Chromium-rich 5.5 Ni steel plate based on TMCP process and preparation method thereof

Abstract

The invention discloses a chromium-rich 5.5 Ni steel plate based on a TMCP process and a preparation method thereof, and belongs to the technical field of steel and iron materials. The steel plate comprises the following chemical components in percentage by weight: 0.03%-0.1% of C, 0.05%-0.12% of Si, 0.52%-0.98% of Mn, 5.2%-5.7% of Ni, 0.1%-0.3% of Mo, 0.2%-0.35% of Cr, less than or equal to 0.006% of P, less than or equal to 0.005% of S and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: 1) selecting raw materials according to the components, smelting, pouring into a cast ingot, and heating and insulating the cast ingot; (2) the heated cast ingot is subjected to two-stage rolling, and after rolling, the cast ingot is subjected to ultrafast cooling to 200 DEG C or below; and (3) the hot-rolled steel plate is quenched and tempered, water cooling or air cooling is conducted to the room temperature after the hot-rolled steel plate is discharged out of the furnace, and the chromium-rich 5.5 Ni type LNG storage tank steel plate with the thickness being 6-20 mm is obtained and has ultrahigh room-temperature strength plasticity and low-temperature toughness, alloy elements are few, and the cost is far lower than that of 9Ni steel.

Description

Technical Field

[0001] The invention belongs to the technical field of ultra-low temperature energy storage material manufacturing, and relates to a chromium-rich 5.5Ni ultra-low temperature steel for a nickel-saving LNG storage tank and a heat treatment process thereof. Background Art

[0002] The development trend of world energy consumption is that the proportion of natural gas in primary energy consumption continues to increase. The transportation of liquefied natural gas requires the construction of a large number of offshore LNG storage tanks, land LNG storage tanks, new energy vehicle fuel tanks, inland transport ship fuel tanks, etc., which requires a large amount of steel for LNG storage tanks.

[0003] LNG is a liquid formed by liquefying natural gas at -162℃ and 0.1 MPa, and its volume is about 1 / 625 of gaseous natural gas. In view of the special physical and chemical properties of LNG, from the perspective of safety, the structural materials of ultra-low temperature storage and transportation facilities are required to have a combination of high strength, excellent ultra-low temperature toughness, good welding performance and easy processing. At present, the materials commonly used in ultra-low temperature environments are austenitic stainless steel, Ni-based alloys, aluminum alloys and Ni-based ultra-low temperature 9Ni steel. Compared with austenitic stainless steel and Ni-based alloys, 9Ni steel has lower alloy cost and higher strength; compared with aluminum alloys, 9Ni steel has higher strength and better welding performance. Therefore, 9Ni steel is widely used as the liner structural material of ultra-low temperature storage and transportation facilities internationally.

[0004] In recent years, in order to reduce the production cost of 9Ni steel and the dependence on the high-priced Ni element, the development of Ni-saving steel plates has become an important development direction for Ni-based ultra-low temperature steel. However, the reduction of Ni content in 7Ni steel is limited, so it is necessary to continue to reduce Ni. Chinese patent CN201410600875.5 discloses a "5Ni steel plate for ultra-low temperature pressure vessels and its production method, which is applicable to the temperature range of -45℃~-120℃; CN111440990A discloses "a method for manufacturing a 5Ni steel plate for ships with low remanence and excellent surface quality", which is suitable for low temperature environments of -130℃. However, neither of them can meet the service requirements of LNG storage tanks. At present, there are few reports on 5.5Ni steel plates with a Ni content of about 5.5%, strength properties equivalent to those of 9Ni steel plates, and low-temperature impact energy of -196℃ ≥150J or more and their preparation processes. Chinese patent CN202310562946.6 proposes "a 5.5Ni steel plate for ultra-low temperature environments and its manufacturing method", but its preparation process is relatively complicated, the steel plate needs to be quenched at high temperature after hot rolling, and the low-temperature impact energy of -196℃ is relatively low. Summary of the invention

[0005] The present invention aims to provide a chromium-rich 5.5Ni steel plate based on the TMCP process and a preparation method thereof, to produce a chromium-rich 5.5Ni steel plate with low-temperature toughness and high-strength plasticity, and the comprehensive mechanical properties reach the level of 9Ni steel. The mechanical properties of the steel plate are as follows: room temperature yield strength of 590-700 MPa, room temperature tensile strength of 750-820 MPa, room temperature elongation of 24%-26%, and impact energy of 170-240J at -196°C; it can replace 9Ni steel for the construction of LNG storage tanks, etc., to achieve low cost of materials.

