A 440mpa grade steel plate having super low temperature toughness and a method for manufacturing the same

Abstract

This invention discloses a 440MPa grade steel plate with ultra-low temperature toughness and its preparation method, belonging to the field of steel materials technology. It solves the contradiction between insufficient strength and toughness grades and excessive nickel content in existing low-temperature steels. The composition of the 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.030%–0.085%, Si: 0.18%–0.38%, Mn: 0.95%–1.35%, P: ≤0.010%, S: ≤0.003%, Cr: 0.01%–0.25%, Mo: 0.01%–0.15%, Ni: 3.20%–4.25%, Cu: 0.12%–0.85%, Nb: 0.008%–0.035%, Al: 0.02%–0.036%, Ti: 0.008%–0.022%, with the balance being Fe and other unavoidable impurities. The steel plate of this invention exhibits 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 440MPa grade steel plate with ultra-low temperature toughness and its preparation method. Background Technology

[0002] The demand for energy gases in my country has driven the rapid development of cryogenic engineering, requiring more steels for use in low-temperature and ultra-low-temperature environments. The application of cryogenic steel can be broadly categorized into four types: low-carbon aluminum-killed steel, high-strength cryogenic steel, nickel-based cryogenic steel, and austenitic stainless steel. The main function of cryogenic steel is to prevent accidental failure due to low-temperature brittleness during service; low-temperature toughness is one of the key technical indicators of cryogenic steel. All ferritic steel materials exhibit cold-brittle transition characteristics; prolonged service in the brittle temperature range will inevitably lead to material and equipment failure.

[0003] Nickel-containing low-temperature steels are the most commonly used type of low-temperature steel, including 2.25% Ni, 3.5% Ni, 5% Ni, 7% Ni, 8% Ni, and 9% Ni steels, with a wide applicable temperature range. As the Ni content increases, the service temperature of the steel decreases significantly. For example, 3.5% Ni steel can serve in environments up to -101℃, 5% Ni steel in environments up to -120℃, and 9% Ni steel and the recently developed 7% Ni steel can safely serve in environments from -163℃ to -196℃. Reverse austenite transformation is an important microstructure in low-temperature steels, maximizing low-temperature toughness and service performance. However, the role and advantages of reverse austenite transformation have not been fully explored and utilized in existing low-temperature steel technologies. Considering the preciousness of nickel resources, how to achieve superior ultra-low-temperature toughness and higher strength in steels using comparable or lower nickel contents, thus providing better cost-effectiveness and structural weight reduction in low-temperature service environments, is an urgent problem to be solved. Summary of the Invention

[0004] In view of the above, the present invention aims to provide a 440MPa grade steel plate with ultra-low temperature toughness and its preparation method, in order to solve the contradiction between insufficient strength and toughness grades and excessive nickel content in existing low temperature steels.

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

[0006] This invention provides a 440MPa grade steel plate with ultra-low temperature toughness. The composition of the 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.030%~0.085%, Si: 0.18%~0.38%, Mn: 0.95%~1.35%, P: ≤0.010%, S: ≤0.003%, Cr: 0.01%~0.25%, Mo: 0.01%~0.15%, Ni: 3.20%~4.25%, Cu: 0.12%~0.85%, Nb: 0.008%~0.035%, Al: 0.02%~0.036%, Ti: 0.008%~0.022%, with the balance being Fe and other unavoidable impurities.

[0007] Furthermore, the content of Ni, Mn, and Cu in the 440MPa grade steel plate with ultra-low temperature toughness satisfies the following relationship with the thickness t of the steel plate: 100Ni + 67Cu + 50Mn ≥ 3.95 + 0.088t 1 / 2 And Ni+Cu>3.85%, where Ni, Mn, and Cu refer to the mass percentage of the elements, and t is in mm.

[0008] Furthermore, the composition of the 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.035%–0.050%, Si: 0.18%–0.30%, Mn: 0.98%–1.33%, P: ≤0.005%, S: ≤0.0015%, Cr: 0.06%–0.20%, Mo: 0.08%–0.15%, Ni: 3.50%–4.20%, Cu: 0.35%–0.82%, Nb: 0.015%–0.030%, Al: 0.02%–0.036%, Ti: 0.008%–0.015%, with the balance being Fe and other unavoidable impurities.

