WO2019037749A1 - Steel for use in low-temperature pressurized vessel and manufacturing method therefor - Google Patents

Abstract

A steel for use in a low-temperature pressurized vessel, the mass percent proportions of the chemical elements of the steel being: C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0-12.0%, N≤0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, Ca and/or Mg 0.001-0.005%, optionally V and/or Ti 0.1-0.3%, and the remainder being Fe and other unavoidable impurities. A manufacturing method for the low-temperature pressurized vessel comprises the steps of: (1) smelting: converter smelting and then LF+RH refining; (2) continuous casting; (3) hot rolling; (4) quenching heat treatment; and (5) tempering. The steel for use in a low-temperature pressurized vessel has great low-temperature impact toughness.

Description

Steel for low temperature pressure vessel and manufacturing method thereof Technical field

The present invention relates to a steel and a method of manufacturing the same, and more particularly to a nickel-containing steel and a method of manufacturing the same for use in a cryogenic pressure vessel.

Background technique

9% Ni steel refers to low carbon steel with a Ni content of about 9%. It was founded in the product research laboratory of the International Nickel Company of the United States, and the minimum operating temperature is -196 °C. In 1952, the first 9% Ni steel storage tank was put into use in the United States. Japan built the first LNG cryogenic storage tank in China in 1969. The maximum tank capacity of the tanks built has reached 20×10 ⁴ m ³ . As the newly added proven reserves of domestic natural gas continue to grow, the government is increasingly paying attention to the development and utilization of natural gas and the design and construction of its cryogenic storage equipment. In the 1980s, in the Daqing Ethylene Project, a large 9% Ni steel vinyl spherical tank was successfully built for the first time. In 2004, the first large-scale low-temperature liquefied natural gas project in China, the Guangdong LNG project, started with a single tank volume of 16×10 ⁴ m ³ . To date, 9% Ni steel has been used in LNG equipment for more than 60 years. Due to its excellent low temperature toughness and good welding performance, 9% Ni steel has become a widely used steel in the field of cryogenic equipment.

The low temperature mechanical properties of 9% Ni steel are mainly determined by the chemical composition, especially the content of Ni and C elements. In addition, the toughness of the steel depends on the purity of the steel as well as the microstructure.

The production of 9% Ni steel adopts the continuous casting steelmaking process. The metallurgical treatment in the steel casting process, the vacuum degassing process and the high purity of the steel play an extremely important role in improving the low temperature toughness of the steel. Since the presence of impurity elements such as P, S deteriorates the low temperature toughness of the steel, it is necessary to strictly control the content of impurity elements such as P, S to a relatively low level.

Japan incorporated 9% Ni steel into the JIS standard in 1977. In the same year, the United States also included 9% Ni steel in ASME and ASTM standards. The code, chemical composition and mechanical properties of 9% Ni steel in major industrial countries are shown in Tables 1 and 2.

Table 1: Chemical composition (wt%) of related typical steel grades in the prior art

Table 2: Mechanical properties of typical steels in the prior art

As can be seen from Tables 1 and 2, prior art steels for cryogenic pressure vessels are increasingly unable to meet increasing use and manufacturing requirements. In view of this, it is desirable to obtain a steel for a low-temperature pressure vessel, the mechanical properties and low-temperature impact toughness of the low-temperature pressure vessel are improved as compared with the prior art, and the production cost of the steel for producing the low-temperature pressure vessel is more economical. .

Summary of the invention

One of the objects of the present invention is to provide a steel for a low temperature pressure vessel which adopts a microalloy addition design without adding excessive expensive elements such as Ni by adding an appropriate amount of Nb, and Ca and/or Mg elements and optionally V and / or Ti, control the lower content of total oxygen, so that the steel for low temperature pressure vessel has higher strength, good molding performance and low temperature impact toughness, and the cost of steel material is lower than that of the prior art. .

Based on the above object, the present invention provides a steel for a low-temperature pressure vessel having a chemical element mass distribution ratio of C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0-12.0%, N. ≤0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, Mg 0-0.005%, Ca 0-0.005%, V 0-0.3% and Ti 0-0.3%; balance is Fe and other unavoidable impurities And the sum of the mass distribution ratios of Ca and Mg is 0.001-0.005%.

In certain embodiments, the steel for a cryogenic pressure vessel of the present invention contains only at least one or two of Ca and Mg, and does not contain V and Ti. In these embodiments, the chemical element mass distribution ratio of the steel for low temperature pressure vessel of the present invention is: C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni7.0-12.0%, N≤0.005 %, Al 0.015-0.05%, Nb 0.1-0.3%, Ca and/or Mg 0.001-0.005%; the balance is Fe and other unavoidable impurities.

In certain embodiments, the steel for low temperature pressure vessel of the present invention further contains at least one or two of V and Ti, and the sum of the mass distribution ratios of V and Ti is in the range of 0.1 to 0.3%. Therefore, in these embodiments, the chemical element mass distribution ratio of the steel for low temperature pressure vessel of the present invention is: C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0-12.0%, N≤ 0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, V and/or Ti 0.1-0.3%, Ca and/or Mg 0.001-0.005%; the balance is Fe and other unavoidable impurities.