[0006] The technical solution of the present invention: The chromium-rich 5.5Ni steel plate based on the TMCP process has a chemical composition of C=0.03%~0.1%, Si=0.05%~0.12%, Mn=0.52%~0.98%, Ni=5.2%~5.7%, Mo=0.1%~0.3%, Cr=0.2%~0.35%, P≤0.006%, S≤0.005%, and the balance is Fe and unavoidable impurities; the steel plate thickness is 6~20 mm, the room temperature yield strength is 590~700 MPa, the room temperature tensile strength is 750~820 MPa, the room temperature elongation is 24%~26%, and the impact energy at -196℃ is 170~240J.

[0007] Furthermore, the microstructure of the prepared chromium-rich 5.5Ni steel plate is ferrite + lath martensite + tempered martensite + reversed austenite + retained austenite, wherein the volume fraction of reversed austenite is 6% to 9%, and the volume fraction of retained austenite is 1% to 3%.

[0008] The preparation method of chromium-rich 5.5Ni steel plate based on TMCP process, the key process steps include: 1) Use vacuum induction melting furnace to produce steel ingots, cut off the riser and forge them into steel billets; 2) The steel billet is kept at 1200℃ for 1.5-2.5h, and then cooled to 990-1020℃ for multiple rough rolling. The rough rolling stage is in the static recrystallization zone of austenite, the cumulative reduction rate is about 55%, and the final rolling temperature of rough rolling is 890-930℃; after the rough rolling, multiple finishing rolling is carried out. The finishing rolling stage is in the non-recrystallization zone of austenite, the cumulative reduction rate is ≥65%, the final rolling temperature is 810-830℃, and the steel billet is water-cooled to room temperature after rolling; 3) The rolled steel plate is subjected to heat treatment of two-phase zone quenching + high-temperature tempering: the two-phase zone sub-temperature quenching temperature is between Ac1 and Ac1; the steel plate after two-phase zone quenching is subjected to high-temperature tempering at a temperature lower than Ac1 and air-cooled to room temperature.

[0009] Furthermore, the thickness of the steel billet in step 1) is 100 mm; in step 2), the rough rolling stage has four passes, the reduction rate of each pass is 20% to 25%, and the thickness of the intermediate billet is 45 to 50 mm; the finishing rolling stage has three passes, the reduction rate of each pass is 20% to 25%.

[0010] Furthermore, in step 3), the two-phase region quenching temperature is 670-730° C., and the holding time is 30-60 min; the high-temperature tempering temperature is 600-640° C., and the holding time is 40-60 min.

[0011] The following is a detailed description of the effects and dosage of the components in the present invention: C is the most important strengthening element in steel and also the element that has the greatest impact on toughness, especially low temperature toughness. For 5.5Ni steel plate, considering that the Ni content is significantly lower than that in 9Ni, the C content is lower than that in conventional 9Ni steel. Based on this, the present invention controls the carbon content to be 0.03-0.06%.

[0012] Si is a weak deoxidizing element in steel and is cheap. Si improves the strength of steel in the form of solid solution strengthening. When the Si content is less than 0.05%, the deoxidation effect is poor; when the Si content is higher, the toughness and welding performance will decrease. The Si content of the present invention is controlled to be 0.05-0.12%.

[0013] Mn plays a role in solid solution strengthening and deoxidation, but its deoxidation ability is relatively weak. Compared with 9Ni steel, due to the lower C content and Ni content, a higher Mn content is designed to improve the strength of the steel plate. The Mn content of the present invention is controlled to be 0.52-0.98%.

[0014] Ni is a beneficial element that can significantly improve low temperature toughness, especially has a significant impact on the impact toughness and tough-to-brittle transition process of steel plates in ultra-low temperature environments, but too high a Ni content will significantly increase the manufacturing cost of the steel plate. Therefore, the present invention controls its content to 5.20-5.80%.

[0015] Mo can improve hardenability, thereby increasing strength; improve the tempering stability of steel; when present together with chromium or manganese, it can reduce or inhibit the temper brittleness caused by other elements.

[0016] Cr can be enriched along the grain boundaries in the solid solution state and cause the solute drag effect to refine the grains. Therefore, a small amount of Cr can compensate for the loss of strength through solid solution strengthening, which is one of the innovations of the present invention.

[0017] P is a harmful impurity element in steel. Phosphorus combines with iron to form iron phosphide, which is easy to form segregation at the grain boundary, weaken the grain boundary and cause cold brittleness. The content of phosphorus is controlled to ≤0.006% in the present invention.

[0018] S is a harmful impurity element in steel, which is easy to form defects such as segregation and inclusions. During the solidification process of the slab, Mn is easy to segregate with S in the center of the slab to form lamellar MnS inclusions, reducing the formation of low-melting FeS at the austenite grain boundary, but MnS inclusions are easy to cause unstable impact toughness in the core of the steel plate. In the later stage of refining, the Ca treatment technology is used to make the inclusion morphology spheroidized and evenly distributed, reduce the viscosity of the molten steel, and promote the floating of the inclusions. The present invention controls its content to ≤0.005%.