[0009] Furthermore, the matrix structure of the entire cross section includes tempered martensite + lath bainite, with an effective grain size of 3.18–3.95 μm; the original austenite grain size is 12–19 μm.

[0010] Furthermore, the microstructure contains a small amount of reverse-transformed austenite, wherein the volume percentage (RA%) of the reverse-transformed austenite is 3%–10%, and the equivalent diameter (DRA) of the reverse-transformed austenite is 6–22 nm.

[0011] Furthermore, the elemental content in the reverse-transformed austenite has the following characteristics: Ni(RA) = 1.6–2.5Ni, Mn(RA) = 1.8–3.2Mn, Cu(RA) = 2.0–3.2Cu, and C(RA) > 0.25%.

[0012] Furthermore, the ductile-brittle transition temperature (FATT50) at 50% fiber content in the impact section of 440MPa grade steel plate with ultra-low temperature toughness is related to the austenite content (RA%) and equivalent diameter (DRA) as follows: FATT50 = a0 - 100 a1(RA%) + a2(DRA). 3 / 2 Where DRA is expressed in nm, a0 = -118, a1: 5.1 to 5.6, a2: 0.1 to 0.3.

[0013] Furthermore, the ductile-brittle transition temperature of the impact section with 50% fiber content of the 440MPa grade steel plate with ultra-low temperature toughness is -130 to -170℃.

[0014] The present invention also provides a method for preparing the above-mentioned 440MPa grade steel plate with ultra-low temperature toughness, comprising the following steps:

[0015] Step 1: Homogenize the steel billet with heat treatment;

[0016] Step 2: Use two-stage or three-stage controlled rolling;

[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, including primary quenching, two-phase quenching, and tempering.

[0019] Furthermore, in step 4, the primary quenching temperature is 30-50°C higher than Ac3, and the two-phase quenching temperature is in the range of (a*Ac3+b*Ac1), where b = 0.2-0.5 and a = 1-b.

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

[0021] a) The 440MPa grade steel plate with ultra-low temperature toughness of this invention, based on nickel as the main toughening element, uses austenite-forming elements such as Ni, Mn, and Cu in combination with C content to provide austenitizing stabilizing chemical factors. Combined with a corresponding two-phase partitioning heat treatment process, this lays the raw material foundation for obtaining beneficial quantities and distributions of reverse-transformed austenite. Furthermore, this invention discovers the elemental equivalent relationship of Ni, Mn, and Cu, which plays a crucial role in maintaining austenite stabilization across 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. This invention significantly increases the hardenability of the steel without significantly increasing costs, and can stably obtain a full low-temperature transformation microstructure of tempered martensite + lath bainite within a wide thickness range. The high-temperature transformation granular bainite, which significantly deteriorates low-temperature toughness, is avoided across the entire thickness cross-section, resulting in stable strength improvement and good cross-sectional uniformity.

[0022] (b) The addition of microalloying elements such as Nb, Al, and Ti to the 440MPa 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 10–100 nm with an average particle size of approximately 24 nm; Nb(CN) particles are mainly distributed in the range of 5–60 nm with an average particle size of approximately 15 nm; and AlN particles are mainly distributed in the range of 3–20 nm with an average particle size of approximately 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 12–19 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.

[0023] c) The 440MPa 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 3% to 10% and the equivalent diameter (DRA) of reverse austenite is 6 to 22 nm through precise control of composition and matching with appropriate heat treatment processes. The number 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, the Mn content is 1.8 to 3.2 times that of the matrix, and the Cu content in the enriched region is also 2.0 to 3.2 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 number 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 440MPa 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. During the two-phase region secondary 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 440MPa grade steel plate of the present invention, possessing ultra-low temperature toughness, exhibits excellent strength and low-temperature toughness. For example, the room temperature properties of the steel plate are: yield strength above 440MPa (e.g., 457-486MPa), tensile strength above 550MPa (e.g., 552-612MPa), and elongation above 28% (e.g., 28.5%-34.5%); the -120℃ low-temperature properties are: yield strength above 610MPa (e.g., 633-686MPa), tensile strength above 730MPa (e.g., 747-821MPa), and elongation above 30% (e.g., 32.5%-39%); the ductile-brittle transition temperature (FATT50) at 50% fiber content of the impact section is -130 to -170℃. Compared with grades with equivalent nickel content as described in standards such as EN10028, GB 713-5, and JIS G3127, the steel plate of this invention has a strength level that is more than 100 MPa higher, significantly improved low-temperature toughness, and a ductile-brittle transition temperature that is more than 30°C lower.