In certain embodiments, the steel for low temperature pressure vessel of the present invention contains V and Ca, and the chemical element mass ratio is: C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0- 12.0%, N≤0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, V 0.1-0.3%, Ca 0.001-0.005%; the balance is Fe and other unavoidable impurities. In some embodiments, the low temperature pressure vessel has a chemical element mass distribution ratio of C: 0.02-0.06%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0-12.0%, N≤0.005% Al 0.015-0.05%, Nb 0.1-0.3%, V 0.1-0.3%, Ca 0.001-0.005%; the balance is Fe and other unavoidable impurities.

In certain embodiments, the steel for low temperature pressure vessel of the present invention contains Ti and Mg having a chemical element mass ratio of C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0- 12.0%, N≤0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, Ti 0.1-0.3%, Mg 0.001-0.005%; the balance is Fe and other unavoidable impurities.

In certain embodiments, the steel for low temperature pressure vessel of the present invention contains only Mg, does not contain Ca, Ti and V, and has a chemical element mass distribution ratio of C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3. -0.8%, Ni 7.0-12.0%, N≤0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, Mg 0.001-0.005% (preferably 0.001-0.003%); balance is Fe and other unavoidable impurities .

Compared with the prior art, the steel for low temperature pressure vessel of the present invention forms Nb (C, N) by adding an appropriate amount of Nb, which is advantageous for improving strength and improving impact toughness; moreover, by adding Ca and/or Mg and optionally V and/or Ti can significantly improve the low temperature impact toughness of the steel while taking into account the effect of increasing the strength of the steel.

Further, in the present case, the microstructure of the steel for the low-temperature pressure vessel is evolved as follows: from the solidification of the continuous casting slab to the austenite structure at room temperature. After hot rolling, the main structures treated by quenching + tempering (QT) heat treatment are low carbon tempered martensite. Among them, the quenching treatment can obtain martensite with fine grain, and the subsequent tempering treatment converts the martensite structure into ferrite and fine precipitated carbide, and a small amount of diffused austenite can be obtained. The toughness of the base metal is greatly improved, and it is especially suitable for the manufacture of parts that are resistant to low temperature and pressure.

The design principle of each chemical element of the steel for low temperature pressure vessel according to the present invention is as follows:

C: Usually, the mass percentage of C mainly affects the precipitation amount and precipitation temperature range of the carbide. In the steel for a low-temperature pressure vessel according to the present invention, carbon has a certain strengthening effect, and controlling the lower mass percentage of C is advantageous for improving the impact toughness of the steel. However, an excessively high mass percentage of carbon reduces the corrosion resistance of the material. In order to balance mechanical properties and impact toughness, the mass percentage of C is controlled to 0.02-0.08%. In certain embodiments, the mass percentage of C is controlled to be 0.02-0.06%.

Si: Si can be increased in strength in steel, but Si is also detrimental to the formability and toughness of steel. The mass percentage of Si to be controlled in the steel for a low temperature pressure vessel according to the present invention is from 0.10 to 0.35%, preferably from 0.10 to 0.30%.

Mn: Mn is an austenite element, which suppresses the harmful effects of S in a nickel-based corrosion-resistant alloy and improves thermoplasticity. However, too high a mass percentage of Mn is detrimental to its corrosion resistance. Therefore, considering the mechanical properties and corrosion resistance, the low temperature pressure vessel of the present invention is limited to a mass percentage of Mn of 0.3 to 0.8%, preferably 0.35 to 0.7%.

Ni: Ni is a main element in the steel for a low-temperature pressure vessel according to the present invention, and has excellent austenite phase stability, and can improve the mechanical properties and impact toughness of the steel for a low-temperature pressure vessel according to the present invention. With the increase of Ni, the tensile strength at high temperature gradually increases. This is because when the mass percentage of Ni is low, most of Ni is dissolved in austenite, expanding the austenite phase region and increasing the recrystallization temperature. The mechanical properties of the alloy are improved and improved. Therefore, the mass percentage of Ni in the steel for a low-temperature pressure vessel according to the present invention is limited to 7.0 to 12.0%, preferably 7.5 to 10.5%.

N: N is a stable austenite element. The N having a lower percentage of controlled mass is advantageous for improving the impact toughness of the steel for the low temperature pressure vessel. However, higher mass percentages of nitrogen tend to result in reduced toughness and ductility of the steel and also reduce the hot workability of the steel. Therefore, the mass percentage of N in the steel for a low-temperature pressure vessel according to the present invention is limited to N ≤ 0.005%.

Al: In the technical solution described in the present invention, the alloy is strengthened mainly by controlling the oxygen content in the steel by Al, thereby affecting the dislocation behavior. Increasing the mass percentage of Al can significantly increase the solution temperature and creep strength, but the excessive mass percentage of Al impairs the plasticity of the steel. In addition, the addition of Al is beneficial to improve the elongation and deformation properties of the steel, thereby improving the processing properties of the steel. However, an Al content of more than 0.05% by mass is required to lower the impact toughness of the steel. Based on the above considerations, the mass percentage of Al in the steel for a low temperature pressure vessel according to the present invention is limited to 0.015 to 0.05%, preferably 0.02 to 0.04%.

