5.5 Ni steel with excellent ultralow-temperature impact toughness as well as preparation method and application of 5.5 Ni steel

Abstract

The invention discloses 5.5 Ni steel with excellent ultralow-temperature impact toughness as well as a preparation method and application of the 5.5 Ni steel. The 5.5 Ni steel is prepared from chemical components in percentage by mass as follows: 0.04%-0.1% of C, 0.04%-0.15% of Si, 0.5%-1.5% of Mn, 5.0%-6.0% of Ni, 0.2%-0.3% of Mo and the balance of Fe and inevitable impurities. The preparation method comprises the following steps: smelting; carrying out thermal mechanical treatment; and carrying out heat treatment by a quenching-two-phase region quenching-tempering process. According to the 5.5 Ni steel provided by the invention, only a certain mass fraction of Mo element is added, so that the problem of insufficient hardenability and strength caused by reduction of Ni element is solved, and the problem of low low-temperature toughness when the 5.5 Ni steel replaces 9Ni steel is solved through a QLT heat treatment process of thermal mechanical treatment and a quenching-two-phase region quenching-tempering process; the impact absorbing energy of the 5.5 Ni steel at the temperature of-196 DEG C reaches 250 J or above, the yield strength at the room temperature is larger than 580 MPa, and the requirements for strength and low-temperature toughness of the steel for the LNG storage tank are met.

Description

Technical Field

[0001] The invention relates to the technical field of steel smelting, and in particular to a 5.5Ni steel with excellent ultra-low temperature impact toughness and a preparation method and application thereof. Background Art

[0002] Energy conservation and environmental protection are two key issues in today's world. With the continuous growth of domestic demand for clean energy, the import volume of liquefied natural gas (LNG) is also increasing. LNG storage tanks are required in the LNG storage and transportation process, so the demand for LNG tank steel will continue to increase. 9Ni steel has a high performance margin, which ensures safety, but the cost of 9Ni steel alloy is high and the weldability is poor, so it is necessary to find a substitute for 9Ni steel.

[0003] With the continuous advancement of smelting technology, it has become possible to further reduce the Ni content in steel used in LNG tanks. Relevant research on 5.5Ni steel focuses on how to make the mechanical properties of 5.5Ni steel meet the requirements of 9Ni steel, especially the ultra-low temperature impact toughness of 5.5Ni steel is very low. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art or related art.

[0005] To this end, in a first aspect of the present invention, a 5.5Ni steel with excellent ultra-low temperature impact toughness has the following chemical compositions by mass percentage: C 0.04%-0.1%, Si 0.04%-0.15%, Mn 0.5%-1.5%, Ni 5.0%-6.0%, Mo 0.2%-0.3%, and the remaining elements are Fe and unavoidable impurities.

[0006] Furthermore, the chemical composition is calculated by mass percentage as follows: C 0.04%-0.08%, Si 0.04%-0.1%, Mn 0.7%-1%, Ni 5.5%-5.8%, Mo 0.2%-0.3%, and the remaining elements are Fe and unavoidable impurities.

[0007] Furthermore, the 5.5Ni steel has a yield strength of 580MPa-600MPa, a tensile strength of 680MPa-700MPa, a total elongation of 22%-24%, and a low-temperature impact absorption energy of more than 250J at -196°C.

[0008] A second aspect of the present invention provides a method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness, comprising:

[0009] Smelting is carried out according to the chemical composition of the above-mentioned 5.5Ni steel with excellent ultra-low temperature impact toughness;

[0010] Thermomechanical treatment: heat the steel billet to 1100℃~1150℃ and keep it warm. Cool it down to 980℃~1020℃ after it comes out of the furnace. Perform the first two rolling passes. After the first two rolling passes, wait for the temperature. When the surface temperature reaches 790℃~810℃, perform the subsequent three rolling passes. The final rolling temperature is 770℃~790℃. After rolling, cool it to room temperature with water.

