A 785mpa grade steel plate having super low temperature toughness and a method for manufacturing the same

Abstract

The application discloses a 785MPa-grade steel plate with ultralow-temperature toughness and a preparation method thereof, and belongs to the technical field of steel materials, and solves the contradiction between the insufficient toughness level of the ultrahigh-strength steel and the excessive matching of the nickel content in the prior art.The 785MPa-grade steel plate with ultralow-temperature toughness comprises the following components in percentage by mass: C: 0.08% to 0.15%, Si: 0.02% to 0.18%, Mn: 0.60% to 1.63%, P: ≤0.010%, S: ≤0.004%, Cr: 0.35% to 0.95%, Mo: 0.32% to 0.60%, Ni: 3.15% to 4.35%, Cu: 0.75% to 1.85%, V: 0.04 to 0.12%, Nb: 0.008% to 0.035%, Al: 0.017% to 0.036%, Ti: 0.008% to 0.022%, and the balance of Fe and other inevitable impurities.The 785MPa-grade steel plate with ultralow-temperature toughness has excellent strength and low-temperature toughness.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, and in particular to a 785MPa grade steel plate with ultra-low temperature toughness and its preparation method. Background Technology

[0002] The demand for energy gases has driven the rapid development of cryogenic engineering, requiring more steels for use in low-temperature and ultra-low-temperature environments. Low-temperature toughness is one of the key technical indicators of cryogenic steel.

[0003] Nickel-containing low-temperature steel is the most commonly used type of low-temperature steel. As the Ni content increases, the service temperature of the steel decreases significantly. Considering the preciousness of nickel resources, how to make steel have better ultra-low temperature toughness and higher strength under the condition of using equivalent or lower nickel content, so that the steel can have better cost performance and structural weight reduction effect in low-temperature service environment, has become an urgent problem to be solved. Summary of the Invention

[0004] In view of the above, the present invention aims to provide a 785MPa grade steel plate with ultra-low temperature toughness and its preparation method, in order to solve the contradiction between insufficient toughness and excessive nickel content in existing ultra-high strength steels.

[0005] The objective of this invention is mainly achieved through the following technical solutions:

[0006] On one hand, the present invention provides a 785MPa grade steel plate with ultra-low temperature toughness. The composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.08% to 0.15%, Si: 0.02% to 0.18%, Mn: 0.60% to 1.63%, P: ≤0.010%, S: ≤0.004%, Cr: 0.35% to 0.95%, Mo: 0.32% to 0.60%, Ni: 3.15% to 4.35%, Cu: 0.75% to 1.85%, V: 0.04% to 0.12%, Nb: 0.008% to 0.035%, Al: 0.017% to 0.036%, Ti: 0.008% to 0.022%, with the balance being Fe and other unavoidable impurities.

[0007] Furthermore, the Ni, Mn, and Cu contents in the 785MPa grade steel plate with ultra-low temperature toughness also satisfy the following condition with respect to the plate thickness t: 100Ni + 67Cu + 50Mn ≥ 3.55 + 0.12t 1 / 2 Where Ni, Mn, and Cu refer to the mass percentage of the elements, and t is expressed in mm.

[0008] Furthermore, in the 785MPa grade steel plate with ultra-low temperature toughness, Ni+Cu>4.2%, where Ni and Cu refer to the mass percentage of the elements.

[0009] Furthermore, the values ​​of Mn, Cr, and Mo in the 785MPa grade steel plate with ultra-low temperature toughness satisfy the following condition with respect to the thickness t of the steel plate: 333Mn+216Cr+300Mo>4.45+1.0(t / 50)+0.2(t / 50) 2 Where Mn, Cr, and Mo refer to the mass percentage of the elements, and t is expressed in mm.

[0010] Furthermore, the composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.09%–0.12%, Si: 0.03%–0.15%, Mn: 0.9%–1.5%, P: ≤0.006%, S: ≤0.002%, Cr: 0.4%–0.9%, Mo: 0.40%–0.55%, Ni: 3.3%–4.05%, Cu: 0.9%–1.8%, V: 0.04%–0.12%, Nb: 0.02%–0.03%, Al: 0.017%–0.03%, Ti: 0.008%–0.02%, with the balance being Fe and other unavoidable impurities.

[0011] Furthermore, the matrix structure of the 785MPa grade steel plate with ultra-low temperature toughness consists of tempered martensite + lath bainite, with an effective grain size of 1.75–2.35 μm.

[0012] Furthermore, the microstructure of the 785MPa grade steel plate with ultra-low temperature toughness contains a small amount of reverse-transformed austenite.

[0013] Furthermore, the volume percentage of reverse-transformed austenite is 4% to 15%.