[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 1;

[0029] Figure 2 The microstructure is shown in Comparative Example 1.

[0030] Figure 3 This is the reverse-transformed austenite morphology of Example 2. Detailed Implementation

[0031] 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.

[0032] This invention provides a 440MPa grade steel plate with ultra-low temperature toughness. The composition of the above-mentioned 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.030%~0.085%, Si: 0.18%~0.38%, Mn: 0.95%~1.35%, P: ≤0.010%, S: ≤0.003%, Cr: 0.01%~0.25%, Mo: 0.01%~0.15%, Ni: 3.20%~4.25%, Cu: 0.12%~0.85%, Nb: 0.008%~0.035%, Al: 0.02%~0.036%, Ti: 0.008%~0.022%, with the balance being Fe and other unavoidable impurities.

[0033] Specifically, the thickness t of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness is 5 to 80 mm.

[0034] The following details the function and dosage selection of the components contained in this invention:

[0035] Ni: Among the various alloying 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 a liquid nitrogen temperature of -196°C, 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 3.20%–4.25%.

[0036] 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 also 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 low-temperature steel, the more appropriate the C content needs to be. Taking all factors into consideration, the C content in this invention is controlled at 0.030%–0.085%.

[0037] 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 0.18% and 0.38%.

[0038] 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 phase 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 0.95%–1.35%.

[0039] 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; Cu is an important component of the reverse-transformed austenite in the steel of this invention. Thirdly, Cu can also 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 is controlled at 0.12%–0.85% in this invention.

[0040] 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%.

[0041] 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 0.02% to 0.036%.

[0042] 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%.

[0043] P: Phosphorus is an impurity element in steel that can impair the toughness of steel plates and weld heat-affected zones, especially reducing the steel's ultra-low temperature toughness. Therefore, the P content should be controlled below 0.010%, and ideally below 0.005% where conditions permit.

[0044] Sulfur is an impurity element in steel that can form sulfide inclusions, becoming crack initiation sites. Therefore, the sulfur content should be controlled below 0.003%, and ideally below 0.0015%.

[0045] Specifically, the Ni, Mn, and Cu contents in the aforementioned 440MPa grade steel plate with ultra-low temperature toughness also satisfy the following condition with respect to the plate thickness t: 100Ni + 67Cu + 50Mn ≥ 3.95 + 0.088t 1 / 2 And Ni+Cu>3.85%, where Ni, Mn, and Cu refer to the mass percentage of the elements, and t is in mm.

[0046] Specifically, the composition of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.035%–0.050%, Si: 0.18%–0.30%, Mn: 0.98%–1.33%, P: ≤0.005%, S: ≤0.0015%, Cr: 0.06%–0.20%, Mo: 0.08%–0.15%, Ni: 3.50%–4.20%, Cu: 0.35%–0.82%, Nb: 0.015%–0.030%, Al: 0.02%–0.036%, Ti: 0.008%–0.015%, with the balance being Fe and other unavoidable impurities; the steel plate thickness t is 30–80 mm.

[0047] Specifically, the matrix structure of the above-mentioned 440MPa grade steel plate with ultra-low temperature toughness includes tempered martensite + lath bainite structure (which can be referred to as tempered M + lath B), with an effective grain size of 3.18 to 3.95 μm and a standard deviation of ≤0.24 μm; the original austenite grain size is 12 to 19 μm and the standard deviation is ≤0.83 μm.

[0048] Specifically, the microstructure of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness contains a small amount of reverse-transformed austenite, with a volume percentage (RA%) of 3% to 10% and an equivalent diameter (DRA) of 6 to 22 nm.

