KR20220004220A - 7Ni steel plate and production process for LNG storage tank - Google Patents

Abstract

The present invention relates to the field of steel smelting technology, and provides a 7Ni steel plate for an LNG storage tank and a production process. Its chemical composition and mass percentage are: C: 0.02%~0.06%, Si: 0.20%~0.35%, Ni: 4.0%~8.0%, Mn: 0.3%~0.7%, P≤0.005%, S≤0.005%, Al : 0.03%~0.05%, Nb: 0.02%~0.05%, Cr: 0.2%~0.4%, the remainder is Fe and unavoidable impurities. The tensile strength of the steel sheet is 690~790Mpa, the yield strength is 590~700MPa, the elongation is ≥24%, and the transverse impact energy at -196℃ is ≥100J, and it can be used to manufacture pressure vessels that store and transport LNG. .

Description

7Ni steel plate and production process for LNG storage tank

The present invention relates to the field of steel smelting technology, and more particularly, to a 7Ni steel plate for an LNG storage tank and a production process.

As the economy develops, environmental and resource issues are emerging as top priorities for many countries, and as a clean, pollution-free and high-calorie energy source, LNG will increasingly account for a larger share of future energy use. Expanding the scope of natural gas use has become an important means for China to optimize its energy structure and protect its ecological environment. Currently, the steel sheet widely used for LNG storage and transportation tools at home and abroad is 9Ni steel. As a Ni poor country, China is one of the important ways to reduce the Ni content based on securing performance in order to reduce costs. Research on nickel content-saving 7Ni steel is still in its infancy, and several patents for 7Ni steel sheet have been reported. Application No. 201711306269.2, "Nickel-saving 7Ni steel for cryogenic environment and method for manufacturing heat treatment process thereof" discloses nickel-saving 7Ni steel for cryogenic environment and heat treatment process thereof, and the 7Ni steel component contains Ni: 7.00% to 7.60%, C: 0.02% to 0.06%, Si: 0.03% to 0.80%, Mn: 0.10% to 0.90%, Cr: 0.30% to 0.60%, the remainder is Fe and unavoidable inclusions. 7Ni steel adopting the QLT heat treatment has a strong combination of plasticity and excellent low-temperature toughness, and its performance has reached a significant level compared to 9Ni steel. The Ni mass fraction of steel mentioned in "High-nickel steel for cryogenic pressure vessel and manufacturing method thereof" of application number 201310285597.4 is 7.00-7.50%, and 7Ni steel sheet obtained by adopting two-time quenching + tempering process replaces 9Ni steel and LNG Can be used to manufacture storage tanks. The Mn mass fraction of the steel mentioned in "Low Ni, high Mn economical low-temperature steel usable at -196 ° C. and its manufacturing method" of Application No. 201310494688.9 is 1.00% to 1.50%, and the Ni mass fraction is 7.00% to 8.00%, twice Ni steel obtained by adopting quenching + tempering process has excellent toughness at -196°C. Application No. 201410369201.9 "Low-cost cryogenic nickel steel and manufacturing method thereof" adopts normalizing + quenching + tempering process to obtain Ni steel having excellent toughness at -196°C. In addition, by adopting the UFC + TMCP process, the steel sheet is quickly cooled to below the martensite transformation temperature after rolling, and the offline quenching process is performed to simplify the heat treatment process and improve the low-temperature toughness of the 7Ni steel sheet.

The present invention provides a 7Ni steel sheet for an LNG storage tank and a production process that overcomes the disadvantages of the prior art in consideration of the above technical problems. An economical and feasible manufacturing method is provided by reducing the Ni content as much as possible on the premise of securing the performance of steel for LNG storage and transportation.