[0014] The present invention also provides a method for preparing the above-mentioned 785MPa grade steel plate with ultra-low temperature toughness, comprising the following steps:

[0015] Step 1: Homogenize the steel billet;

[0016] Step 2: Two-stage controlled rolling is adopted;

[0017] Step 3: After rolling, the steel plate is immersed in water to accelerate cooling;

[0018] Step 4: Perform heat treatment on the steel plate, which includes quenching and tempering.

[0019] Furthermore, in step 4, quenching includes single quenching or single quenching plus two-phase quenching; the single quenching temperature is 50-150℃ higher than Ac3.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] a) The 785MPa grade steel plate with ultra-low temperature toughness of this invention, based on nickel as the main toughening element, utilizes austenite-forming elements such as Ni, Mn, and Cu in combination with C content to provide austenitizing stabilizing chemical factors. This lays the raw material foundation for obtaining a beneficial quantity and distribution of reverse-transformed austenite. This invention discovers the elemental equivalent relationship of Ni, Mn, and Cu, which plays an important role in maintaining austenite stabilization within different thickness ranges. The minimum content inequality of Ni, Mn, and Cu in this invention is an important compositional basis for obtaining steel plates with different strength grades and ultra-low temperature toughness. Under the same grade conditions, the greater the steel plate thickness, the higher the required Ni, Mn, and Cu content. Simultaneously, this invention also discovers the elemental equivalent relationship between Mn, Cr, and Mo and thickness, which plays an important role in maintaining the hardenability of steel within different thickness ranges. This invention significantly increases the hardenability of steel without significantly increasing costs. It can stably obtain a full low-temperature transformation structure of tempered martensite + lath bainite within a wide range of thickness specifications. The high-temperature transformation granular bainite, which has a significant deteriorating effect on low-temperature toughness, is avoided throughout the entire thickness section range, resulting in a stable increase in the strength of the steel and good cross-sectional uniformity.

[0022] (b) The addition of microalloying elements such as Nb, Al, and Ti to the 785MPa grade steel plate with ultra-low temperature toughness of this invention interacts with interstitial elements such as C and N to precipitate TiN, Nb(CN), and AlN. These elements can suppress the growth of austenite grain size and refine the original austenite grains during rolling heating, TMCP rolling, and heat treatment reheating, respectively. Compared with the prior art, this invention utilizes a combination of multiple types, scales, and distributions of dispersed precipitated particles. TiN particles are mainly distributed in the range of 12–100 nm with an average particle size of about 25 nm; Nb(CN) particles are mainly distributed in the range of 5–50 nm with an average particle size of about 16 nm; and AlN particles are mainly distributed in the range of 3–15 nm with an average particle size of about 6 nm. This multi-stage comprehensive suppression can stabilize the original austenite grain size of the final product within a fine range of 8.5–12 μm, resulting in a fine final-state original austenite grain size. The fine austenite grain size is also one of the fundamental sources of the good ultra-low temperature toughness achieved in this invention.

[0023] c) The 785MPa grade steel plate with ultra-low temperature toughness of the present invention ensures that the content (RA%) of reverse austenite in the steel plate is 4% to 15% and the equivalent diameter (DRA) of reverse austenite is 5 to 25 nm through precise control of composition and matching with appropriate heat treatment processes. The quantity and distribution of reverse austenite are significantly increased. On the other hand, the element enrichment degree of reverse austenite is significantly increased. Compared with the element content of the matrix, the average Ni content in the enriched region can reach 1.6 to 2.5 times that of the matrix, the Mn content is 1.5 to 2.8 times that of the matrix, and the Cu content in the enriched region is 2.0 to 3.0 times that of the matrix, further improving the stability level of reverse austenite. Compared with the prior art, the volume content of reverse austenite in the steel plate of the present invention is increased, the size is reduced, the quantity is significantly increased, and the stability is significantly improved. It effectively hinders the crack tip propagation rate under impact loads at low temperatures and improves the low temperature toughness of the steel.

[0024] d) In the preparation method of the 785MPa grade steel plate with ultra-low temperature toughness of the present invention, a two-phase region elemental distribution heat treatment process is adopted, namely, one-time quenching + two-phase region elemental distribution quenching + tempering. The one-time quenching heating process completely austenitizes the steel, and the quenching yields a lath martensite + lath bainite structure with high dislocation density and a small amount of retained austenite. In the two-phase region two-time quenching heating process, a mixture of tempered martensite and austenite is formed, and an element-enriched region is formed at the austenite position of the two phases. This element-enriched region has the following characteristics: 1) The refinement of the original austenite in the previous process makes the enriched region small and dispersed; 2) The composition of austenite-stabilizing elements such as Ni, Mn, Cu, and C significantly improves the austenite stability of the element-enriched region. After tempering again, the elements in the element-enriched region are redistributed again, and the element enrichment degree is higher in a smaller area, resulting in a more stable reverse-transformed austenite.