[0049] Specifically, the M(RA) content in the reverse austenite of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness has the following characteristics: Ni(RA) = 1.6~2.5Ni, Mn(RA) = 1.8~3.2Mn, Cu(RA) = 2.0~3.2Cu, C(RA) > 0.25%.

[0050] Specifically, the microstructure of the aforementioned 440MPa 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 10–100 nm with an average particle size of about 24 nm, Nb(CN) particles are mainly distributed in the range of 5–60 nm with an average particle size of about 15 nm, and AlN particles are mainly distributed in the range of 3–20 nm with an average particle size of about 6 nm. The content of TiN precipitates is approximately 0.013%–0.019%, the content of AlN precipitates is approximately 0.011%–0.016%, and the content of Nb(CN) precipitates is approximately 0.03%–0.039%.

[0051] Specifically, the ductile-brittle transition temperature (FATT50) at 50% fiber content of the impact section of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness 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 volume percentage, DRA is in nm, a0 = -118, a1: 5.1~5.6, a2: 0.1~0.3.

[0052] Specifically, the ductile-brittle transition temperature (FATT50) of the 440MPa grade steel plate with ultra-low temperature toughness at 50% fiber content on the impact section is -130 to -170℃.

[0053] Specifically, the room temperature properties of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness are: yield strength above 440MPa (e.g., 457-486MPa), tensile strength above 550MPa (e.g., 552-612MPa), and elongation above 28% (e.g., 28.5%-34.5%).

[0054] Specifically, the -120℃ low-temperature performance of the aforementioned 440MPa grade steel plate with ultra-low temperature toughness is as follows: yield strength above 610MPa (e.g., 633~686MPa), tensile strength above 730MPa (e.g., 747~821MPa), and elongation above 30% (e.g., 32.5%~39%).

[0055] On the other hand, the present invention also provides a method for preparing the above-mentioned 440MPa grade steel plate with ultra-low temperature toughness, comprising the following steps:

[0056] Step 1: Heat the steel billet to 1080-1160℃ and keep it at that temperature to ensure uniformity;

[0057] Step 2: Use two-stage or three-stage controlled rolling;

[0058] Step 3: After rolling, the steel plate is immersed in water for accelerated cooling, with a cooling rate of not less than 15℃ / s;

[0059] Step 4: Perform heat treatment on the steel plate, including primary quenching, two-phase quenching, and tempering.

[0060] Specifically, in step 2 above, when using three-stage controlled rolling, the rolling temperature range of the first stage is 1060-950℃, and the average reduction per pass is about 15%; the rolling temperature range of the second stage is 860-800℃, and the deformation of the large deformation pass is about 15%; the rolling temperature range of the third stage is 770-740℃, and the average deformation per pass is about 12%.

[0061] Specifically, in step 4 above, the primary quenching temperature is 30-60℃ higher than Ac3, 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)℃.

[0062] Specifically, in step 4 above, the primary quenching temperature is 830–860℃, and the two-phase quenching temperature is 710–740℃.

[0063] Specifically, in step 4 above, the quenching and holding time is generally 2 to 3 minutes per minute.

[0064] 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.

[0065] 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)℃.

[0066] 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.

[0067] 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.

[0068] Compared with existing technologies, the 440MPa 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, and Nb in its composition design. This significantly increases the hardenability of the steel without significantly increasing costs, allowing for a stable 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.

[0069] The addition of microalloying elements such as Nb, Al, and Ti to the 440MPa 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 10–100 nm with an average particle size of about 24 nm; Nb(CN) particles are mainly distributed in the range of 5–60 nm with an average particle size of about 15 nm; and AlN particles are mainly distributed in the range of 3–20 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 12–19 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.

[0070] The 440MPa 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 3% to 10% and the equivalent diameter (DRA) of reverse austenite is 6 to 22 nm through precise control of composition and matching with appropriate heat treatment process. 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 1.8 to 3.2 times that of the matrix, and the Cu content in the enriched region is also 2.0 to 3.2 times that of the matrix, further improving the stability level of reverse austenite. Compared with the prior art, the steel plate of the present invention has increased volume content, reduced size, and significantly increased quantity of reverse austenite, resulting in significantly improved stability. It effectively hinders the crack tip propagation rate under impact loads at low temperatures and improves the low temperature toughness of the steel.