[0011] The heat treatment adopts the quenching-two-phase zone quenching-tempering process; wherein the quenching temperature is 790°C to 820°C, the two-phase zone quenching temperature is 660°C to 700°C, and the tempering temperature is 580°C to 610°C.

[0012] Furthermore, the thermomechanical treatment comprises:

[0013] The steel billet with a cross-section of (45-55) mm × (45-55) mm is placed in a heating furnace and heated to 1100°C-1150°C and kept warm for 110min-130min. It is then taken out of the furnace and cooled to 980°C-1020°C. The first two rolling passes are performed. After the first two rolling passes, the temperature is kept at 790°C-810°C. The subsequent three rolling passes are performed. The final rolling temperature is 770°C-790°C. After rolling, it is water-cooled to room temperature to obtain a 5.5Ni steel plate with a thickness of 12±1 mm.

[0014] Furthermore, the heat treatment comprises:

[0015] The quenching temperature is 790℃~820℃, and the quenching holding time is 35min~45min;

[0016] The quenching temperature of the two-phase zone is 660℃~700℃, and the quenching holding time of the two-phase zone is 35min~45min;

[0017] The tempering temperature is 580℃~610℃, the tempering holding time is 50min~70min, and it is water-cooled.

[0018] Furthermore, the quenching temperature is 800° C. to 810° C., and the quenching holding time is 40 min to 45 min.

[0019] Furthermore, the two-phase region quenching temperature is 680° C. to 690° C., and the two-phase region quenching holding time is 40 min to 45 min.

[0020] Furthermore, the tempering temperature is 580° C. to 600° C., and the tempering holding time is 50 min to 60 min.

[0021] A third aspect of the present invention provides an application of 5.5Ni steel having excellent ultra-low temperature impact toughness, wherein the 5.5Ni steel is used for medium and thick plates of LNG storage tanks, and the thickness of the medium and thick plates is 6 mm to 30 mm.

[0022] Compared with the prior art, the present invention has at least the following beneficial effects:

[0023] The 5.5Ni steel with excellent ultra-low temperature impact toughness provided by the present invention significantly reduces the alloy cost, and only adds 0.2% to 0.3% by mass of Mo element, which makes up for the insufficient hardenability and strength caused by the reduction of Ni element, and avoids the deterioration of low temperature impact toughness caused by adding elements such as Cr and Nb. The preparation method solves the problem of low low temperature toughness when 5.5Ni steel replaces 9Ni steel through a QLT heat treatment process of thermomechanical treatment and quenching-two-phase zone quenching-tempering process, and obtains a 5.5Ni steel with a yield strength of 580MPa to 600MPa, a tensile strength of 680MPa to 700MPa, a total elongation of 22% to 24%, and an impact absorption energy of more than 250J at -196°C, so that the 5.5Ni steel of the present invention can be used for the production of medium and thick plates for LNG storage tanks. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the EBSD-BC diagram of the 5.5Ni steel plate provided in Example 1 of the present application after QLT treatment.

[0025] Figure 2 This is an enlarged EBSD-BC image of the 5.5Ni steel plate provided in Example 1 of the present application after QLT treatment.

[0026] Figure 3 This is the EBSD phase distribution diagram of the 5.5Ni steel plate provided in Example 1 of the present application after QLT treatment. DETAILED DESCRIPTION

[0027] In order to better understand the above technical solution, the technical solution of the present invention is described in detail below through specific embodiments.

[0028] In a first aspect of an embodiment of the present invention, a 5.5Ni steel with excellent ultra-low temperature impact toughness is provided, wherein the chemical composition is calculated by mass percentage as follows: C 0.04% to 0.1%, Si 0.04 to 0.15%, Mn 0.5% to 1.5%, Ni 5.0% to 6.0%, Mo 0.2% to 0.3%, and the remaining elements are Fe and inevitable impurities. Preferably, the chemical composition of the 5.5Ni steel with excellent ultra-low temperature impact toughness is calculated by mass percentage as follows: C 0.04% to 0.08%, Si 0.04 to 0.1%, Mn 0.7% to 1%, Ni 5.5% to 5.8%, Mo 0.2% to 0.3%, and the remaining elements are Fe and inevitable impurities.