[0025] e) The 785MPa grade steel plate with ultra-low temperature toughness of the present invention exhibits excellent strength and low-temperature toughness. For example, the room temperature properties of the steel plate are: yield strength above 785MPa (e.g., 803-910MPa), tensile strength above 880MPa (e.g., 890-1000MPa), yield strength ratio not exceeding 0.96, and elongation above 21% (e.g., 21.5%-26%); impact energy at -80℃ is above 200J (e.g., 205-255J); the impact energy at -120℃ of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness is above 150J (e.g., 150-220J); and the ductile-brittle transition temperature (FATT50) at 50% fiber content of the impact section is below -120℃ (e.g., -125--196℃). Compared with conventional technology, the steel plate of the present invention has a higher level of low-temperature toughness, better strength-toughness matching and stability, requires lower nickel content, and can stably produce products with a larger maximum thickness.

[0026] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0027] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0028] Figure 1 The microstructure of Example 3;

[0029] Figure 2 This is the EBSD map of Example 3. Detailed Implementation

[0030] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0031] This invention provides a 785MPa grade steel plate with ultra-low temperature toughness. The composition of the above-mentioned 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.08%~0.15%, Si: 0.02%~0.18%, Mn: 0.60%~1.63%, P: ≤0.010%, S: ≤0.004%, Cr: 0.35%~0.95%, Mo: 0.32%~0.60%, Ni: 3.15%~4.35%, Cu: 0.75%~1.85%, V: 0.04%~0.12%, Nb: 0.008%~0.035%, Al: 0.017%~0.036%, Ti: 0.008%~0.022%, with the balance being Fe and other unavoidable impurities.

[0032] Specifically, the thickness t of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness is 10-150mm.

[0033] The following details the function and dosage selection of the components contained in this invention:

[0034] Ni: Among the various elements added to low-temperature steel, Ni is the most important. Ni is a non-carbide-forming element and does not form carbides. With increasing Ni content, the Ar3 phase transformation temperature decreases upon cooling, and the stability of austenite increases. When the Ni content is sufficiently high, even at liquid nitrogen temperature of -196℃, the γ→α transformation does not occur, resulting in a single-phase austenite structure. Ni is the most important alloying element in reversible austenite, and its enrichment in reversible austenite is the main source of stability. However, excessive Ni content is not only uneconomical but also impairs weldability and other processing properties. While achieving good low-temperature toughness, the amount of Ni added should be controlled as much as possible to improve the distribution and utilization efficiency of Ni in each phase. Considering all factors, the Ni content in this invention is controlled at 3.15%–4.35%.

[0035] Carbon (C) is essential for increasing strength, but it also reduces the toughness and weldability of materials and increases the ductile-brittle transition temperature. In low-temperature steels, C can accumulate in the reverse-transformed austenite, improving the stability of austenite, reducing the C content in the matrix, and improving the toughness and plasticity of the steel matrix. The higher the strength grade of the low-temperature steel, the more appropriate the C content needs to be. Considering all factors, the C content is controlled at 0.08% to 0.15% in this invention.

[0036] Silicon (Si): As a deoxidizing element and a solid solution strengthening element, silicon can improve the strength of steel. However, when the silicon content exceeds 0.4%, it reduces the low-temperature toughness and weldability of the steel. Therefore, for low-temperature steel, the Si content should be controlled below 0.38%, and ideally below 0.25%. Taking all factors into consideration, this invention controls the Si content to be between 0.02% and 0.18%.

[0037] Mn: Manganese is an essential element for ensuring the strength and toughness of steel. It not only delays the high-temperature phase transformation time and lowers the transformation temperature, improving the hardenability of steel, but also plays a crucial role in this invention by enriching in the reverse-transformed austenite, enhancing its stability, and thus improving the toughness of the steel. The Mn content can be controlled at different levels to achieve different ultra-low temperature toughness requirements. Considering all factors, the Mn content in this invention is controlled at 0.60%–1.63%.

[0038] Cu: Copper is a non-carbide-forming element and plays a beneficial role in at least three aspects in this invention. Firstly, Cu dissolves in supercooled austenite, improving the hardenability of the steel. Secondly, Cu, in combination with elements such as Ni, Mn, and C, enhances the stability of the reverse-transformed austenite, making Cu a crucial component of the reverse-transformed austenite in the steel of this invention. Thirdly, Cu can precipitate during aging in martensite and bainite, thereby strengthening the steel through precipitation. Under different strength levels and low-temperature toughness requirements, the performance requirements can be achieved by adjusting the amount of Cu added. Considering all factors, the Cu content in this invention is controlled at 0.75%–1.85%.

[0039] Cr: Chromium is an effective element for improving hardenability, especially in thick plates. Adding Cr shifts the phase transformation curve of the steel to the right, inhibiting high-temperature phase transformation. The greater the thickness, the higher the required Cr content. Considering all factors, the Cr content in this invention is controlled at 0.35%–0.95%.