Nb: Nb is one of the commonly used solid solution strengthening elements. The atomic radius of Nb is 15-18% larger than that of Ni, Co, and Fe atoms. In addition, Nb is a strong carbonitride forming element, and combines with carbon and nitrogen to form Nb (C, N), which is beneficial to improve strength and improve impact. toughness. At the same time, carbon and nitrogen have a certain strengthening effect. Some Nb in the steel forms Nb (C, N), which can strengthen the austenite phase matrix, refine the austenite grains, and strengthen the austenite grain boundaries. It is advantageous to improve the low temperature impact toughness of the steel for low temperature pressure vessels. Therefore, the mass percentage of Nb in the steel for a low temperature pressure vessel according to the present invention is limited to 0.1 to 0.3%, preferably 0.1 to 0.2%.

Mg: Micro-magnesium segregation at grain boundaries reduces grain boundary energy and phase boundary energy, improves and refines the morphology of grain boundary carbides and other grain boundary precipitation phases, such as blocking or spheroidizing carbides, effectively inhibiting grain boundary sliding Reduce the grain boundary stress concentration and eliminate the notch sensitivity. In addition, magnesium and sulfur and other harmful impurities form high melting point compounds MgO and MgS, etc., purify the grain boundary, so that the concentration of impurity elements such as S, O, P, etc. at the grain boundary is significantly reduced, and the harm of impurity elements is reduced. During the solidification process, MgO and MgS in the steel can be used as nucleation sites to refine grains. Trace magnesium improves plasticity, improves high temperature tensile ductility, and increases impact toughness and fatigue strength.

Ca: Calcium can change the composition, quantity and morphology of non-metallic inclusions in steel. In addition, calcium can refine the grain of steel, deoxidize and desulfurize, and CaO and CaS can be used as nucleation sites to refine the solidified structure. Improve steel corrosion resistance, wear resistance, high temperature resistance, low temperature resistance; improve steel plasticity, impact toughness, fatigue strength and welding performance; enhance steel resistance to thermal cracking, hydrogen cracking and delamination resistance .

The steel for low temperature pressure vessel of the present invention contains any one or both of Ca and Mg, and the content of Ca is 0-0.005%, such as 0.001-0.005%; the content of Mg is 0-0.005%, such as 0.001-0.005 %; the condition is that the sum of the contents of Ca + Mg is in the range of 0.001 - 0.005%. In certain embodiments, the steel for low temperature pressure vessel of the present invention contains only Mg in an amount ranging from 0.001 to 0.005%, preferably from 0.001 to 0.003%.

V:V refines the grain of the structure and improves strength and toughness. In order to obtain fine-grained martensite after quenching, the addition of vanadium is a more effective means. Vanadium is a strong carbide forming element and has a strong binding bond with carbon to form a stable VC. It is a typical high melting point, high hardness, high dispersion carbide, and is an element that strongly improves wear resistance. The particles that form during the tempering process or form VC at other stages are finely dispersed. The rhodium-vanadium compound is added in a higher strength than the Nb alone. At the same time, the austenite grains can be further refined, so that the ferrite grains after cooling are finer, which is advantageous for improving strength and toughness.

Ti: Ti has solid solution strengthening and precipitation strengthening in steel, and it has strong binding ability with O, which can reduce the oxygen content in steel. In addition, Ti combines with C and N to form Ti(C, N), which can refine the solidified structure. In alloys containing higher Ni, especially under the combined action of Nb and Al, the addition of Ti forms Ni ₃ (Al, Ti, Nb), which improves the strength and toughness of the steel.

The steel for low temperature pressure vessel of the present invention may further contain any one or both of V and Ti, and the content of V is 0-0.3%, such as 0.1-0.3%; and the content of Ti is 0-0.3%, such as 0.1-0.3%. In certain embodiments, when V and/or Ti are included, the sum of the V+Ti contents is in the range of 0.1-0.3%.

It should be noted that, in the technical solution described in the present invention, the unavoidable impurity elements include O, P, and S. For the technical solution of the present invention, O is mainly present as an oxide inclusion, and a high total oxygen content indicates that there are many inclusions, and reducing the total oxygen content is advantageous for improving the comprehensiveness of the material, and thus the above-mentioned unavoidable impurity elements are described above. The mass percentage of the steel for the low temperature pressure vessel is controlled by: total oxygen ≤ 0.001%, P ≤ 0.010%, S ≤ 0.005%.

Further, in the steel for a low-temperature pressure vessel according to the present invention, the chemical element further has a rare earth element having a mass distribution ratio of ≤1%, such as 0.1-1%. In the present invention, the rare earth element includes Ce, Hf, La, Re, Sc, and Y. At least one of Ce, Hf, La, Re, Sc, and Y may be added to the steel for a low-temperature pressure vessel of the present invention, and the total mass percentage of the rare earth element to be added is ≤1%.

In the technical solution of the present invention, the rare earth element acts as a purifying agent, and has the functions of deoxidation and desulfurization, thereby reducing the harmful effects of oxygen and sulfur at the grain boundary; in addition, the rare earth element is segregated at the grain boundary as a microalloying element, To strengthen the grain boundary; and, as an active element, the rare earth element improves the oxidation resistance of the alloy and improves the surface stability.