[0071] The method for preparing 440MPa grade steel plates with ultra-low temperature toughness of the present invention employs a two-phase region elemental distribution heat treatment process, namely, primary quenching + two-phase region elemental distribution quenching + tempering. The primary quenching heating process completely austenitizes the steel, resulting in a high dislocation density lath martensite (+ lath bainite) structure and a small amount of retained austenite. During the secondary quenching heating process in the two-phase region, a mixture of tempered martensite and austenite is formed, creating an element-enriched region at the austenite positions of the two phases. This element-enriched region has the following characteristics: 1) the refinement of the original austenite in the previous process gives the enriched region a fine and dispersed characteristic; 2) the compositional combination of austenite-stabilizing elements such as Ni, Mn, Cu, and C significantly improves the austenite stability of the element-enriched region. After a second tempering, the elements in this element-enriched region are redistributed again, resulting in a higher degree of element enrichment in a smaller area, and the formed reverse-transformed austenite exhibits higher stability.

[0072] The 440MPa grade steel plate of this invention exhibits excellent strength and low-temperature toughness. For example, the room temperature properties of the steel plate are: yield strength above 440MPa (e.g., 457-486MPa), tensile strength above 550MPa (e.g., 552-612MPa), and elongation above 28% (e.g., 28.5%-34.5%); the -120℃ low-temperature properties are: yield strength above 610MPa (e.g., 633-686MPa), tensile strength above 730MPa (e.g., 747-821MPa), and elongation above 30% (e.g., 32.5%-39%); the ductile-brittle transition temperature (FATT50) at 50% fiber content of the impact section is -130 to -170℃.

[0073] Examples 1-4

[0074] 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 and comparative examples.

[0075] Examples 1-4 of the present invention provide a 440MPa grade steel plate with ultra-low temperature toughness and its preparation method. The chemical composition of the steel plates in Examples 1-4 is shown in Table 1.

[0076] The preparation method of Example 1 includes:

[0077] Step 1, Heating: Heat the steel billet to 1130℃ and maintain the temperature to ensure uniformity;

[0078] Step 2, Rolling: Perform two-stage rolling (TMCP): the first stage rolling is at 1050-950℃, 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.

[0079] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 20℃ / s;

[0080] Step 4, Heat treatment: The steel plate is austenitized at 855℃, held at that temperature for 2 hours, and then water-cooled after being taken out of the furnace; then heated at 715℃, held at that temperature for 1.5 hours, and water-cooled after being taken out of the furnace; finally, it is held at 580℃ for 4 hours, and then air-cooled after being taken out of the furnace to obtain the steel plate.

[0081] The steel plate obtained in Example 1 has a thickness of 45 mm.

[0082] The preparation method of Example 2 includes:

[0083] Step 1, Heating: Heat the steel billet to 1150℃ and maintain the temperature to ensure uniformity;

[0084] Step 2, Rolling: Three-stage rolling (TMCP) is performed: the first stage rolling is at 1057-952℃, with an average reduction of about 15% per pass; the second stage rolling is at 854-815℃, with a deformation of about 15% in the large deformation passes; the intermediate stage is air-cooled or cooled by high-pressure water using the rolls; the third stage rolling is at 762-741℃, with an average deformation of about 12% per pass.

[0085] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 15℃ / s;

[0086] Step 4, Heat treatment: The steel plate is austenitized at 835℃, held for 3 hours, and then water-cooled after being taken out of the furnace; then heated at 730℃, held for 2.5 hours, and water-cooled after being taken out of the furnace; finally, it is first held at 520℃ for 3 hours, then air-cooled after being taken out of the furnace, and then held at 580℃ for 5.5 hours, and air-cooled after being taken out of the furnace.

[0087] The steel plate obtained in Example 2 has a thickness of 80 mm.