[0040] Mo: Molybdenum is also an important element for improving hardenability, and its hardenability effect is even higher than that of Cr. Adding Mo to thick and extra-thick plates causes a strong rightward shift in the phase transformation curve of the steel. In particular, when combined with Cr, it ensures good cross-sectional uniformity in thick and even extra-thick plates. Taking all factors into consideration, the Mo content is controlled at 0.32% to 0.60% in this invention.

[0041] Vanadium (V) is an important supplementary element for improving cross-sectional uniformity in extra-thick plates. VC precipitation can improve the differences in grain size and microstructure between the core and other locations of the steel plate. Taking all factors into consideration, the V content is controlled at 0.04%–0.12% in this invention.

[0042] Niobium (Nb) is added to inhibit austenite recrystallization during steel rolling, causing the austenite to flatten during rolling and increasing the area of ​​deformed austenite. Simultaneously, fine Nb(CN) particles precipitate during rolling, inhibiting austenite grain growth during rolling and post-rolling cooling, thus refining the grain size. Therefore, considering all factors, the Nb content in this invention should be controlled at a level of 0.008% to 0.035%.

[0043] Al: The addition of aluminum promotes the precipitation of the AlN second phase. Fine AlN particles precipitate during the reheating process of heat treatment, preventing the growth of austenite grains during heat treatment and refining the grain size. Therefore, considering all factors, the Al content in this invention should be controlled at a level of 0.017% to 0.036%.

[0044] Ti: The addition of trace amounts of titanium, combined with nitrogen, forms TiN precipitates. The precipitation temperature is controlled within the range of 1250–1350°C to prevent excessive particle growth due to excessively high precipitation temperatures. This inhibits excessive austenite growth during the pre-rolling heating process, providing a basis for microstructure refinement in subsequent rolling and heat treatment. Therefore, considering all factors, the Ti content in this invention should be controlled at a level of 0.008%–0.022%.

[0045] Phosphorus (P) is an impurity element in steel that can impair the toughness of steel plates and weld heat-affected zones, especially reducing the steel's low-temperature toughness. Therefore, the P content should be controlled below 0.010%, and ideally below 0.006% where conditions permit.

[0046] S: Sulfur is an impurity element in steel that can form sulfide inclusions, becoming crack initiation sites. Therefore, the S content should be controlled below 0.004%, and ideally below 0.002%.

[0047] Specifically, the contents of Ni, Mn, Cu, Cr, and Mo in the aforementioned 785MPa grade steel plate with ultra-low temperature toughness also satisfy the following condition with respect to the plate thickness t: 100Ni + 67Cu + 50Mn ≥ 3.55 + 0.12t 1 / 2 ,333Mn+216Cr+300Mo>4.45+1.0(t / 50)+0.2(t / 50) 2 And Ni+Cu>4.20%, where Ni, Mn, Cu, Cr, and Mo refer to the mass percentage of the elements, and t is in mm.

[0048] Specifically, the composition of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.09%–0.12%, Si: 0.03%–0.15%, Mn: 0.9%–1.5%, P: ≤0.006%, S: ≤0.002%, Cr: 0.4%–0.9%, Mo: 0.40%–0.55%, Ni: 3.3%–4.05%, Cu: 0.9%–1.8%, V: 0.04%–0.12%, Nb: 0.02%–0.03%, Al: 0.017%–0.03%, Ti: 0.008%–0.02%, with the balance being Fe and other unavoidable impurities; the steel plate thickness t is 36–120 mm.

[0049] Specifically, the matrix structure of the entire cross-section of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness includes tempered martensite + lath bainite (which can be referred to as tempered M + lath B), with an effective grain size of 1.75 to 2.35 μm and a standard deviation of ≤0.19 μm; the original austenite grain size is 8.5 to 12 μm and the standard deviation is ≤0.45 μm.

[0050] Specifically, the microstructure of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness contains a small amount of reverse-transformed austenite, with a volume percentage (RA%) of 4% to 15% and an equivalent diameter (DRA) of 5 to 25 nm.

[0051] Specifically, the M(RA) content in the reverse austenite of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness has the following characteristics: Ni(RA) = 1.6~2.5Ni, Mn(RA) = 1.5~2.8Mn, Cu(RA) = 2.0~3.0Cu, C(RA) > 0.32%.

[0052] Specifically, the microstructure of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness includes multiple types, scales, and distributions of dispersed precipitates. The precipitates mainly include TiN, Nb(CN), and AlN. TiN particles are mainly distributed in the range of 12–100 nm with an average particle size of about 25 nm, Nb(CN) particles are mainly distributed in the range of 5–50 nm with an average particle size of about 16 nm, and AlN particles are mainly distributed in the range of 3–15 nm with an average particle size of about 6 nm. The content of TiN precipitates is about 0.013%–0.020%, the content of AlN precipitates is about 0.011%–0.016%, and the content of Nb(CN) precipitates is about 0.032%–0.045%.