[0029] The 5.5Ni steel with excellent ultra-low temperature impact toughness provided by the embodiment of the present invention reduces the Ni content by 3.5% compared with 9Ni steel, significantly reduces the alloy cost, and only adds a certain mass fraction of Mo element to make up for the deficiency caused by the reduction of Ni element. In addition, only Mo element is added to avoid the influence of other elements such as Cr, Nb, etc. on the low-temperature impact toughness, especially the low-temperature impact toughness of welded joints, so that the yield strength of 5.5Ni steel at room temperature is greater than 580MPa, and the impact absorption work at -196℃ reaches more than 250J, which meets the strength and low-temperature toughness requirements of steel for LNG storage tanks.

[0030] Specifically, C: Carbon has an important influence on the strength, toughness and welding performance of steel, and carbon has a strong effect of stabilizing austenite. Therefore, by appropriately increasing the C content and enriching it in the reverse austenite, the stability of the reverse austenite is improved, thereby obtaining high low-temperature toughness. At the same time, the C content does not exceed 0.1%, which also ensures that the steel has excellent welding performance. Mn: It mainly plays a role in solid solution strengthening, but excessive Mn addition is not good for toughness. Si: Silicon dissolved in ferrite can produce a certain "carbon removal" effect, causing C atoms to segregate in austenite and improve its stability. Mo: The addition of molybdenum can significantly improve hardenability, and can form ordered clusters after heat treatment, which can improve strength without compromising low-temperature toughness. Ni, as an austenite stabilizing element, can be enriched in the reverse austenite during tempering, thereby improving its stability. Stable austenite helps to improve the low-temperature toughness of steel.

[0031] A second aspect of the embodiments of the present invention provides a method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness, comprising:

[0032] Smelting is carried out according to the chemical composition of the above-mentioned 5.5Ni steel with excellent ultra-low temperature impact toughness;

[0033] Thermomechanical treatment: heat the steel billet to 1100℃~1150℃ and keep it warm. Cool it down to 980℃~1020℃ after it comes out of the furnace. Perform the first two rolling passes. After the first two rolling passes, wait for the temperature. When the surface temperature reaches 790℃~810℃, perform the subsequent three rolling passes. The final rolling temperature is 770℃~790℃. After rolling, cool it to room temperature with water.

[0034] The heat treatment adopts the quenching-two-phase zone quenching-tempering process; wherein the quenching temperature is 790°C to 820°C, the two-phase zone quenching temperature is 660°C to 700°C, and the tempering temperature is 580°C to 610°C.

[0035] The embodiment of the present invention provides a preparation method of 5.5Ni steel with excellent ultra-low temperature impact toughness. Under the premise of significantly reducing the alloy cost, the 5.5Ni steel reaches the mechanical level of 9Ni through microalloying, thermomechanical treatment (thermomechanical control process TMCP), and QLT heat treatment process of quenching-two-phase zone quenching-tempering process, which solves the problem of low low-temperature toughness when 5.5Ni steel replaces 9Ni steel. It has excellent low-temperature impact toughness, and obtains 5.5Ni steel with a yield strength of 580MPa to 600MPa, a tensile strength of 680MPa to 700MPa, a total elongation of 22% to 24%, and an impact absorption energy of more than 250J at -196°C, so that the 5.5Ni steel of the present invention can be used for medium and thick plates of LNG storage tanks.