In order to solve the above technical problem, the present invention provides a 7Ni steel sheet for an LNG storage tank. Its chemical composition and mass percentage are: C: 0.02%~0.06%, Si: 0.20%~0.35%, Ni: 4.0%~8.0%, Mn: 0.3%~0.7%, P≤0.005%, S≤0.005%, Al : 0.03%~0.05%, Nb: 0.02%~0.05%, Cr: 0.2%~0.4%, the remainder is Fe and unavoidable impurities.

The technical effects are as follows. The present invention adopts the Nb, Cr microalloy Ni-saving alloy design, and innovates the normalizing rolling + two-phase region quenching + tempering production process, and omits the rapid cooling process after rolling. This not only saves energy, but also provides an excellent plate shape for the smooth progress of the subsequent quenching process, and finally, a steel plate for an LNG storage tank with excellent mechanical properties was obtained.

The technical solution further limited in the present invention is as follows.

Furthermore, the thickness is 8-30 mm.

Another object of the present invention is to provide a 7Ni steel plate production process for an LNG storage tank. Here,

Smelting the raw material according to the chemical composition, performing the smelting in a vacuum smelting furnace, after casting into an ingot, forging the ingot into a square slab, and a slab manufacturing step of performing air cooling in an unventilated place;

The slab is maintained at a temperature of 1150 to 1250 ℃ for 2-3 hours, and two-stage rolling is performed on the slab, wherein the initial rolling temperature of the rough rolling is 1000 to 1100 ℃, and the total compression ratio is 40% ~ 60%, the initial rolling temperature of the finish rolling is 850 ~ 900 ℃, the total compression ratio is 40% ~ 70%, the final rolling temperature is 750 ~ 850 ℃, then normalizing rolling step of performing air cooling; and

The rolled steel sheet is quenched at 600~700℃ to 300℃ or lower, and again tempered at 500~580℃, the tempering rate is 5~20℃/s, the temperature holding time is 0.5~2 hours, and air cooling is performed. A two-phase zone quenching + tempering step is included.

In the above-described 7Ni steel sheet production process for an LNG storage tank, the chemical composition and mass percentage of the steel sheet is C: 0.05%, Si: 0.25%, Mn: 0.60%, Ni: 7.2%, P: 0.0045%, S: 0.0030% , Al: 0.038%, Cr: 0.28%, Nb: 0.043%, the remainder being Fe and unavoidable impurities.

Smelting raw materials according to chemical composition, performing smelting in a vacuum smelting furnace, casting into an ingot, forging the ingot into a square slab with a thickness of 80 mm, and performing air cooling in a non-ventilated place. step;

The slab is transferred to a heating furnace, heated to 1156° C., maintained at the temperature for 2.6 hours, taken out and rolled, and two-stage rolling is performed on the slab, where rough rolling is rolled in the austenite complete recrystallization region. The initial rolling temperature is 1100 ° C, the rolling reduction is 60%, the finish rolling is rolled in the austenite non-recrystallization region, the initial rolling temperature is 900 ° C, the rolling reduction is 63%, and the final rolling temperature is 790 ° C, Thereafter, normalizing rolling step of performing air cooling to form a hot-rolled steel sheet having a thickness of 12 mm; and

The rolled steel sheet is quenched at 660 ° C. to 300 ° C. or less, and again tempered at 560 ° C., the tempering rate is 10 ° C / s, and the two-phase region quenching + tempering step is performed while maintaining the temperature for 1 hour. .

In the above-described 7Ni steel sheet production process for an LNG storage tank, the chemical composition and mass percentage of the steel sheet is C: 0.06%, Si: 0.23%, Mn: 0.55%, Ni: 7.3%, P: 0.0043%, S: 0.0031% , Al: 0.035%, Cr: 0.25%, Nb: 0.040%, the remainder being Fe and unavoidable impurities.