[0088] The preparation method of Example 3 includes:

[0089] Step 1, Heating: Heat the steel billet to 1120℃ and maintain the temperature to ensure uniformity;

[0090] Step 2, Rolling: Three-stage rolling (TMCP) is performed: the first stage rolling is at 1040-970℃, 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; the intermediate stage is air-cooled or cooled by high-pressure water using the rolls; the third stage rolling is at 760-745℃, with an average deformation of about 12% per pass.

[0091] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 15℃ / s;

[0092] Step 4, Heat treatment: The steel plate is austenitized at 845℃, held at that temperature for 2.5 hours, and then water-cooled after being taken out of the furnace; then heated at 720℃, held at that temperature for 2.5 hours, and water-cooled after being taken out of the furnace; finally, it is held at 600℃ for 5 hours and then air-cooled after being taken out of the furnace.

[0093] The steel plate obtained in Example 3 has a thickness of 60 mm.

[0094] The preparation method of Example 4 includes:

[0095] Step 1, Heating: Heat the steel billet to 1100℃ and maintain the temperature to ensure uniformity;

[0096] Step 2, Rolling: Two-stage rolling (TMCP) is performed: the first stage rolling is at 1050-955℃, with an average reduction of about 15% per pass; the second stage rolling is at 855-800℃, with a deformation of about 15% in the large deformation passes.

[0097] Step 3, Cooling: The steel plate is immersed in water to accelerate cooling, with an average cooling rate of 25℃ / s;

[0098] Step 4, Heat treatment: The steel plate is austenitized at 850℃, held for 1.5 hours, and then water-cooled after being taken out of the furnace; then it is heated at 710℃, held for 2 hours, and then water-cooled after being taken out of the furnace; finally, it is first held at 540℃ for 2 hours, then air-cooled after being taken out of the furnace, and then held at 600℃ for 3 hours, and then air-cooled after being taken out of the furnace.

[0099] The steel plate obtained in Example 4 has a thickness of 30 mm.

[0100] The heat treatment process parameters for Examples 1-4 are shown in Table 2 below.

[0101] In Example 2, AlN, TiN, and NbCN precipitates were extracted from the steel using an electrochemical extraction method and quantitatively analyzed. The TiN precipitate content in Example 2 was 0.0172% by mass fraction, indicating that most of the Ti in the steel exists in precipitated form. The AlN precipitate content was 0.014%, indicating that AlN also precipitates to a certain extent, and due to the low precipitation temperature, it exhibits a fine and dispersed distribution. The Nb(CN) precipitate content was 0.0334%, indicating that most of the Nb also exists in precipitate form, playing a role in preventing grain growth during austenite rolling. Through the control of elements such as Ti, Nb, and Al, the second-phase precipitates in this invention play a corresponding role in preventing grain growth during the heating process before steel plate rolling, the steel plate rolling process, and the heat treatment process, respectively, thus refining the grains and improving low-temperature toughness.

[0102] Figure 1 The microstructure of Example 1; Figure 2 The microstructure is shown in Comparative Example 1. Figure 3 This is the reverse-transformed austenite morphology of Example 2.

[0103] The low-temperature impact properties of Examples 1-4 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.

[0104] Table 1 Chemical composition, wt%

[0105]

[0106] Table 2 Process Parameters

[0107]

[0108] Table 3. Results of Low Temperature Impact Performance Test

[0109]

[0110]

[0111] Table 4 Tensile property test results

[0112]

[0113] Table 5 Microstructure of Steel

[0114]

[0115] The inventors conducted extensive experimental research during the research process, and the existing technology solutions are now used as comparative examples.

[0116] Comparative Example 1

[0117] This comparative example provides a steel plate with certain low-temperature toughness. The steel composition is shown in Table 1 above, and the preparation method is as follows:

[0118] Step 1, Heating: Heat the steel billet to 1200℃ and maintain the temperature to ensure uniformity;

[0119] Step 2, Rolling: Two-stage rolling is performed: the first stage rolling is at 1110-1030℃; the second stage rolling is at 900-840℃, and the deformation amount of the above rolling is about 12-15%.

[0120] Step 3, Cooling: Air cooling of the steel plate after rolling;

[0121] Step 4, Heat treatment: The steel plate is austenitized at 850℃, held at that temperature for 2.5 hours, and then water-cooled after being removed from the furnace; it is then held at 600℃ for 6 hours and air-cooled after being removed from the furnace.