[0036] In a feasible implementation, the thermomechanical treatment (i.e., thermomechanical control process TMCP) includes: placing a steel billet with a cross-section of (45-55) mm×(45-55) mm in a heating furnace, heating it to 1100°C-1150°C, and keeping it warm for 110-130 minutes, cooling it to 980°C-1020°C after it is taken out of the furnace, and performing the first two rolling passes, wherein the thickness is rolled to 39±1 mm and 27±1 mm respectively in the first two rolling passes, and waiting for the temperature after the first two rolling passes, and when the surface temperature reaches 790°C-810°C, performing the subsequent three rolling passes, wherein the gaps between the subsequent three rolls are 19±1 mm, 14±1 mm, and 11±1 mm respectively, and the final rolling temperature is 770°C-790°C, and after rolling, cooling it to room temperature, and obtaining a 5.5Ni steel plate with a thickness of 12±1 mm. Then, heat treatment is performed.

[0037] Heat treatment, including:

[0038] The quenching temperature is 790°C to 820°C, and the quenching holding time is 35min to 45min; preferably, the quenching temperature is 800°C to 810°C, and the quenching holding time is 40min to 45min; most preferably, the quenching temperature is 810°C, and the quenching holding time is 40min.

[0039] The two-phase region quenching temperature is 660°C to 700°C, and the two-phase region quenching holding time is 35min to 45min; preferably, the two-phase region quenching temperature is 680°C to 690°C, and the two-phase region quenching holding time is 40min to 45min. Most preferably, the two-phase region quenching temperature is 680°C, and the two-phase region quenching holding time is 40min.

[0040] The tempering temperature is 580°C to 610°C, the tempering holding time is 50min to 70min, and the steel is water-cooled. Preferably, the tempering temperature is 580°C to 600°C, and the tempering holding time is 50min to 60min. Most preferably, the tempering temperature is 580°C, and the tempering holding time is 60min.

[0041] Specifically, by placing a steel billet with a cross-section of (45-55) mm × (45-55) mm into a heating furnace and heating it to 1100-1150°C for 110-130 min, the structure can be made uniform and refined; the furnace cooling is controlled to 980-1020°C, and the first two rollings are performed to obtain an optimized rolling effect; after the first two rollings are completed, the surface temperature is controlled to reach 790-810°C, and the subsequent three rollings are performed, which is conducive to promoting the transformation of austenite to ferrite; the final rolling temperature is 770-790°C, and a 5.5Ni steel plate with a thickness of 12±1 mm of better quality can be obtained. In the heat treatment, as the most preferred, 810°C is in the austenite zone of the experimental 5.5Ni steel, which is a temperature with a good combination of austenite nucleation rate and growth rate. The holding time of 40min ensures uniform austenitization and avoids coarse grains. After research, it was found that as the most preferred, two-phase quenching at 680℃×40min can obtain an appropriate amount of austenite, which can allow Ni atoms to enrich austenite to obtain austenite with a high Ni concentration. The 5.5Ni steel after heat treatment has the best low-temperature toughness. As the most preferred, tempering at 580℃×1h can obtain reversed austenite with the best quantity and morphological distribution.

[0042] According to a third aspect of an embodiment of the present invention, an application of 5.5Ni steel with excellent ultra-low temperature impact toughness is provided. The 5.5Ni steel is used for medium and thick plates of LNG storage tanks. The thickness of the medium and thick plates is 6mm to 30mm. The yield strength at room temperature is greater than 580MPa. It has excellent low-temperature toughness, and the low-temperature impact absorption energy at -196°C reaches more than 250J, which solves the problem of low low-temperature toughness when 5.5Ni steel replaces 9Ni steel.

[0043] Example 1 A 12 mm thick 5.5 Ni steel plate with excellent ultra-low temperature impact toughness and a preparation method thereof (I) Chemical composition of 5.5 Ni steel

[0044] The weight percentage is: C 0.05%, Si 0.05%, Mn 0.8%, Ni 5.5%, Mo 0.25%, and the balance is Fe and inevitable impurity elements.

[0045] (II) Preparation method

[0046] (1) Smelting: Smelting according to the chemical composition of 5.5Ni steel in Example 1.