Further, in the steel for low temperature pressure vessel according to the present invention, there are (Nb) CN particles, MgO and/or MgS particles and/or CaO and/or CaS particles in the microstructure, optionally containing V ( C, N) particles and / or Ti (C, N) particles.

When an element selected from the group consisting of V and Ti or a combination thereof and Mg and Ca or a combination thereof is added to the steel for a low-temperature pressure vessel according to the present invention, a small amount of V (C, N) is formed in the alloy during cooling and solidification. And/or Ti(C, N) and CaO and/or MgO and/or CaS particles and/or MgS particles. The above particles are advantageous for refining and stabilizing the austenite grains, thereby avoiding the formation of crack defects on the surface of the continuous casting billet or the hot rolled sheet by the steel for the low temperature pressure vessel, and also improving the low temperature impact toughness of the material.

Further, in the steel for a low-temperature pressure vessel according to the present invention, when V(C, N) particles are contained, the particles have a diameter of about 0.2 to 5 μm; when CaO and/or CaS particles are contained, the particles are The diameter is about 0.2-5 μm; when containing Ti(C,N) particles, the particles have a diameter of about 0.1-8 μm; when MgO and/or MgS particles are contained, the particles have a diameter of about 0.1-8 μm.

Further, in the steel for a low-temperature pressure vessel according to the present invention, when it is contained, the number of V(C, N) particles in the cross section of the steel for the low-temperature pressure vessel is 5 to 20/mm ² . The number of CaO and/or CaS particles is 5 to 20 / mm ² , the number of Ti (C, N) particles is 5 to 25 / mm ² , and the number of MgO and / or MgS particles is 5 to 25 / mm ² . When only Mg and/or Ca is contained without V and/or Ti, the amount of MgO and/or MgS particles and/or CaO and/or CaS is 15 to 55 / mm ² .

Further, in the steel for a low-temperature pressure vessel according to the present invention, when V is contained only, the mass percentage thereof is 0.1-0.2%; and when Ti is contained only, the mass percentage thereof is 0.1-0.2%; or When both V and Ti are contained, the sum of the mass percentages of the two is 0.1-0.2%.

Further, in the steel for a low-temperature pressure vessel according to the present invention, when Ca is contained only, the mass percentage thereof is 0.001 to 0.003%; or when only Mg is contained, the mass percentage thereof is 0.001 to 0.003%; When both Ca and Mg are contained, the sum of the mass percentages of the two is 0.001 to 0.003%.

Accordingly, in certain embodiments, the chemical element mass percent ratio of the steel for cryogenic pressure vessels of the present invention is:

C: 0.02-0.08%, preferably 0.02-0.06%;

Si: 0.10-0.35%, preferably 0.1-0.3%;

Mn: 0.3-0.8%, preferably 0.35-0.7%;

Ni: 7.0-12.0%, preferably 7.5-10.5%;

N: ≤ 0.005%;

Al: 0.015-0.05%, preferably 0.02-0.04%;

Nb: 0.1-0.3%, preferably 0.1-0.2%;

Mg: 0.001-0.005%, preferably 0.001-0.003%, or Ca: 0.001-0.005%, preferably 0.001-0.003%, or Mg+Ca: 0.001-0.005%, preferably 0.001-0.003%;

Optionally 0.1-0.3%, preferably 0.1-0.2% V;

Optionally 0.1-0.3%, preferably 0.1-0.2% Ti; and

Rare earth elements: ≤1%

The balance is Fe and other unavoidable impurities.

In certain embodiments, the chemical element mass ratio of the steel for cryogenic pressure vessels of the present invention is:

C: 0.02-0.06%;

Si: 0.1-0.3%;

Mn: 0.35-0.7%;

Ni: 7.5-10.5%;

N: ≤ 0.005%;

Al: 0.02-0.04%;

Nb: 0.1-0.2%;

Mg: 0.001-0.003%, or Ca: 0.001-0.003%, or Mg+Ca: 0.001-0.003%;

Optionally 0.1-0.3%, preferably 0.1-0.2% V;

Optionally 0.1-0.3%, preferably 0.1-0.2% Ti; and

Rare earth elements: ≤1%

The balance is Fe and other unavoidable impurities.

Further, in the steel for low temperature pressure vessel according to the present invention, the tensile strength is ≥ 850 MPa, the yield strength is ≥ 625 MPa, the elongation is ≥ 25%, and the impact toughness at -196 ° C is ≥ 150 J. In some embodiments, the steel for low temperature pressure vessel according to the present invention has a tensile strength of 850-870 MPa, a yield strength of 625-650 MPa, an elongation of 25-30%, and an impact toughness at -196 °C. 150-170J.

Accordingly, another object of the present invention is to provide a method for producing a steel for a low temperature pressure vessel as described above, which comprises the steps of:

(1) Smelting: converter smelting, then LF+RH refining;

(2) continuous casting;

(3) hot rolling;

(4) quenching heat treatment;

(5) Tempering treatment.