[0122] The performance of the comparative examples is shown in Tables 3 and 4 above, and the microstructure is shown in Table 5.

[0123] As shown in Table 3, Examples 1-4 of the present invention all achieved good low-temperature toughness, with impact energy exceeding 150 J at temperatures ranging from -101°C to -140°C. For example, the impact energy at -101°C was 280-340 J, at -120°C it was 250-310 J, and at -140°C it was 160-290 J. The ductile-brittle transition temperature (FATT50) of Examples 1-4 was below -140°C, for example, between -165°C and -140°C. The differences in impact energy and ductile-brittle transition temperature at the 1 / 4 and 1 / 2 positions of the thickness section were not significant, indicating good cross-sectional uniformity. In Comparative Example 1, the ductile-brittle transition temperature (FATT50) ranged from -95°C to -115°C. There were certain differences in impact energy and ductile-brittle transition temperature at the 1 / 4 and 1 / 2 positions of the thickness section, which is related to the acquisition of a certain amount of granular bainite in the core. The ductile-brittle transition temperature of the comparative example was more than 30°C higher than that of the examples.

[0124] As shown in Table 4, Examples 1-4 of the present invention all achieved yield strengths of over 440 MPa, ranging from 457 to 486 MPa, which is 40 to 80 MPa higher than the yield strengths of the comparative examples (380 to 415 MPa). Furthermore, the tensile properties of the thickness sections in the examples showed little difference at the 1 / 4 and 1 / 2 positions, within 15 MPa, while the difference in the comparative examples exceeded 30 MPa. In addition, in the tensile properties at -120°C, the elongation of the examples all reached over 32%, even exceeding the elongation at room temperature. This is related to the fact that the examples obtained a larger volume content of reverse-transformed austenite, resulting in a strain-induced phase transformation (TRIP) effect during low-temperature tensile testing. In contrast, the comparative examples, containing very little reverse-transformed austenite, had relatively lower elongation.

[0125] As shown in Table 5 above, the good mechanical properties of Examples 1-4 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 was observed; the effective grain size was extremely fine, reaching below 4 μm, and the original austenite grain size was also below 19 μm. The microstructure of Comparative Example 1 consisted of tempered martensite + lath bainite + granular bainite. Figure 2 The steel plate contains approximately 10% granular bainite, suggesting that it was not fully hardened. The volume fraction of reverse-transformed austenite in the comparative sample is only about 1%, insufficient to provide adequate stability and low-temperature toughness.

[0126] Further analysis was conducted on the elemental enrichment of the reverse-transformed austenite in the embodiments. Samples from Example 2 were taken, and the elemental composition enrichment of the reverse-transformed austenite was detected using high-resolution transmission electron microscopy. Figure 3 The test results show that Ni, Mn, and Cu elements are enriched to a certain extent in the reverse-transformed austenite in different regions. This enrichment is the source and basis of the stability of the reverse-transformed austenite, and also the source of the good low-temperature toughness of this invention.

[0127] Table 6. Element enrichment of reverse-transformed austenite (Example 2)

[0128]

[0129] 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 440 MPa grade steel plate having ultra-low temperature toughness, characterized in that, The composition of the 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.030%~0.085%, Si: 0.18%~0.38%, Mn: 0.95%~1.35%, P: ≤0.010%, S: ≤0.003%, Cr: 0.01%~0.17%, Mo: 0.01%~0.15%, Ni: 3.20%~4.25%, Cu: 0.55%~0.85%, Nb: 0.008%~0.035%, Al: 0.02%~0.036%, Ti: 0.008%~0.022%, with the balance being Fe and other unavoidable impurities; The content of Ni, Mn, Cu in the 440 MPa grade steel plate with super low temperature toughness satisfies: 100Ni+67Cu+50Mn≥3.95+0.088t 1 / 2 , and Ni+Cu>3.85%, wherein Ni, Mn, Cu refer to the mass percentage of elements, and t is in mm. The microstructure of the 440MPa grade steel plate with ultra-low temperature toughness consists of tempered martensite + lath bainite, with an effective grain size of 3.18~3.95μm. The microstructure contains a small amount of reverse-transformed austenite, with a volume percentage (RA%) of 3%~10%. The microstructure also includes dispersed precipitates: TiN, Nb(CN), and AlN; TiN particles are distributed at 10~100 nm, Nb(CN) particles at 5~60 nm, and AlN particles at 3~20 nm; the content of TiN precipitates is 0.013%~0.019%, the content of AlN precipitates is 0.011%~0.016%, and the content of Nb(CN) precipitates is 0.03%~0.039%. The 440MPa grade steel plate with ultra-low temperature toughness has an impact section ductile-brittle transition temperature of -130~-170℃ at 50% fiber content; the steel plate's low-temperature performance at -120℃ is: yield strength above 610MPa, tensile strength above 730MPa, and elongation above 30%.