[0047] (2) TMCP: A steel billet with a cross-section of 50 mm × 50 mm was placed in a heating furnace and heated to 1130 °C for 120 min. After being taken out of the furnace and cooled to 1000 °C, rolling was started. The first two passes rolled the steel billet to a thickness of 39 mm and 27 mm, respectively. After the two passes, the steel billet was kept warm. When the surface temperature reached 800 °C, rolling was started. The roll gaps in the next three passes were 19 mm, 14 mm, and 11 mm. The final rolling temperature was 780 °C. After rolling, the steel billet was cooled to room temperature and a 12 mm thick 5.5 Ni steel plate was obtained.

[0048] (3) Heat treatment: The rolled steel plate is subjected to QLT treatment. The specific heat treatment parameters are:

[0049] The quenching temperature is 810℃ and the quenching holding time is 40min;

[0050] The quenching temperature of the two-phase zone is 680°C, and the quenching holding time of the two-phase zone is 40min;

[0051] The tempering temperature is 580℃, the tempering holding time is 60min, and the steel is water-cooled.

[0052] (III) Mechanical properties are shown in Table 1.

[0053] Comparative Example 1 A 12 mm thick 5.5 Ni steel plate and its preparation method

[0054] (I) Chemical composition of 5.5Ni steel

[0055] Same as Example 1.

[0056] (II) Preparation method

[0057] (1) Smelting: Smelting is carried out according to the chemical composition of the 5.5Ni steel of Comparative Example 1.

[0058] (2) TMCP: Same as Example 1.

[0059] (3) Heat treatment: QT treatment is performed on the rolled steel plate. The specific heat treatment parameters are:

[0060] The quenching temperature is 810℃ and the quenching holding time is 40min;

[0061] The tempering temperature is controlled at 540°C, 560°C, 580°C, 600°C, 620°C, 650°C, 680°C or 700°C respectively, the tempering holding time is 60 minutes, and water cooling is used.

[0062] (III) Mechanical properties are shown in Table 1.

[0063] Comparative Example 2 A 12 mm thick 5.5 Ni steel plate and its preparation method

[0064] (I) Chemical composition of 5.5Ni steel

[0065] The weight percentage is: C 0.05%, Si 0.05%, Mn 0.8%, Ni 5.5%, Mo 0.5%, and the balance is Fe and inevitable impurity elements.

[0066] (II) Preparation method

[0067] (1) Smelting: Smelting is carried out according to the chemical composition of the 5.5Ni steel of Comparative Example 2.

[0068] (2) TMCP: Same as Example 1.

[0069] (3) Heat treatment: Same as Example 1.

[0070] (III) Mechanical properties are shown in Table 1.

[0071] Comparative Example 3 A 12 mm thick 5.5 Ni steel plate and its preparation method

[0072] (I) Chemical composition of 5.5Ni steel

[0073] The weight percentage is: C 0.05%, Si 0.05%, Mn 0.8%, Ni 5.5%, and the balance is Fe and inevitable impurity elements.

[0074] (II) Preparation method

[0075] (1) Smelting: Smelting is carried out according to the chemical composition of the 5.5Ni steel of Comparative Example 3.

[0076] (2) TMCP: Same as Example 1.

[0077] (3) Heat treatment: Same as Example 1.

[0078] (III) Mechanical properties are shown in Table 1.

[0079] Results and Discussion

[0080] As can be seen from Table 1, the QLT heat treatment process of Example 1 enables 5.5Ni steel to obtain excellent low-temperature impact toughness, while the QT heat treatment process of Comparative Example 1, no matter how the tempering temperature is adjusted, the low-temperature impact toughness of 5.5Ni steel is relatively poor. Comparative Examples 2 and 3 adjust the content of Mo in 5.5Ni steel and find that the low-temperature impact toughness is best when the Mo content in Example 1 is 0.25wt%. Figure 1 It can be seen that the 5.5Ni steel structure of Example 1 is fine and evenly distributed. Figure 2 and Figure 3 It is shown that the 5.5Ni steel of Example 1 has many fine reversed austenites dispersed at the grain boundaries.