[0019] Elements such as Mn, Mo, and Ni in steel are all elements that improve the hardenability of steel plates. A lamellar martensitic structure is obtained by water cooling after hot rolling. During the sub-temperature quenching process in the two-phase zone, part of the original martensite is re-austenitized, and new martensite is obtained in the subsequent water cooling process, which refines the grains. During this process, the stability of the original martensite is reduced. During the high-temperature tempering process, C and Ni have certain diffusion and migration capabilities, and part of the unstable original martensite phase decomposes, breaking through the potential energy barrier under the action of carbon and nickel elements, and inverting to form austenite.

[0020] Cr in steel can be enriched along the grain boundary in the solid solution state and cause the solute drag effect to refine the grains. Therefore, a small amount of Cr can compensate for the loss of strength through solid solution strengthening, which is one of the innovations of the present invention. In addition, when it coexists with molybdenum or manganese, it can reduce or suppress the temper brittleness caused by other elements.

[0021] In step 2), the steel billet is cooled to 990-1020°C and then subjected to multiple rough rolling. Studies have shown that when the temperature is ≥810°C, the hot rolling deformation resistance does not change significantly and does not increase the hot rolling compliance. Therefore, controlled rolling in this temperature range can refine the original austenite grains.

[0022] The chromium-rich 5.5Ni steel plate and its preparation method of the present invention have the following beneficial effects compared with the prior art: (1) The chemical composition design increases the Cr element content, which can be enriched along the grain boundary in the solid solution state and cause the solute drag effect to refine the grains. Therefore, a small amount of Cr can compensate for the loss of strength through solid solution strengthening, which is one of the innovations of the present invention. In addition, when it coexists with molybdenum or manganese, it can reduce or suppress the temper brittleness caused by other elements.

[0023] (2) Compared with the traditional QLT process, the process of water cooling after hot rolling + LT can not only improve the heritability of hot rolling structure such as fine grain and precipitation, but also eliminate the high-energy consumption step of high-temperature quenching, which is more green and efficient.

[0024] (3) The corresponding mechanical properties of the required steel were achieved. The room temperature yield strength of the steel was 590-700 MPa, the room temperature tensile strength was 750-820 MPa, the room temperature elongation was 24%-26%, and the impact energy at -196°C was 170-240 J. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 SEM microstructure of typical chromium-rich 5.5Ni steel No. 1 prepared in Example 1 of the present invention.

[0026] Figure 2 SEM microstructure of typical chromium-rich 5.5Ni steel 2# steel prepared in Example 2 of the present invention.

[0027] Figure 3 SEM microstructure of typical chromium-rich 5.5Ni steel 3# steel prepared in Example 3 of the present invention.

[0028] Figure 4 SEM microstructure of typical chromium-free 5.5Ni steel 4# steel prepared in Example 4 of the present invention.

[0029] Figure 5 The SEM microstructure of the 5# steel prepared in Example 5 of the present invention is a typical chromium-containing 5.5Ni steel. DETAILED DESCRIPTION

[0030] The following examples 1 to 5 are preparation methods of chromium-rich 5.5Ni steel plates based on the TMCP process. The hot rolling forming process is carried out on a 450mm two-roll reversible hot rolling experimental rolling mill. The chemical composition of the steel is shown in Table 1 by weight percentage, and the remainder is Fe and unavoidable impurities. The process steps include: 1) According to the above composition design, a vacuum induction melting furnace is used to refine steel ingots, which are cut off from the riser and forged into steel billets 1#, 2#, 3#, 4# and 5# with a thickness of 100 mm; 1#, 2#, 3#, 4#, and 5# represent the steel billets prepared in Example 1, Example 2, Example 3, Example 4, and Example 5, respectively; 2) Keep the 1#~5# steel billets at 1200℃ for 2h, and then rough roll the steel billets in the austenite dynamic recrystallization zone. The rolling passes, final rolling temperature, single pass reduction rate, total reduction rate and post-rolling cooling method are shown in Table 2; after the rough rolling, multiple passes of finishing rolling are carried out. The finishing rolling stage is in the austenite non-recrystallization zone. There are three passes in the finishing rolling stage. The rolling passes, final rolling temperature, single pass reduction rate, total reduction rate, finished rolled steel plate thickness and post-rolling cooling method are shown in Table 3.

[0031] 3) The rolled steel plate is subjected to two-phase zone sub-temperature quenching: the heat treatment temperature, holding time and cooling method are shown in Table 4; the steel plate after the above two-phase zone sub-temperature quenching is subjected to high-temperature tempering heat treatment: the heat treatment temperature, holding time and cooling method are shown in Table 5.