[0053] Specifically, the ductile-brittle transition temperature (FATT50) of the impact section of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness at 50% fiber content is related to the volume percentage of reverse-transformed austenite (RA%) and the equivalent diameter (DRA) as follows: FATT50 = a0 - 100 a1(RA%) + a2(DRA) 3 / 2 Where RA% is the volume percentage, DRA is in nm, a0 = -105, a1: 5.5~6.5, a2: 0.2~0.4.

[0054] Specifically, the ductile-brittle transition temperature (FATT50) of the 785MPa grade steel plate with ultra-low temperature toughness at 50% fiber content in the impact section is below -120℃, for example -125 to -196℃.

[0055] Specifically, the room temperature properties of the aforementioned 785MPa grade steel plate with ultra-low temperature toughness are as follows: yield strength above 785MPa (e.g., 803–910MPa), tensile strength above 880MPa (e.g., 890–1000MPa), yield-to-tensile ratio not higher than 0.96 (e.g., 0.89–0.96), and elongation above 21% (e.g., 21.5%–26%). Specifically, the aforementioned 785MPa grade steel plate with ultra-low temperature toughness has an impact energy of above 200J at -80℃ (e.g., 205–255J); and an impact energy of above 150J at -120℃ (e.g., 150–220J).

[0056] On the other hand, the present invention also provides a method for preparing the above-mentioned 785MPa grade steel plate with ultra-low temperature toughness, comprising the following steps:

[0057] Step 1: Heat the steel billet to 1080-1160℃ and keep it at that temperature to ensure uniformity;

[0058] Step 2: Two-stage controlled rolling is adopted;

[0059] Step 3: After rolling, the steel plate is immersed in water for accelerated cooling, with a cooling rate of not less than 5℃ / s;

[0060] Step 4: Perform heat treatment on the steel plate, including quenching and tempering.

[0061] Specifically, in step 2 above, the rolling temperature range for the first stage is 950–1060℃, and the rolling temperature range for the second stage is 790–850℃.

[0062] Specifically, in step 4 above, quenching includes single quenching or single quenching plus two-phase quenching.

[0063] Specifically, in step 4 above, the quenching temperature is 50-150℃ higher than Ac3. When two-phase quenching is used, the two-phase quenching temperature is in the range of (a*Ac3+b*Ac1), where b=0.2-0.5, a=1-b, and the tempering temperature is Ac1-(50-150)℃.

[0064] Specifically, in step 4 above, the primary quenching temperature is 820–920℃, and when two-phase quenching is used, the two-phase quenching temperature is 720–780℃.

[0065] Specifically, in step 4 above, the quenching and holding time is generally 2 to 3 minutes per minute.

[0066] Specifically, in step 4 above, in order to further increase the element enrichment effect and achieve the stabilization and enhancement of reverse austenite, the tempering can be done more than once. When the tempering is done twice, the first tempering temperature is lower than the second tempering temperature.

[0067] Specifically, in step 4 above, when the tempering is performed twice, the first tempering temperature is Ac1-(100~150)℃, and the second tempering temperature is Ac1-(50~100)℃.

[0068] Specifically, in step 4 above, when tempering is performed once, the tempering holding time is 4 to 6 min / mm; when tempering is performed twice in stages, the holding time for the first tempering is 2 to 4 min / mm, and the holding time for the second tempering is 4 to 6 min / mm.

[0069] Specifically, in step 4 above, the quantity, distribution, and element enrichment effect of reversible austenite are further enhanced through stepwise tempering. The principle is to first stimulate element enrichment and migration dynamics through short-duration low-temperature tempering, increasing the nucleation rate of reversible austenite, and then create kinetic enrichment conditions through long-duration tempering at relatively high temperatures. This achieves stronger kinetic enrichment capabilities and effects in each reversible austenite region, further optimizing the volume fraction, quantity, distribution, and element enrichment degree of reversible austenite.

[0070] Compared with existing technologies, the 785MPa grade steel plate with ultra-low temperature toughness of the present invention, in addition to the hardenability provided by elements such as Ni, Mn, and Cu, also incorporates small amounts of elements such as Cr, Mo, V, and Nb in its composition design. This significantly increases the hardenability of the steel without significantly increasing costs, allowing for a stable and complete low-temperature transformation microstructure of tempered martensite + lath bainite across a wide range of thicknesses. The high-temperature transformation granular bainite, which significantly deteriorates low-temperature toughness, is avoided across the entire thickness cross-section, resulting in a stable increase in steel strength and good cross-sectional uniformity. Furthermore, the addition of an appropriate amount of Cu in this invention has at least three beneficial effects: Cu dissolves in supercooled austenite, improving the hardenability of the steel; Cu, in combination with elements such as Ni, Mn, and C, improves the stability of the reverse-transformed austenite; and Cu precipitates during aging in martensite and bainite, enhancing the strength of the steel through precipitation strengthening.