[0081] Table 1 Mechanical properties of 5.5Ni steel of the embodiment and comparative example

[0082]

[0083] It is easy for those skilled in the art to understand that, under the premise of no conflict, the above-mentioned advantageous methods can be freely combined and superimposed. The above are only preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application. The above are only preferred implementation methods of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present application, and these improvements and variations should also be regarded as the protection scope of the present application.

Claims

1. A 5.5Ni steel with excellent ultra-low temperature impact toughness, characterized in that: The chemical composition by mass percentage is: C 0.04% ~ 0.1%, Si 0.04 ~ 0.15%, Mn 0.5% ~ 1.5%, Ni 5.0% ~ 6.0%, Mo 0.2% ~ 0.3%, and the remaining elements are Fe and inevitable impurities.

2. The 5.5Ni steel having excellent ultra-low temperature impact toughness according to claim 1, characterized in that: The chemical composition by mass percentage is: C 0.04%~0.08%, Si 0.04~0.1%, Mn 0.7%~1%, Ni5.5%~5.8%, Mo0.2%~0.3%, and the remaining elements are Fe and inevitable impurities.

3. The 5.5Ni steel having excellent ultra-low temperature impact toughness according to claim 1 or 2, characterized in that: The 5.5Ni steel has a yield strength of 580MPa-600MPa, a tensile strength of 680MPa-700MPa, a total elongation of 22%-24%, and a low-temperature impact absorption energy of more than 250J at -196°C.

4. A method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness, characterized in that: include: Smelting, smelting according to the chemical composition of 5.5Ni steel with excellent ultra-low temperature impact toughness as claimed in any one of claims 1 to 3; Thermomechanical treatment: heat the steel billet to 1100℃~1150℃ and keep it warm. Cool it down to 980℃~1020℃ after it comes out of the furnace. Perform the first two rolling passes. After the first two rolling passes, wait for the temperature. When the surface temperature reaches 790℃~810℃, perform the subsequent three rolling passes. The final rolling temperature is 770℃~790℃. After rolling, cool it to room temperature with water. The heat treatment adopts the quenching-two-phase zone quenching-tempering process; wherein the quenching temperature is 790°C to 820°C, the two-phase zone quenching temperature is 660°C to 700°C, and the tempering temperature is 580°C to 610°C.

5. The method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness as claimed in claim 4, characterized in that: The thermomechanical treatment comprises: The steel billet with a cross-section of (45-55) mm × (45-55) mm is placed in a heating furnace and heated to 1100°C-1150°C and kept warm for 110min-130min. It is then taken out of the furnace and cooled to 980°C-1020°C. The first two rolling passes are performed. After the first two rolling passes, the temperature is kept at 790°C-810°C. The subsequent three rolling passes are performed. The final rolling temperature is 770°C-790°C. After rolling, it is water-cooled to room temperature to obtain a 5.5Ni steel plate with a thickness of 12±1 mm.

6. The method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness according to claim 5, characterized in that: The heat treatment comprises: The quenching temperature is 790℃~820℃, and the quenching holding time is 35min~45min; The quenching temperature of the two-phase zone is 660℃~700℃, and the quenching holding time of the two-phase zone is 35min~45min; The tempering temperature is 580℃~610℃, the tempering holding time is 50min~70min, and it is water-cooled.

7. The method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness according to claim 6, characterized in that: The quenching temperature is 800° C. to 810° C., and the quenching holding time is 40 min to 45 min.

8. The method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness according to claim 6, characterized in that: The two-phase region quenching temperature is 680° C. to 690° C., and the two-phase region quenching holding time is 40 min to 45 min.

9. The method for preparing 5.5Ni steel with excellent ultra-low temperature impact toughness according to claim 6, characterized in that: The tempering temperature is 580° C. to 600° C., and the tempering holding time is 50 min to 60 min.

10. Application of 5.5Ni steel with excellent ultra-low temperature impact toughness, characterized in that: The 5.5Ni steel according to any one of claims 1 to 3 is used for medium and thick plates of LNG storage tanks, and the thickness of the medium and thick plates is 6 mm to 30 mm.

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