2. The 440 MPa grade steel sheet having ultra-low temperature toughness according to claim 1, characterized by, The composition of the 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.035%~0.050%, Si: 0.18%~0.30%, Mn: 0.98%~1.33%, P: ≤0.005%, S: ≤0.0015%, Cr: 0.06%~0.17%, Mo: 0.08%~0.15%, Ni: 3.50%~4.20%, Cu: 0.55%~0.82%, Nb: 0.015%~0.030%, Al: 0.02%~0.036%, Ti: 0.008%~0.015%, with the balance being Fe and other unavoidable impurities.

3. The 440MPa grade steel plate with ultra-low temperature toughness according to claim 2, characterized in that, The composition of the 440MPa grade steel plate with ultra-low temperature toughness, by mass percentage, includes: C: 0.041%~0.050%, Si: 0.18%~0.23%, Mn: 1.12%~1.33%, P: ≤0.005%, S: ≤0.0015%, Cr: 0.06%~0.17%, Mo: 0.08%~0.15%, Ni: 3.50%~4.20%, Cu: 0.55%~0.82%, Nb: 0.015%~0.030%, Al: 0.02%~0.036%, Ti: 0.008%~0.015%, with the balance being Fe and other unavoidable impurities.

4. The 440MPa grade steel plate with ultra-low temperature toughness according to claim 1, characterized in that, The original austenite grain size is 12~19μm.

5. The 440MPa grade steel plate with ultra-low temperature toughness according to claim 4, characterized in that, The equivalent diameter DRA of reverse-transformed austenite is 6~22nm.

6. The 440MPa grade steel plate with ultra-low temperature toughness according to claim 5, characterized in that, The elemental composition of reverse-transformed austenite has the following characteristics: Ni(RA) = 1.6~2.5 Ni, Mn(RA) = 1.8~3.2 Mn, Cu(RA) = 2.0~3.2 Cu, and C(RA) > 0.25%.

7. The 440MPa grade steel plate with ultra-low temperature toughness according to claim 5, characterized in that, The impact fracture 50% fiber rate ductile-brittle transition temperature FATT50 of the 440 MPa grade steel plate with super low temperature toughness and the reversed austenite content RA% and the equivalent diameter DRA exist the following relationship: FATT50 = a0-100 a1 (RA%) + a2 (DRA) 3 / 2 Wherein DRA is in nm, a0=-118, a1: 5.1~5.6, a2: 0.1~0.

3.

8. The 440MPa grade steel plate with ultra-low temperature toughness according to any one of claims 1 to 7, characterized in that, The 440MPa grade steel plate with ultra-low temperature toughness has a ductile-brittle transition temperature of -130~-160℃ at 50% fiber content on the impact section.

9. A method for preparing a 440MPa 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 with heat treatment; Step 2: Use two-stage or three-stage controlled rolling; Step 3: After rolling, the steel plate is immersed in water to accelerate cooling; Step 4: Perform heat treatment on the steel plate, including primary quenching, two-phase quenching, and tempering.

10. The preparation method according to claim 9, characterized in that, In step 4, the primary quenching temperature is 30-50°C higher than Ac3, and the two-phase quenching temperature is in the range of (a*Ac3+b*Ac1), where b=0.2~0.5 and a=1-b.

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