In the manufacturing method of the present invention, a small amount of ferrovanadium and/or ferrotitanium is added at the end of RH refining to add V and/or Ti, and a calcium wire is fed to add Ca and/or a nickel-magnesium alloy to be added to add Mg. After further controlling the mass percentage of each element in the steel to satisfy the range defined by the present invention, soft agitation of the argon blowing gas is performed, and the flow rate of the argon gas is controlled at 5 to 8 liters/min.

Further, in the manufacturing method of the present invention, there is also a grinding step before the hot rolling step.

Further, in the manufacturing method of the present invention, in the step (2), the control of the pulling speed is 0.9 to 1.2 m/min.

Further, in the manufacturing method of the present invention, in the step (2), electromagnetic stirring is performed by a crystallizer during continuous casting, and the control current is 500-1000 A and the frequency is 2.5-3.5 Hz, so that after continuous casting The slab equiaxed crystal ratio is ≥40%.

Further, in the manufacturing method of the present invention, the step (3) includes rough rolling and finish rolling, wherein the rough rolling temperature is controlled to be 1150 to 1250 ° C, and the finishing rolling temperature is 1050 to 1150 ° C.

Further, in the production method of the present invention, in the step (3), the total reduction ratio is controlled to be 60 to 95%, such as 60 to 90%.

Further, in the manufacturing method of the present invention, in the step (4), the quenching heat treatment temperature is 750 to 850 ° C, the holding time is 60 to 90 min, and water cooling is performed at the time of discharging.

Further, in the manufacturing method of the present invention, in the step (5), the tempering treatment temperature is 550 to 650 ° C, the holding time is 40 to 120 min, and air cooling is performed after the furnace is discharged. The parameter setting of the above scheme is beneficial to improve the room temperature mechanical properties and low temperature impact toughness of the steel, thereby obtaining a hot rolled product whose comprehensive performance meets the production requirements.

The steel for low temperature pressure vessel according to the present invention adopts a microalloy addition design, and does not need to add excessive expensive elements such as Ni, and is controlled by adding an appropriate amount of Nb, V and/or Ti, Ca and/or Mg elements. The low content of total oxygen makes the steel for low temperature pressure vessel have higher strength, good molding performance and low temperature impact toughness, and the steel material cost is lower than that of the prior art.

Detailed ways

The steel for low temperature pressure vessel and the method for producing the same according to the present invention will be further explained and explained below in conjunction with specific embodiments. However, the explanation and description are not intended to unduly limit the technical solutions of the present invention.

Examples 1-6 and Comparative Examples 1-3

The steel for the low temperature pressure vessel of Examples 1-6 was obtained by the following steps:

(1) Smelting: converter smelting, then LF+RH refining, controlling the mass percentage of each chemical element as shown in Table 3;

(2) Continuous casting: control pull speed control is 0.9~1.2m/min, electromagnetic stirring is used in continuous casting, control current is 500-1000A, frequency is 2.5~3.5Hz, so that slab after continuous casting, etc. Axial crystal ratio ≥ 40%;

(3) Hot rolling: including rough rolling and finish rolling, wherein the rough rolling temperature is controlled to be 1150 to 1250 ° C, the finishing rolling temperature is 1050 to 1150 ° C, and the total pressure control efficiency is 60 to 90%;

(4) Quenching heat treatment: the temperature is 750-850 ° C, the holding time is 60-90 min, and the water is cooled;

(5) tempering treatment: the temperature is 550 ~ 650 ° C, the holding time is 40-120 min, and the air is cooled.

It should be noted that the steel for a low temperature pressure vessel of Examples 1-6 further has a grinding step before the hot rolling step. Comparative steels of Comparative Examples 1-3 were prepared using the prior art.

Table 3 lists the mass ratios of the respective chemical elements in the steel for the low temperature pressure vessel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

Table 3: (wt%, balance is Fe and other inevitable impurity elements other than O, P and S)

Table 4 lists the specific process parameters of the manufacturing method of each example.

Table 4

When the microstructure of the steel for low temperature pressure vessel of the above Examples 1-6 was observed, it was found that the microstructure of each of the examples in the present case was austenite structure from the solidification of the continuous casting slab to the room temperature, and was subjected to hot rolling. After quenching + tempering (QT) heat treatment, the main structure of the case is low carbon tempered martensite, in which quenching treatment can obtain fine grained martensite, followed by tempering treatment to make martensite The structure is transformed into ferrite and fine precipitated carbides, and a small amount of dispersed austenite is obtained. This structure can greatly improve the toughness of the base metal, and is particularly suitable for manufacturing parts that are resistant to low temperature and pressure. Wherein the microstructure of each embodiment has V(C, N) particles and CaO and/or CaS particles, the V(C, N) particles, CaO and/or CaS particles having a diameter of about 0.2-5 μm, The number of V (C, N) particles and CaO and/or CaS particles in the cross section of the steel for the low temperature pressure vessel is 5 to 20 / mm ² .