[0071] The addition of microalloying elements such as Nb, Al, and Ti to the 785MPa grade steel plate with ultra-low temperature toughness of this invention interacts with interstitial elements such as C and N to precipitate TiN, Nb(CN), and AlN. These elements can suppress the growth of austenite grain size and refine the original austenite grains during rolling heating, TMCP rolling, and heat treatment reheating, respectively. Compared with the prior art, this invention utilizes a combination of multiple types, scales, and distributions of dispersed precipitates. TiN particles are mainly distributed in the range of 12–100 nm with an average particle size of about 25 nm; Nb(CN) particles are mainly distributed in the range of 5–50 nm with an average particle size of about 16 nm; and AlN particles are mainly distributed in the range of 3–15 nm with an average particle size of about 6 nm. This multi-stage comprehensive suppression ensures that the original austenite grain size of the final product is stabilized in a fine range of 8.5–12 μm, resulting in a fine final-state original austenite grain size. The fine austenite grain size is also one of the fundamental sources of the excellent ultra-low temperature toughness achieved in this invention.

[0072] The 785MPa grade steel plate with ultra-low temperature toughness of the present invention ensures that the content (RA%) of reverse austenite in the steel plate is 4% to 15% and the equivalent diameter (DRA) of reverse austenite is 5 to 25 nm through precise control of composition and matching with appropriate heat treatment process. The amount of reverse austenite is significantly increased and the distribution is optimized. On the other hand, the element enrichment degree of reverse austenite is significantly increased. Compared with the element content of the matrix, the average Ni content in the enriched region can reach 1.6 to 2.5 times that of the matrix, for example 1.8 to 2.4 times; the Mn content is 1.5 to 2.8 times that of the matrix, for example 1.5 to 2.5 times; and the Cu content in the enriched region also reaches 2.0 to 3.0 times that of the matrix, for example 2.05 to 3.0 times; further improving the stability level of reverse austenite. Compared with the prior art, the steel plate of the present invention has increased volume content of reverse-transformed austenite, reduced size, significantly increased quantity, and significantly improved stability, effectively hindering the crack tip propagation rate under impact load at low temperature and improving the low temperature toughness of the steel.

[0073] In the preparation method of the 785MPa grade steel plate with ultra-low temperature toughness of the present invention, a two-phase region elemental distribution heat treatment process can be adopted, namely, one-time quenching + two-phase region elemental distribution quenching + tempering. The one-time quenching heating process completely austenitizes the steel, and the quenching yields a high dislocation density lath martensite (+ lath bainite) structure and a small amount of retained austenite. In the two-phase region two-time quenching heating process, a mixture of tempered martensite and austenite is formed, and an element-enriched region is formed at the austenite position of the two phases. This element-enriched region has the following characteristics: 1) The refinement of the original austenite in the previous process makes the enriched region small and dispersed; 2) The composition of austenite-stabilizing elements such as Ni, Mn, Cu, and C significantly improves the austenite stability of the element-enriched region. After tempering again, the elements in the element-enriched region are redistributed again, and the element enrichment degree is higher in a smaller area, resulting in a more stable reverse-transformed austenite.

[0074] The 785MPa grade steel plate with ultra-low temperature toughness of the present invention exhibits excellent strength and low-temperature toughness. The room temperature properties of the 785MPa grade steel plate with ultra-low temperature toughness are as follows: yield strength above 785MPa (e.g., 803-910MPa), tensile strength above 880MPa (e.g., 890-1000MPa), yield-to-tensile ratio not higher than 0.96, and elongation above 21% (e.g., 21.5%-26%).

[0075] The 785MPa grade steel plate with ultra-low temperature toughness of the present invention has an impact energy of more than 200J at -80℃, for example, 205 to 255J; the 785MPa grade steel plate with ultra-low temperature toughness has an impact energy of more than 150J at -120℃, for example, 150 to 220J.

[0076] Examples 1-3

[0077] The advantages of precise control over the composition and process parameters of the steel plate of the present invention will be demonstrated below with specific embodiments.

[0078] Examples 1-3 of the present invention provide a 785MPa grade steel plate with ultra-low temperature toughness and its preparation method. The chemical composition of the steel plates in Examples 1-3 is shown in Table 1.

[0079] The preparation method of Example 1 includes:

[0080] Step 1, Heating: Heat the steel billet to 1150℃ and maintain the temperature to ensure uniformity;

[0081] Step 2, Rolling: Two-stage rolling (TMCP) is performed: the first stage rolling is at 1050-970℃, with an average reduction of about 15% per pass; the second stage rolling is at 850-800℃, with a deformation of about 15% in the large deformation passes.