Smelting raw materials according to chemical composition, performing smelting in a vacuum smelting furnace, casting into an ingot, forging the ingot into a square slab with a thickness of 80 mm, and performing air cooling in a non-ventilated place. step;

The slab is transferred to a heating furnace, heated to 1239° C., maintained at the temperature for 2.1 hours, taken out and rolled, and two-stage rolling is performed on the slab, where rough rolling is rolled in the austenite complete recrystallization region. The initial rolling temperature is 1100 ℃, the rolling reduction is 60%, the finish rolling is rolled in the austenite non-recrystallization region, the initial rolling temperature is 900 ℃, the rolling reduction is 45%, the final rolling temperature is 790 ℃, After that, normalizing rolling step of performing air cooling to form a hot-rolled steel sheet having a thickness of 18 mm; and

The rolled steel sheet is quenched at 66 ° C. to 300 ° C. or less, and again tempered at 560 ° C., the tempering rate is 10 ° C / s, and the two-phase zone quenching + tempering step is performed while maintaining the temperature for 1 hour. .

Advantageous effects of the present invention are as follows.

(1) In the present invention, nickel is a non-carbide forming element, an austenite forming and stabilizing element that can expand the γ-phase region, and can improve low-temperature toughness without lowering strength. Nickel can shift the CCT curve of the steel to the right, so the hardenability can be improved by lowering the critical reduction quenching rate. Since nickel plays an important role in improving low-temperature toughness and lowering the toughness-brittle transition temperature, Ni is the most important alloying element in the present invention.

(2) In the present invention, carbon as a strong solid solution strengthening element and a strong austenite stabilizing element has a positive effect on the strength of the steel sheet, but has a negative effect on toughness, plasticity and welding performance. Therefore, in order to impart excellent impact toughness and welding performance to the low-temperature steel sheet, it is necessary to control the carbon content in a relatively low range.

(3) In the present invention, manganese mainly plays a role of solid solution strengthening, and can compensate for the decrease in strength due to a decrease in carbon content. At the same time, manganese, like nickel, can lower the phase transition temperature of steel, and appropriately increasing Mn/C and Mn/S is beneficial for toughness. Therefore, the present invention uses Mn as one of the main alloying elements.

(4) In the present invention, silicon as a solid solution strengthening element and deoxidizing element can increase the strength of steel and reduce the content of oxygen, which is a harmful element in the steelmaking process. Silicon is present in steel in a certain ratio with manganese, and in addition to suppressing manganese segregation, it can suppress segregation of phosphorus in the crystal system. However, since silicon lowers the low-temperature toughness of the weld heat-affected region of steel, the silicon content is controlled to 0.15-0.3%.

(5) The addition of chromium in the present invention can improve the hardenability of the steel sheet and improve the strength of the steel sheet. If a small amount of Nb is added and a certain Nb/Si range is controlled, it is not only harmless to strength and plasticity, but also helps to improve welding performance of wide and thick plates. S and P are harmful elements of steel, which are easy to cause segregation, reduce the low-temperature toughness of steel, and increase the sensitivity of hot cracking during welding. Therefore, the P and S content in the steel must be strictly controlled.

(6) In the cross-sectional structure of the present invention, martensite and austenite are distributed on a ferrite matrix. The mechanical performance indicators are the yield strength of 590~700MPa, the tensile strength of 690~790MPa, the elongation ≥24%, and the transverse impact energy at -196℃ ≥100J.

1 is a photograph of the metal structure of Example 1.
2 is a photograph of the metal structure of Example 2.

Example 1

In the 7Ni steel sheet for LNG storage tank and the production process provided in this embodiment, the chemical composition and mass percentage of the steel sheet is C: 0.05%, Si: 0.25%, Mn: 0.60%, Ni: 7.2%, P: 0.0045% , S: 0.0030%, Al: 0.038%, Cr: 0.28%, Nb: 0.043%, the remainder being Fe and unavoidable impurities.