Further, the steel for low temperature pressure vessel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 were sampled, and various performance tests were performed on the samples, and the results obtained by the tests are shown in Table 5.

table 5

Numbering	Yield strength R el (MPa)	Tensile strength R m (MPa)	Elongation rate (%)	-196 °C impact toughness (J)
Example 1	625	854	26	153
Example 2	632	850	25	165
Example 3	629	862	26	158
Example 4	628	858	27	161
Example 5	626	865	26	156
Example 6	627	853	28	157
Comparative example 1	576	695	20	108
Comparative example 2	584	734	18	105
Comparative example 3	593	721	19	107

As can be seen from Table 5, the yield strength, tensile strength, elongation and impact toughness at -196 °C of the examples in this case were significantly higher than the yield strength, tensile strength, elongation and -196 ° C of each comparative example. Impact toughness, indicating that the mechanical properties and low temperature impact toughness of the examples in the present case are high. Further, in each of the examples, the tensile strength was ≥ 850 MPa, the yield strength was ≥ 625 MPa, the elongation was ≥ 25%, and the impact toughness at -196 ° C was ≥ 150 J.

Example 7-12

The steel for the low temperature pressure vessel of Examples 7-12 was obtained by the following steps:

(1) Smelting: converter smelting, then LF+RH refining, controlling the mass percentage of each chemical element as shown in Table 3;

(2) Continuous casting: control pull speed control is 0.9~1.2m/min, electromagnetic stirring is used in continuous casting, control current is 500A, frequency is 2.5～3.5Hz, so that the slab equiaxed crystal after continuous casting The ratio is ≥40%;

(3) Hot rolling: including rough rolling and finish rolling, wherein the rough rolling temperature is controlled to be 1150 to 1250 ° C, the finishing rolling temperature is 1050 to 1150 ° C, and the total pressure control efficiency is 60 to 90%;

(4) Quenching heat treatment: the temperature is 750-850 ° C, the holding time is 60-90 min, and the water is cooled;

(5) tempering treatment: the temperature is 550 ~ 650 ° C, the holding time is 40-120 min, and the air is cooled.

It should be noted that the steel for a low temperature pressure vessel of Examples 7 to 12 also has a grinding step before the hot rolling step.

Table 6 lists the mass ratios of the chemical elements of the steel for the low temperature pressure vessel of Examples 7-12.

Table 6: (wt%, balance is Fe and other inevitable impurity elements other than O, P and S)

Numbering	C	Si	Mn	Ni	P	S	N	Al	Nb	O	Ti	Mg	Rare earth element
Example 7	0.04	0.10	0.5	11.0	0.008	0.004	0.002	0.015	0.10	0.0009	0.2	0.003	Ce: 0.4
Example 8	0.08	0.30	0.3	9.0	0.003	0.003	0.003	0.03	0.13	0.0007	0.1	0.002	-
Example 9	0.06	0.22	0.7	10.0	0.007	0.004	0.004	0.035	0.28	0.0009	0.3	0.001	Hf: 0.5
Example 10	0.02	0.35	0.8	7.0	0.99	0.005	0.005	0.05	0.2	0.0008	0.1	0.002	Ce, La: 0.7
Example 11	0.05	0.28	0.4	12.0	0.006	0.002	0.004	0.04	0.3	0.0010	0.2	0.001	-
Example 12	0.03	0.18	0.6	8.0	0.004	0.005	0.003	0.025	0.18	0.0007	0.3	0.003	Se: 0.3

Table 7 lists the specific process parameters of the manufacturing method of each example.

Table 7

The microstructure of the steel for low temperature pressure vessel of the above Examples 7-12 was observed, and it can be found that the microstructure of each embodiment of the present invention is austenite structure from the solidification of the continuous casting slab to the room temperature, and the heat is passed through the heat. After rolling, after quenching + tempering (QT) heat treatment, the main structure of the case is low carbon tempered martensite, in which quenching treatment can obtain fine grained martensite, and then tempering treatment makes Markov The bulk structure is transformed into ferrite and fine precipitated carbides, and a small amount of dispersed austenite is obtained. This structure can greatly improve the toughness of the base metal, and is particularly suitable for manufacturing parts that are resistant to low temperature and pressure. Wherein the microstructure of each embodiment has Ti(C,N) particles and MgO and/or MgS particles, the Ti(C,N) particles, MgO and/or MgS particles having a diameter of about 0.1-8 μm, The number of Ti (C, N) particles and MgO and/or MgS particles in the cross section of the steel for the low temperature pressure vessel is 5 to 25 / mm ² .

Further, the steels for the low-temperature pressure vessel of the above Examples 7 to 12 and the conventional steels of Comparative Examples 1-3 were sampled, and various performance tests were carried out, and the results obtained by the tests are shown in Table 8.

Table 8

Numbering	Yield strength R el (MPa)	Tensile strength R m (MPa)	Elongation rate (%)	-196 °C impact toughness (J)
Example 7	635	863	27	167
Example 8	630	858	26	157
Example 9	625	857	25	163
Example 10	640	862	28	150
Example 11	638	868	29	169
Example 12	642	858	27	155
Comparative example 1	576	695	20	108
Comparative example 2	584	734	18	105
Comparative example 3	593	721	19	107

As can be seen from Table 8, the yield strength, tensile strength, elongation and impact toughness at -196 °C of the examples in this case were significantly higher than the yield strength, tensile strength, elongation and -196 ° C of each comparative example. Impact toughness, indicating that the mechanical properties and low temperature impact toughness of the examples in the present case are high. Further, in each of the examples, the tensile strength was ≥ 850 MPa, the yield strength was ≥ 625 MPa, the elongation was ≥ 25%, and the impact toughness at -196 ° C was ≥ 150 J.