[0032] Table 1 Chemical composition of steel prepared in Example (wt.%) .

[0033] Table 2 Rough rolling process parameters of the embodiment .

[0034] Table 3 Finishing rolling process parameters of the embodiment .

[0035] Table 4 Process parameters of two-phase sub-temperature quenching in the embodiment .

[0036] Table 5 High temperature tempering process parameters of the embodiment .

[0037] The chromium-rich 5.5Ni steel plates with excellent room temperature ductility and low temperature impact toughness prepared in each of the above embodiments were subjected to tensile and impact test specimens along the rolling direction. The room temperature tensile strength and -196°C impact energy are shown in Table 6.

[0038] Table 6 Mechanical properties test results of steel prepared in Example .

[0039] The results of each embodiment show that the preparation method of the chromium-rich 5.5Ni steel plate based on the TMCP process of the present invention can make the chromium-rich 5.5Ni steel plate obtain excellent room temperature strength and plasticity, and -196°C impact energy while taking into account simplicity, high efficiency and low cost. And by comparing the results of Example 1 with those of Example 2 and Example 3, it is shown that the quenching temperature of the critical zone of the combined control can effectively improve the room temperature strength and plasticity and -196°C impact energy. By comparing the results of Example 1 and Example 4, it is shown that within an appropriate range, increasing the Cr content can greatly improve the room temperature strength without reducing its room temperature plasticity and -196°C impact energy.

[0040] SEM microstructure of 1# steel after final heat treatment Figure 1 As shown, the SEM microstructure of the final heat treatment structure of 2# steel is as follows Figure 2 As shown, the SEM microstructure of the final heat treatment structure of 3# steel is as follows Figure 3 As shown, the SEM microstructure of the final heat treatment structure of 4# steel is as follows Figure 4 As shown, the SEM microstructure of the final heat treatment structure of 5# steel is as follows Figure 5 shown.

Claims

1. Chromium-rich 5.5Ni steel plate based on TMCP process, characterized by: The chemical composition of the steel plate is C=0.03%~0.1%, Si=0.05%~0.12%, Mn=0.52%~0.98%, Ni=5.2%~5.7%, Mo=0.1%~0.3%, Cr=0.2%~0.35%, P≤0.006%, S≤0.005%, and the balance is Fe and unavoidable impurities; the thickness of the steel plate is 6~20mm, the room temperature yield strength is 590~700 MPa, the room temperature tensile strength is 750~820 MPa, the room temperature elongation is 24%~26%, and the impact energy at -196℃ is 170~240J.

2. The chromium-rich 5.5Ni steel plate based on the TMCP process according to claim 1, characterized in that: The microstructure of the prepared chromium-rich 5.5Ni steel plate is ferrite+lath martensite+tempered martensite+reversed austenite+retained austenite, wherein the volume fraction of reversed austenite is 6% to 9%, and the volume fraction of retained austenite is 1% to 3%.

3. A method for preparing a chromium-rich 5.5Ni steel plate based on the TMCP process, characterized in that The key process steps include: 1) Use vacuum induction melting furnace to produce steel ingots, cut off the riser and forge them into steel billets; 2) The steel billet is kept at 1200℃ for 1.5-2.5h, and then cooled to 990-1020℃ for multiple rough rolling. The rough rolling stage is in the static recrystallization zone of austenite, the cumulative reduction rate is about 55%, and the final rolling temperature of rough rolling is 890-930℃; after the rough rolling, multiple finishing rolling is carried out. The finishing rolling stage is in the non-recrystallization zone of austenite, the cumulative reduction rate is ≥65%, the final rolling temperature is 810-830℃, and the steel billet is water-cooled to room temperature after rolling; 3) The rolled steel plate is subjected to heat treatment of two-phase zone quenching + high-temperature tempering: the two-phase zone sub-temperature quenching temperature is between Ac1 and Ac1; the steel plate after two-phase zone quenching is subjected to high-temperature tempering at a temperature lower than Ac1 and air-cooled to room temperature.

4. The method for preparing a chromium-rich 5.5Ni steel plate based on the TMCP process according to claim 3, characterized in that: In step 1), the thickness of the steel billet is 100 mm; in step 2), the rough rolling stage has four passes, the reduction rate of each pass is 20% to 25%, and the thickness of the intermediate billet is 45 to 50 mm; the finishing rolling stage has three passes, the reduction rate of each pass is 20% to 25%.

5. The method for preparing a chromium-rich 5.5Ni steel plate based on the TMCP process according to claim 3, characterized in that: In step 3), the two-phase region quenching temperature is 670-730°C, and the holding time is 30-60 min; the high-temperature tempering temperature is 600-640°C, and the holding time is 40-60 min.

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