Casting slab manufacturing stage: smelting raw materials according to chemical composition, performing smelting in a vacuum smelting furnace, casting into ingots, forging the ingots into square slabs with a thickness of 80mm, and air cooling in a non-ventilated place carry out

Normalizing rolling step: The slab is transferred to a heating furnace, heated to 1156° C., maintained at the temperature for 2.6 hours, taken out and rolled, and two-stage rolling is performed on the slab, where rough rolling is complete austenite Rolling in the recrystallization region, the initial rolling temperature is 1100 ℃, the rolling reduction is 60%, the finish rolling is rolling in the austenite non-recrystallization region, the initial rolling temperature is 900 ℃, the rolling reduction is 63%, the final rolling temperature is 790 ° C., and then air cooling is performed to form a hot rolled steel sheet having a thickness of 12 mm.

Two-phase region quenching + tempering step: The rolled steel sheet is quenched at 660 ° C. to 300 ° C. or less, and again tempered at 560 ° C., the tempering rate is 10 ° C./s, and air cooling is performed while maintaining the temperature for 1 hour.

The low-temperature steel sheet obtained in this example was detected according to the relevant national standard, and the results are shown in Table 1.

Table 1: Example 1 low temperature steel sheet performance

Example 2

In the 7Ni steel sheet for LNG storage tank and the production process provided in this embodiment, the chemical composition and mass percentage of the steel sheet are C: 0.06%, Si: 0.23%, Mn: 0.55%, Ni: 7.3%, P: 0.0043% , S: 0.0031%, Al: 0.035%, Cr: 0.25%, Nb: 0.040%, the remainder being Fe and unavoidable impurities.

Casting slab manufacturing stage: smelting raw materials according to chemical composition, performing smelting in a vacuum smelting furnace, casting into ingots, forging the ingots into square slabs with a thickness of 80mm, and air cooling in a non-ventilated place carry out

Normalizing rolling step: The slab is transferred to a heating furnace, heated to 1239° C., maintained at the temperature for 2.1 hours, and then taken out and rolled, and two-stage rolling is performed on the slab, where rough rolling is austenite complete Rolling in the recrystallization region, the initial rolling temperature is 1100 ℃, the rolling reduction is 60%, the finish rolling is rolling in the austenite non-recrystallization region, the initial rolling temperature is 900 ℃, the rolling reduction is 45%, the final rolling temperature is 790°C, and then air-cooling to form a hot-rolled steel sheet having a thickness of 18 mm.

Two-phase region quenching + tempering step: The rolled steel sheet is quenched at 660 ° C. to 300 ° C. or less, and again tempered at 560 ° C., the tempering rate is 10 ° C./s, and air cooling is performed while maintaining the temperature for 1 hour.

The low-temperature steel sheet obtained in this example was detected according to the relevant national standard, and the results are shown in Table 2.

Table 2: Example 2 low temperature steel sheet performance

Compared with the prior art, the present invention adopts normalizing rolling + two-phase zone quenching + tempering production process through nickel reduction alloy design, and in performance fully reaches 9Ni standard among GB 3531-2014 low-temperature pressure vessel steel materials, and the production cost is significantly higher savings are made

In addition to the above embodiments, the present invention may have other embodiments. Any technical solutions formed by equivalent substitution or equivalent transformation all fall within the protection scope of the present invention.

Claims (5)