Example 13-18

The steel for the low temperature pressure vessel of Examples 13-18 was obtained by the following steps:

(1) Smelting: converter smelting, then LF+RH refining, controlling the mass percentage of each chemical element as shown in Table 3;

(2) Continuous casting: control pull speed control is 0.9~1.2m/min, electromagnetic stirring is used in continuous casting, control current is 500A, frequency is 2.5～3.5Hz, so that the slab equiaxed crystal after continuous casting The ratio is ≥40%;

(3) Hot rolling: including rough rolling and finish rolling, wherein the rough rolling temperature is controlled to be 1150 to 1250 ° C, the finishing rolling temperature is 1050 to 1150 ° C, and the total pressure control efficiency is 60 to 90%;

(4) Quenching heat treatment: the temperature is 750-850 ° C, the holding time is 60-90 min, and the water is cooled;

(5) tempering treatment: the temperature is 550 ~ 650 ° C, the holding time is 40-120 min, and the air is cooled.

It should be noted that the steel for the low temperature pressure vessel of Examples 13 to 18 also has a grinding step before the hot rolling step.

Table 9 lists the mass ratios of the chemical elements of the steel for the low temperature pressure vessel of Examples 13-18.

Table 9. (wt%, balance is Fe and other inevitable impurity elements other than O, P and S)

Numbering	C	Si	Mn	Ni	P	S	N	Al	Nb	O	Mg
Example 13	0.04	0.10	0.5	11.0	0.008	0.004	0.002	0.015	0.10	0.0009	0.002
Example 14	0.08	0.30	0.3	9.0	0.003	0.003	0.003	0.03	0.13	0.0007	0.001
Example 15	0.06	0.22	0.7	10.0	0.007	0.004	0.004	0.035	0.28	0.0009	0.003
Example 16	0.02	0.35	0.8	7.0	0.99	0.005	0.005	0.05	0.2	0.0008	0.003
Example 17	0.05	0.28	0.4	12.0	0.006	0.002	0.004	0.04	0.3	0.0010	0.002
Example 18	0.03	0.18	0.6	8.0	0.004	0.005	0.003	0.025	0.18	0.0007	0.001

Table 10 lists the specific process parameters of the manufacturing method of each example.

Table 10

The microstructure of the steel for low temperature pressure vessel of the above-mentioned Examples 13-18 in the present invention was observed, and it was found that the microstructure of each embodiment of the present invention was austenite structure from the solidification of the continuous casting slab to the room temperature, and the heat was passed through. After rolling, after quenching + tempering (QT) heat treatment, the main structure of the case is low carbon tempered martensite, in which quenching treatment can obtain fine grained martensite, and then tempering treatment makes Markov The bulk structure is transformed into ferrite and fine precipitated carbides, and a small amount of dispersed austenite is obtained. This structure can greatly improve the toughness of the base metal, and is particularly suitable for manufacturing parts that are resistant to low temperature and pressure. Wherein the microstructure of each embodiment has MgO and/or MgS particles having a diameter of about 0.1-8 μm, in the cross section of the steel for the low temperature pressure vessel, MgO and/or MgS particles The number is 15 to 55 / mm ² .

Further, the steels for the low-temperature pressure vessel of the above Examples 13 to 18 and the conventional steels of Comparative Examples 1-3 were sampled, and various performance tests were carried out, and the results obtained by the tests are shown in Table 11.

Table 11

Numbering	Yield strength R el (MPa)	Tensile strength R m (MPa)	Elongation rate (%)	-196 °C impact toughness (J)
Example 13	637	865	26	166
Example 14	632	856	27	159
Example 15	625	858	28	160
Example 16	643	864	25	155

Example 7	636	865	27	166
Example 18	640	859	29	157
Comparative example 1	576	695	20	108
Comparative example 2	584	734	18	105
Comparative example 3	593	721	19	107

As can be seen from Table 11, the yield strength, tensile strength, elongation and impact toughness at -196 °C of the examples in this case were significantly higher than the yield strength, tensile strength, elongation and -196 ° C of each comparative example. Impact toughness, indicating that the mechanical properties and low temperature impact toughness of the examples in the present case are high. Further, in each of the examples, the tensile strength was ≥ 850 MPa, the yield strength was ≥ 625 MPa, the elongation was ≥ 25%, and the impact toughness at -196 ° C was ≥ 150 J.

It should be noted that the prior art in the scope of protection of the present invention is not limited to the embodiments given in the present application, and all prior art that does not contradict the solution of the present invention includes, but is not limited to, prior Patent documents, prior publications, prior disclosure, and the like can be included in the scope of protection of the present invention.

In addition, the combination of the technical features in the present invention is not limited to the combination described in the claims of the present invention or the combination described in the specific embodiments, and all the technical features described in the present invention can be freely combined or combined in any manner unless There is a contradiction between each other.

It is to be noted that the above is only specific embodiments of the present invention, and it is obvious that the present invention is not limited to the above embodiments, and there are many similar variations. All modifications that are directly derived or associated by those of ordinary skill in the art are intended to be within the scope of the invention.

