties during the production are shown in Table 2.

TABLE 1 comparison of chemical compositions (wt%) of inventive and comparative examples

TABLE 2 production process control parameters and steel performance condition tables of examples of the present invention and comparative examples

As can be seen from tables 1 and 2, the yield strength of the steel produced by the chemical components and the mass percentage of the steel of examples 1 to 5 and the process parameters controlled by the production process is not lower than 395MPa, the impact toughness at-110 ℃ is not lower than 125J, the HS environmental corrosion rate is not higher than 0.3 mm/year, the tensile strength is in the range of 560 to 695MPa, the surface quality is good, the yield ratio is low, and the process performance is good;

while the yield strength of the comparative example 1 reaches 416MPa and the tensile strength reaches 606MP, the impact energy at the temperature of-110 ℃ does not reach 125J, the HS environmental corrosion rate reaches 5.5 mm/year and is far more than the requirement of 0.3 mm/year;

the yield strength of the steel produced by the comparative examples 2 and 3 is 395MPa and lower than 382MPa, the tensile strength is not higher than 560MPa and lower than 528MPa, the impact energy at-110 ℃ is not higher than 125J and not higher than 41J, and the HS environmental corrosion rate is not higher than 0.3 mm/year and not lower than 4.2 mm/year;

the yield strength of the steel prepared in the embodiment 2 of the invention is 421MPa, the tensile strength is 575MPa, the impact energy reaches 258J at minus 110 ℃, the HS environmental corrosion rate is 0.12 mm/year, and the steel is the best embodiment.

The above are only preferred embodiments of the present invention, and the scope of the present invention is not limited to the above examples, and all technical solutions that fall under the spirit of the present invention belong to the scope of the present invention. It should be noted that modifications and adaptations to those skilled in the art without departing from the principles of the present invention may be apparent to those skilled in the relevant art and are intended to be within the scope of the present invention.

Claims (6)

1. The Mn-Ni series ultralow temperature steel is characterized by comprising the following chemical components in percentage by weight: 0.056 to 0.086 percent of C, 0.05 to 0.095 percent of Si, 1.95 to 2.25 percent of Mn, less than or equal to 0.007 percent of P, less than or equal to 0.003 percent of S, less than or equal to 0.012 percent of Cr, ni:1.85 to 2.06%, 0.46 to 0.88% of Cu, mo: 0.011-0.018%, nb 0.076-0.098%, N not more than 0.0020%, and Fe and inevitable impurities in balance.

2. The method for preparing Mn-Ni based ultra-low temperature steel according to claim 1, characterized by comprising the following steps:

step (1), smelting in a converter;

step (2), rolling;

and (3) carrying out laminar cooling after finish rolling.

3. The method of manufacturing Mn-Ni based ultra-low temperature steel according to claim 2, wherein,

in the step (1), the converter smelting comprises the following specific processes:

1. loading molten iron subjected to pre-S removal treatment into a top-bottom combined blowing converter, blowing oxygen, heating, oxidizing, removing C, sampling and inspecting;

2. refining in an LF electric furnace outside a furnace, adding MnFe, siFe, nbFe, niFe and MoFe alloy materials for smelting, and adding high-quality lime to remove the S content until the S content is less than or equal to 0.003%;

3. sampling and inspecting the content of the alloy elements, and adjusting the content of each alloy element;

4. RH vacuum refining, wherein the treatment time under ultimate vacuum is more than or equal to 16 minutes to remove harmful impurities in the steel;

5. sampling and checking the N content, and controlling the N content to be below 0.0020%;

6. and (4) continuous casting.

4. The manufacturing method of Mn-Ni based ultra-low temperature steel according to claim 3, characterized in that,

in step six, the conditions of the continuous casting are:

casting temperature: hoisting the molten steel to a continuous casting platform at the temperature of 1529-1541 ℃;

the cross section of the continuous casting rectangular billet is 260 multiplied by 350mm or the slab size is 230 multiplied by 1860mm;

the plate blank is used for rolling a steel plate with the thickness less than or equal to 60 mm.

5. The method of manufacturing Mn-Ni based ultra-low temperature steel according to claim 2, characterized in that,

in the step (2), the rolling conditions are as follows:

heating the casting blank at a temperature: 1160-1285 ℃, soaking temperature: 1165 to 1265 ℃, and the soaking holding time is more than or equal to 68 minutes;

the finishing rolling initial rolling temperature: the finishing temperature is less than or equal to 916 ℃, and the finishing temperature is as follows: 798-816 ℃;

final rolling pass reduction: 28 to 36 percent; thickness of blank before finish rolling: 76-126mm.

6. The method of manufacturing Mn-Ni based ultra-low temperature steel according to claim 2, wherein,

in the step (3), in the finish-rolling post-laminar cooling,

controlling the cooling rate of the final rolling by a laminar cooling system, and controlling the temperature of the red returning of the steel after the steel exits from the laminar cooling device to be 516-616 ℃;

the steel enters a laminar cooling device to completely exit the laminar cooling device, and the cooling rate is controlled to be 4.1-6.6 ℃/s;

the device comprises steel (1), a roller way (6), a temperature measuring device (2) connected with each other through a line, a water nozzle (3), a computer (4) and an electronic intelligent system (5);

the specific operation flow is as follows: placing steel (1) on a roller way (6) for cooling, wherein in the cooling and advancing process of the steel (1) on the roller way (6), temperature measuring devices (2) are arranged at the head, the middle part and the tail part of the steel, the water flow of a water nozzle (3) is controlled by a computer (4), the temperature measuring devices (2) are controlled by an electronic intelligent system (5), the cooling rate is calculated by the intelligent control system and is compared with a set value, a deviation value is fed back to the water nozzle (3) to control the computer (4), the water flow of the water nozzle (3) is accurately controlled, and the cooling rate of the steel is controlled within a set range; then air-cooled to room temperature.

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