In the 7Ni steel sheet for an LNG storage tank,
Its chemical composition and mass percentage are: C: 0.02%~0.06%, Si: 0.20%~0.35%, Ni: 4.0%~8.0%, Mn: 0.3%~0.7%, P≤0.005%, S≤0.005%, Al : 0.03%~0.05%, Nb: 0.02%~0.05%, Cr: 0.2%~0.4%, 7Ni steel sheet for LNG storage tank, the remainder being Fe and unavoidable impurities. The method of claim 1,
7Ni steel sheet for LNG storage tank, characterized in that the thickness is 8 ~ 30mm. In the 7Ni steel plate production process for an LNG storage tank,
Smelting the raw material according to the chemical composition, performing the smelting in a vacuum smelting furnace, after casting into an ingot, forging the ingot into a square slab, and a slab manufacturing step of performing air cooling in an unventilated place;
The slab is maintained at a temperature of 1150 to 1250 ℃ for 2-3 hours, and two-stage rolling is performed on the slab, wherein the initial rolling temperature of the rough rolling is 1000 to 1100 ℃, and the total compression ratio is 40% ~ 60%, the initial rolling temperature of the finish rolling is 850 ~ 900 ℃, the total compression ratio is 40% ~ 70%, the final rolling temperature is 750 ~ 850 ℃, then normalizing rolling step of performing air cooling; and
The rolled steel sheet is quenched at 600~700℃ to 300℃ or lower, and again tempered at 500~580℃, the tempering rate is 5~20℃/s, the temperature holding time is 0.5~2 hours, and air cooling is performed. 7Ni steel sheet production process for LNG storage tank, characterized in that it comprises; two-phase region quenching + tempering step. 4. The method of claim 3,
The chemical composition and mass percentage of the steel sheet are: C: 0.05%, Si: 0.25%, Mn: 0.60%, Ni: 7.2%, P: 0.0045%, S: 0.0030%, Al: 0.038%, Cr: 0.28%, Nb: 0.043%, the remainder being Fe and unavoidable impurities,
Smelting raw materials according to chemical composition, performing smelting in a vacuum smelting furnace, casting into an ingot, forging the ingot into a square slab with a thickness of 80 mm, and performing air cooling in a non-ventilated place. step;
The slab is transferred to a heating furnace, heated to 1156° C., maintained at the temperature for 2.6 hours, taken out and rolled, and two-stage rolling is performed on the slab, where rough rolling is rolled in the austenite complete recrystallization region. The initial rolling temperature is 1100 ° C, the rolling reduction is 60%, the finish rolling is rolled in the austenite non-recrystallization region, the initial rolling temperature is 900 ° C, the rolling reduction is 63%, and the final rolling temperature is 790 ° C, After that, normalizing rolling step of performing air cooling to form a hot-rolled steel sheet having a thickness of 12 mm; and
Quenching the rolled steel sheet at 660 ° C. to 300 ° C. or less, and tempering again at 560 ° C., the tempering rate is 10 ° C./s, and two-phase region quenching + tempering step of performing air cooling while maintaining the temperature for 1 hour. 7Ni steel plate production process for LNG storage tank, characterized in that. 4. The method of claim 3,
The chemical composition and mass percentage of the steel sheet are C: 0.06%, Si: 0.23%, Mn: 0.55%, Ni: 7.3%, P: 0.0043%, S: 0.0031%, Al: 0.035%, Cr: 0.25%, Nb: 0.040%, the remainder being Fe and unavoidable impurities,
Smelting raw materials according to the chemical composition, performing smelting in a vacuum smelting furnace, casting into an ingot, forging the ingot into a square slab with a thickness of 80 mm, and performing air cooling in a non-ventilated place. step;
The slab is transferred to a heating furnace, heated to 1239° C., maintained at the temperature for 2.1 hours, taken out and rolled, and two-stage rolling is performed on the slab, where rough rolling is rolled in the austenite complete recrystallization region. The initial rolling temperature is 1100 ℃, the rolling reduction is 60%, the finish rolling is rolled in the austenite non-recrystallization region, the initial rolling temperature is 900 ℃, the rolling reduction is 45%, the final rolling temperature is 790 ℃, After that, normalizing rolling step of performing air cooling to form a hot-rolled steel sheet having a thickness of 18 mm; and
Quenching the rolled steel sheet at 660 ° C. to 300 ° C. or lower, and tempering again at 560 ° C., the tempering rate is 10 ° C / s, and two-phase region quenching + tempering step of performing air cooling while maintaining the temperature for 1 hour; 7Ni steel plate production process for LNG storage tank, characterized in that.

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