Claims (17)

1. A steel for a low temperature pressure vessel, characterized in that the chemical element mass distribution ratio is:

   C: 0.02-0.08%;

   Si: 0.10-0.35%;

   Mn: 0.3-0.8%;

   Ni: 7.0-12.0%;

   N: ≤ 0.005%;

   Al: 0.015-0.05%;

   Nb: 0.1-0.3%;

   Mg or Ca: 0.001-0.005%, or Mg+Ca: 0.001-0.005%;

   V: ≤ 0.3%;

   Ti: ≤ 0.3%; and

   Rare earth element: ≤1%;

   The balance is Fe and other unavoidable impurities.
2. The steel for a low-temperature pressure vessel according to claim 1, wherein the chemical element mass distribution ratio is C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0-12.0%, N ≤ 0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, V 0.1-0.3%, Ca 0.001-0.005%, and rare earth element ≤ 1%, the balance being Fe and other unavoidable impurities; or its chemical elements The mass distribution ratio is: C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3-0.8%, Ni 7.0-12.0%, N ≤ 0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, Ti 0.1- 0.3%, Mg 0.001-0.005% and rare earth element ≤1%, the balance is Fe and other unavoidable impurities; or its chemical element mass ratio is: C 0.02-0.08%, Si 0.10-0.35%, Mn 0.3 -0.8%, Ni 7.0-12.0%, N≤0.005%, Al 0.015-0.05%, Nb 0.1-0.3%, Mg 0.001-0.005%, and rare earth element ≤1%, the balance being Fe and other unavoidable impurities.
3. The steel for a low temperature pressure vessel according to claim 1 or 2, wherein the rare earth element is at least one selected from the group consisting of Ce, Hf, La, Re, Sc, and Y.
4. The steel for low temperature pressure vessel according to claim 1 or 2, wherein the microstructure has (1) MgO and/or MgS particles, and/or (2) CaO and/or CaS particles, and optionally V (C, N) particles and / or Ti (C, N) particles.
5. The steel for a low-temperature pressure vessel according to claim 4, wherein said V (C, N) particles, CaO and/or CaS particles have a diameter of about 0.2 to 5 μm; said Ti (C, N) particles, The MgO and/or MgS particles have a diameter of about 0.1-8 μm.
6. The steel for a low temperature pressure vessel according to claim 4, wherein

   The steel for the low-temperature pressure vessel contains V and Ca, and the number of V(C, N) particles and CaO and/or CaS particles in the cross section of the steel for the low-temperature pressure vessel is 5 to 20/mm ² ;

   The steel for low temperature pressure vessel contains Ti and Mg, and the amount of Ti (C, N) particles and MgO and/or MgS particles is 5 to 25 / mm ² ;

   The steel for a low-temperature pressure vessel contains Mg, does not contain Ca, Ti, and V, and the amount of MgO and/or MgS particles is 15 to 55 / mm ² .
7. The steel for a low-temperature pressure vessel according to claim 2, wherein a mass ratio of V is 0.1 to 0.2%; and a mass ratio of Ti is 0.1 to 0.2%.
8. The steel for a low-temperature pressure vessel according to claim 2, wherein the mass ratio of Ca is 0.001 to 0.003%; and the mass ratio of Mg is 0.001 to 0.003%.
9. The steel for a low-temperature pressure vessel according to claim 1, which has a tensile strength ≥ 850 MPa, a yield strength ≥ 625 MPa, an elongation ≥ 25%, and an impact toughness at -196 ° C ≥ 150 J.
10. The method for producing a steel for a low-temperature pressure vessel according to any one of claims 1 to 9, comprising the steps of:

    (1) Smelting: converter smelting, then LF+RH refining;

    (2) continuous casting;

    (3) hot rolling;

    (4) quenching heat treatment;

    (5) Tempering treatment.
11. The manufacturing method according to claim 10, further comprising a grinding step before the hot rolling step.
12. The manufacturing method according to claim 10 or 11, wherein in the step (2), the control of the pulling speed is 0.9 to 1.2 m/min.
13. The manufacturing method according to claim 9 or 10, characterized in that in the step (2), electromagnetic stirring is performed by a crystallizer during continuous casting, and the control current is 500-1000 A and the frequency is 2.5-3.5 Hz, so that The equiaxed crystal ratio of the slab after continuous casting is ≥40%.
14. The method according to claim 10 or 11, wherein the step (3) comprises rough rolling and finish rolling, wherein the rough rolling temperature is controlled to be 1150 to 1250 ° C, and the finishing rolling temperature is 1050 to 1150 ° C.
15. The manufacturing method according to claim 10 or 11, wherein in the step (3), the total reduction ratio is controlled to be 60 to 95%.
16. The method according to claim 10 or 11, wherein in the step (4), the quenching heat treatment temperature is 750 to 850 ° C, the holding time is 60 to 90 min, and water cooling is performed at the time of discharging.
17. The manufacturing method according to claim 10 or 11, wherein in the step (5), the tempering treatment temperature is 550 to 650 ° C, the holding time is 40 to 120 minutes, and air cooling is performed after the furnace is discharged.