[0082] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 20℃ / s;

[0083] Step 4, Heat treatment: The steel plate is austenitized at 850℃, held at that temperature for 2 hours, and then water-cooled after being taken out of the furnace; then heated at 750℃, held at that temperature for 2 hours, and water-cooled after being taken out of the furnace; finally, it is held at 600℃ for 3.5 hours, and then air-cooled after being taken out of the furnace to obtain the steel plate.

[0084] The steel plate obtained in Example 1 has a thickness of 40 mm.

[0085] The preparation method of Example 2 includes:

[0086] Step 1, Heating: Heat the steel billet to 1130℃ and maintain the temperature to ensure uniformity;

[0087] Step 2, Rolling: Two-stage rolling (TMCP) is performed: the first stage rolling is at 1050-950℃, with an average reduction of about 15% per pass; the second stage rolling is at 845-790℃, with a deformation of about 15% in the large deformation passes.

[0088] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 15℃ / s;

[0089] Step 4, Heat treatment: The steel plate is austenitized at 860℃, held at that temperature for 3.5 hours, and then water-cooled after being removed from the furnace; it is then held at 620℃ for 6 hours and air-cooled after being removed from the furnace.

[0090] The steel plate obtained in Example 2 has a thickness of 80 mm.

[0091] The preparation method of Example 3 includes:

[0092] Step 1, Heating: Heat the steel billet to 1150℃ and maintain the temperature to ensure uniformity;

[0093] Step 2, Rolling: Two-stage rolling (TMCP) is performed: the first stage rolling is at 1050-980℃, with an average reduction of about 15% per pass; the second stage rolling is at 850-810℃, with a deformation of about 15% in the large deformation passes.

[0094] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 6℃ / s;

[0095] Step 4, Heat treatment: The steel plate is austenitized at 840℃, held for 4.5 hours, and then water-cooled after being taken out of the furnace; then heated at 730℃, held for 4 hours, and water-cooled after being taken out of the furnace; the first tempering is carried out at 550℃ for 5 hours, and air-cooled after being taken out of the furnace; the second tempering is carried out at 600℃ for 8 hours, and air-cooled after being taken out of the furnace.

[0096] The steel plate obtained in Example 3 has a thickness of 120 mm.

[0097] The heat treatment process parameters for Examples 1-3 are shown in Table 2 below.

[0098] Figure 1 The microstructure of Example 3 (1 / 2 section position) Figure 2 The EBSD map is for Example 3 (1 / 2 section position).

[0099] The low-temperature impact properties of Examples 1-3 are shown in Table 3, the tensile properties test results are shown in Table 4, and the microstructure (all taken at 1 / 4 of the thickness section) is shown in Table 5.

[0100] Table 1 Chemical composition, wt%

[0101]

[0102] Table 2 Process Parameters

[0103]

[0104] Table 3. Results of Low Temperature Impact Performance Test

[0105]

[0106] Table 4 Tensile property test results

[0107]

[0108]

[0109] Table 5 Microstructure of Steel

[0110]

[0111] As shown in Tables 3 and 4, Examples 1-3 of the present invention all achieved good low-temperature toughness levels, with impact energy exceeding 200J at -80℃ and exceeding 150J at -120℃. The ductile-brittle transition temperature (FATT50) was below -130℃. The differences in strength, impact energy, and ductile-brittle transition temperature at the 1 / 4 and 1 / 2 positions of the thickness section were not significant, and good cross-sectional uniformity was observed even at a thickness of 120mm.

[0112] As shown in Table 5 above, the good mechanical properties of Examples 1-3 are matched by the refined microstructure and good reverse-transformation austenite configuration. The microstructure of all examples is tempered martensite + lath bainite. Figure 1 No incompletely quenched granular bainite structure was observed; the microstructure was significantly segmented and refined, with an effective grain size that was extremely small, reaching below 2.2 μm. Figure 2 The original austenite grain size was also less than 12 μm.

[0113] Further analysis was conducted on the elemental enrichment of the reverse-transformed austenite in the embodiments. Samples from Example 3 were used, and high-resolution transmission electron microscopy was employed to detect the elemental enrichment of the reverse-transformed austenite. The results showed that Ni, Mn, and Cu elements were enriched to a certain extent in different regions of the reverse-transformed austenite. This enrichment level is the source and basis of the stability of the reverse-transformed austenite, and also the design source of the good low-temperature toughness of this invention.

[0114] Table 6. Element enrichment of reverse-transformed austenite (Example 3)

[0115]

[0116]

[0117] The above analysis shows that the 785MPa grade steel plate with ultra-low temperature toughness of the present invention has a high toughness level, good strength-toughness matching and stability, low nickel content, and can be stably produced with a larger limit thickness. It can be used not only for pressure vessels for cryogenic gases, but also for ultra-high strength, ultra-low temperature toughness extra-thick steel plates for equipment such as deep-sea submersibles under special working conditions.

[0118] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A 785MPa grade steel plate with ultra-low temperature toughness, characterized in that, The composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.08%~0.15%, Si: 0.06%~0.12%, Mn: 0.60%~1.63%, P: ≤0.010%, S: ≤0.004%, Cr: 0.35%~0.95%, Mo: 0.32%~0.60%, Ni: 3.15%~4.35%, Cu: 0.75%~1.85%, V: 0.04~0.11%, Nb: 0.008%~0.035%, Al: 0.017%~0.036%, Ti: 0.008%~0.012%, with the balance being Fe and other unavoidable impurities; The content of Ni, Mn, and Cu in the 785MPa grade steel plate with ultra-low temperature toughness also satisfies the following condition in relation to the thickness t of the steel plate: 100Ni + 67Cu + 50Mn ≥ 3.55 + 0.12t 1 / 2 Where Ni, Mn, and Cu refer to the mass percentage of the elements, and t is expressed in mm. Ni+Cu>4.2%, where Ni and Cu refer to the mass percentage of the elements; The microstructure of the 785MPa grade steel plate with ultra-low temperature toughness consists of tempered martensite + lath bainite, with an effective grain size of 1.75~2.35μm. The microstructure contains a small amount of reverse-transformed austenite, with a volume percentage of 4%~15%. The reverse-transformed austenite is enriched with high concentrations of Ni, Mn, and Cu. The average Ni content in the enriched regions is 1.6~2.5 times that of the matrix, the Mn content is 1.5~2.8 times that of the matrix, and the Cu content is 2.0~3.0 times that of the matrix. The microstructure includes dispersed precipitates, including TiN, Nb(CN), and AlN. TiN particles are distributed at 12~100 nm, Nb(CN) particles at 5~50 nm, and AlN particles at 3~15 nm. nm; TiN precipitate content is 0.013%~0.020%, AlN precipitate content is 0.011%~0.016%, and Nb(CN) precipitate content is 0.032%~0.045%; The 785MPa grade steel plate with ultra-low temperature toughness has an impact energy of over 150J at -120℃, and the ductile-brittle transition temperature of the 50% fiber content of the impact section of the steel plate is below -120℃.

2. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The content of Mn, Cr, and Mo in the 785MPa grade steel plate with ultra-low temperature toughness satisfies the following relationship with the thickness t of the steel plate: 333Mn + 216Cr + 300Mo > 4.45 + 1.0(t / 50) + 0.2(t / 50) 2 Where Mn, Cr, and Mo refer to the mass percentage of the elements, and t is expressed in mm.

3. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.09%~0.12%, Si: 0.06%~0.12%, Mn: 0.9%~1.5%, P: ≤0.006%, S: ≤0.002%, Cr: 0.4%~0.9%, Mo: 0.40%~0.55%, Ni: 3.3%~4.05%, Cu: 0.9%~1.8%, V: 0.04~0.11%, Nb: 0.02%~0.03%, Al: 0.017%~0.03%, Ti: 0.008%~0.012%, with the balance being Fe and other unavoidable impurities.

4. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.09%, Si: 0.06%, Mn: 0.95%, P: ≤0.006%, S: ≤0.002%, Cr: 0.45%, Mo: 0.42%, Ni: 3.37%, Cu: 0.91%, V: 0.043%, Nb: 0.024%, Al: 0.027%, Ti: 0.011%, with the balance being Fe and other unavoidable impurities.

5. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.10%, Si: 0.10%, Mn: 1.17%, P: ≤0.006%, S: ≤0.002%, Cr: 0.56%, Mo: 0.49%, Ni: 3.57%, Cu: 1.26%, V: 0.082%, Nb: 0.029%, Al: 0.022%, Ti: 0.009%, with the balance being Fe and other unavoidable impurities.

6. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The composition of the 785MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.12%, Si: 0.12%, Mn: 1.48%, P: ≤0.006%, S: ≤0.002%, Cr: 0.85%, Mo: 0.55%, Ni: 4.03%, Cu: 1.68%, V: 0.11%, Nb: 0.021%, Al: 0.017%, Ti: 0.012%, with the balance being Fe and other unavoidable impurities.

7. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The effective grain size is 1.85~2.35μm.

8. The 785MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The volume percentage of reverse-transformed austenite is 4.5% to 15%.

9. A method for preparing a 785MPa grade steel plate with ultra-low temperature toughness as described in any one of claims 1 to 8, characterized in that, Includes the following steps: Step 1: Homogenize the steel billet; Step 2: Two-stage controlled rolling is adopted; Step 3: After rolling, the steel plate is immersed in water to accelerate cooling; Step 4: Perform heat treatment on the steel plate, which includes quenching and tempering.

10. The preparation method according to claim 9, characterized in that, In step 4, quenching includes single quenching or single quenching plus two-phase quenching; the single quenching temperature is 50-150℃ higher than Ac3.

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